Read the article “Practical Advancement of Multipollutant Scientific and Risk Assessment Approaches for Ambient Air Pollution,” and respond to the questions below.

Your article review must be at least three pages in length. Contain four references and in-text citations.  Four references including article, attached pdf and two additional resources All sources used, including the article, must be referenced in APA style. Title and reference pages do not count towards minimum page requirement.

Unit II Article Review

· Weight: 7% of course grade


Risk Assessment Article Review

Read the article “Practical Advancement of Multipollutant Scientific and Risk Assessment Approaches for Ambient Air Pollution,” and respond to the questions below.

Johns, D. O., Stanek, L. W., Walker, K., Benromdhane, S., Hubbell, B., Ross, M., . . . Greenbaum, D. S. (2012). Practical advancement of multipollutant scientific and risk assessment approaches for ambient air pollution. Environmental Health Perspectives, 120(9), 1238–1242.

After reading the article, write an article review that answers the following questions:

· What is the authors’ main point?

· Who is the authors’ intended audience?

· How will a multipollutant approach provide a better assessment of the health impacts of air pollution?

· What are the problems associated with evaluating the human health effects of multipollutant exposures?

· What aspects of the article are related to hazard identification, dose-response assessment, and exposure assessment? Provide a two-sentence quote from the article and a three to four sentence explanation in your own words about how your quote shows how the article addressed that specific step of the risk assessment and its relationship to pollution regulation.

Your article review must be at least three pages in length. Contain four references and in-text citations. All sources used, including the article, must be referenced in APA style. Title and reference pages do not count towards minimum page requirement.


Carlson, A. E. (2018). The Clean Air Act’s Blind Spot: Microclimates and Hotspot Pollution. UCLA Law Review, 65(5), 1036–1088. Retrieved from

Unit Lesson

The effect an air pollutant will have on human health and the environment is determined by its inherent properties in combination with its surroundings. As you learned in Unit I, there are forces in the atmosphere, such as the Coriolis force and local wind patterns that influence the fate and transport of a particle. The temperature inversion that occurred to make the 1948 Donora air pollution tragedy so deadly is an example of how local atmospheric conditions, and even the topography of a location, can impact the ultimate effect of air pollutants. Air pollutants also have inherent physical and chemical properties that play a role in their fate and transport. For example, in the presence of sunlight, tropospheric ozone (O3) is formed in a series of reactions between NOx, volatile organic compounds (VOCs), and carbon monoxide (CO). It is the chemical properties of the ozone precursors that cause them to react to form ozone. To understand these chemical processes, it is necessary to be familiar with some basic chemistry concepts.

You are all likely familiar with the periodic table of the elements. An element is the simplest form of a substance, and it cannot be broken down further by either chemical or physical means. The smallest unit of an element is an atom, which consists of protons, neutrons, and electrons. The protons and neutrons make up the nucleus of the atom, and the electrons surround the nucleus. The number of protons in the nucleus corresponds to the atomic number of an element. The periodic table is arranged by the increasing atomic number. Russian chemist Dimitri Mendeleev is credited with developing the periodic table. Mendeleev’s table was far from complete, but he left spaces where he predicted elements should be and even proposed their properties (Royal Society of Chemistry, n.d.). The properties of these elements could be predicted because elements in the same group of the periodic table share similar chemical properties. For example, the halogens, which are Group 17, all have seven electrons surrounding the nucleus. This makes them reactive because they only need one electron to form a more stable octet of electrons in their outer shell (LibreTexts, 2013).

Being able to predict the way a pollutant will react in the environment is important to mitigation efforts. There are several important reactions that form some key pollutants. The first type of reaction is a combustion reaction. A combustion reaction occurs when a combustible compound is oxidized, producing carbon dioxide (CO2) and water (H2O). For example, propane (C3H8) burns in the presence of oxygen to form CO2 and H2O:

C3H8 + 5O2  3CO2 + 4H2O

Although this combustion reaction would increase the amount of CO2, a greenhouse gas, no toxic air pollutants are formed in a complete combustion reaction. However, incomplete combustion can form carbon monoxide (CO), which is a poisonous gas. Incomplete combustion of hydrocarbons in the presence of chlorine can form dioxin, another poisonous gas. Other air pollutants can also be formed during combustion reactions if conditions are right to allow for incomplete combustion. Controlling conditions in industrial, power generation and waste incineration processes can help to reduce the level of toxic emissions (Vallero, 2014).

Another important class of reactions in air quality are acid/base reactions. In acid/base reactions, the acid is a proton-donor. The proton comes in the form of a hydrogen atom. Remember that the atomic number of an element corresponds to its number of protons. Therefore, a hydrogen atom consists of one proton and one neutron in its nucleus. The base is the proton acceptor. An example of an acid/base reaction that occurs as a result of air pollution is the reaction between sulfuric acid (H2SO4), which is found in acid rain, and calcium

carbonate (CaCO3). There are many older buildings made of limestone, which is CaCO3. Acid rain breaks down limestone in the following acid/base reaction:

CaCO3 + H2SO4  Ca SO4 + H2CO3

In this case, the acid (H2SO4) donates protons, and the base (CaCO3) accepts them. H2CO3 is unstable and will form carbon dioxide and water, and CaSO4 dissolves in water. This results in the limestone structure degrading and crumbling (Casiday & Frey, 1998).

The degradation of structures by acid rain is one example of a hazard posed by air pollution. Other hazards are presented to human health and ecosystems. Identifying the type of hazard presented by a pollutant is the first step in assessing the overall risk (Vallero, 2014). The U.S. Environmental Protection Agency (EPA) separates risk assessment into human health risk assessment and ecological risk assessment. Human health risk assessment consists of planning, hazard identification, dose-response assessment, exposure assessment, and risk characterization. In the planning stage, factors such as who is at risk, the nature of the hazard, the source of the hazard, as well as general information about exposure pathways and health effects are gathered. During the hazard identification, scientific evidence regarding the potential health effects is weighed to determine the potential for adverse impacts on human health. The dose-response step focuses on how much of the pollutant causes what effect on human health. In the exposure assessment, risk assessors seek to identify how much of the pollutant people would be exposed to and how frequently those exposures would occur. Lastly, the risk characterization brings the information in the hazard identification, the dose-response assessment, and the exposure assessment together to create an overall picture of the risk posed by the pollutant (EPA, 2014b)

Similarly, the ecological risk assessment consists of a planning stage during which risk assessors gather information about species at risk, the sources of the hazard, exposure pathways, and ecological effects of a pollutant. The next step in an ecological risk assessment is the problem formulation. The goal of the problem formulation step is to determine what ecosystem components are most important to protect. Next, the analysis phase of an ecological risk assessment is similar to the exposure assessment and dose-response assessment steps of the human health risk assessment. During the analysis phase of the ecological risk assessment, the assessor tries to determine which species will be exposed to the hazard, to what extent they will be exposed, and what the effects of the exposure will be. The last step, risk characterization, brings all the information together to determine the overall risk to the ecological entity that was identified a priority in the problem formulation stage (EPA, 2014a).

Although we can present human health and ecological risk assessment in a straightforward way, the actual process is extremely complex. In some cases, dose-response information is difficult to obtain. Scientists must extrapolate data from animal studies, which often look at much higher doses than occur in the environment. Not only is the dosage a source of error, but the fact that the studies are on animals instead of people also introduces uncertainly into the final determination of a safe dose (Belton & Conrad, 2016). Adverse health effects can appear many years after exposure for carcinogenic chemicals, making the effects difficult to track. Some pollutants have much more severe effects on sensitive individuals, such as children and those with respiratory problems. There can also be gaps in the data needed for a risk assessment, and the assessor must make assumptions that may or may not be accurate. The cumulative effects of low dose exposure to the numerous pollutants we encounter in our everyday lives is also unknown and difficult to study.

Fortunately, advances in molecular and computational biology are helping us to solve some of these risk assessment problems. Computational analysis is allowing scientists to sift through large amounts of data on potentially dangerous chemicals, comparing their chemical structure to known toxicants. New laboratory test methods and toxicity models are also generating more accurate and complete data for use by risk assessors in evaluating a pollutant’s risk to human health and the environment (Cote et al., 2016).

Continued advancement in risk assessment techniques is important not only to provide a greater understanding of the health risks we face from pollutants, but also to allow for better judgments regarding which risks are most important to mitigate through regulation. There is an economic cost to controlling pollution. So do we protect the most sensitive individuals or the average citizen? The lower the threshold is set, the costlier it is to meet. If large amounts of money are spent to decrease air concentrations of one pollutant to extremely low levels, perhaps another, more harmful chemical will be left at concentrations that are too high. More accurate risk assessment data means that we can come closer to determining more optimal levels of pollutant thresholds with the goal of increasing our overall high.

65 UCLA L. Rev. 1036 (2018)

U.C.L.A. Law Review The Clean Air Act’s Blind Spot: Microclimates and Hotspot Pollution

Ann E. Carlson


The Clean Air Act (CAA) has delivered enormous pollution reductions in its almost fifty-year history, produced huge health benefits, and saved thousands of lives. Its ambient focus has led to the almost complete elimination of lead and dramatic drops in the other pollutants it regulates. Nevertheless, the CAA has a major blind spot: small “microclimates” that contain levels of deadly pollutants that can far exceed federal standards. These hotspots contain pollutants that exacerbate asthma, increase respiratory and cardiac deaths, may cause developmental problems in children, and increase cancer risks. The most prevalent of these pollution hotspots occur in predictable patterns around heavily trafficked roads and industrial facilities. Low-income communities and communities of color are much more likely to live in polluted microclimates and suffer health effects as a result.

This Article argues that the ambient focus of the CAA, which requires the monitoring and regulation of large air districts to produce background levels of pollution that meet stringent federal standards, actually masks pollution hotspots. Residents who live in air districts that receive the imprimatur of “attainment” under the CAA may nevertheless experience air quality that is considerably worse than federal air standards. Paradoxically, residents of air districts that are out of attainment, even with extreme designations, may be breathing background air that is cleaner than residents in attainment zones who are exposed to hotspot pollution. Our regulatory system helps shape our understanding of health and safety risks, in other words, in ways that are inconsistent with scientific reality. Unfortunately, using the traditional CAA mechanisms, primarily the National Ambient Air Quality Standards (NAAQS), to regulate conventional pollutants is an awkward fit for hotspot pollution. States are the primary regulators of these pollutants, yet they have little authority over the biggest causes of hotspot pollution: cars, heavy-duty trucks, and other vehicles.

Nevertheless, the Article offers suggestions for how states and the Environmental Protection Agency (EPA) may be able to address microclimate pollution using existing statutory authority. Indeed the most effective strategy for dramatically reducing hotspot pollution, electrifying the transportation fleet, also happens to be one of the most effective ways to reduce greenhouse gas emissions. Recognizing the immediate health benefits that greening the fleet would deliver could make action on climate change mitigation more palatable.



Shirley Shapiro Professor of Environmental Law, UCLA School of Law. I thank participants in workshops at University of Denver Law School, University of Notre Dame Law School, UCLA School of Law, and the University of Colorado/Duke Law School Climate and Energy Works in Progress Workshop, David Adelman, William Boyd, Jonathan Cannon, Brigham Daniels, Sean Hecht, Bruce Huber, Sarah Krakoff , Doug Kysar, Sarah Light, Suzanne Paulsen, Jed Purdy, Noah Sachs, Seana Shiff rin, Jim Salzman, Alex Wang, and Jonathan Zasloff . I received excellent research assistance from Paul Arata and Elizabeth Kent along with the fabulous research librarians at UCLA School of Law.


Introduction ………………………………………………………………………………………………………………………………1040 I. Setting NAAQS, Evaluating the Public Health Effects of Pollution Exposure ……..1049

A. The NAAQS Statutory Framework …………………………………………………………………………………..1049 B. Air Pollution Accomplishments ………………………………………………………………………………………..1050 C. Remaining Health Effects From Pollution Exposure ………………………………………………………..1051

II. Evidence of Near-Source Exposure…………………………………………………………………………………..1056 A. Mobile Sources ………………………………………………………………………………………………………………….1056 B. Stationary Sources……………………………………………………………………………………………………………..1059

III. Why the NAAQS Are Not Well-Designed to Tackle Near-Source Pollution ………..1060 A. The Concept of Ambient Air Pollution …………………………………………………………………………….1060 B. EPA Measures to Address Near-Source Pollution ……………………………………………………………..1061

1. Defi nition of Scale for Monitoring Purposes ……………………………………………………………..1061 2. NO2 One-Hour Standard …………………………………………………………………………………………..1064 3. Improvements in Stationary Source Monitoring ………………………………………………………..1065

IV. Unsuitability of NAAQS Regulation for Targeting Hotspot Pollution …………………1067 A. State v. Federal Authority Over Sources and the Problem of Near-Road Exposure ………….1067 B. Directly Reducing Emissions From Mobile Sources ………………………………………………………….1068

1. Tailpipe Emissions From New Mobile Sources …………………………………………………………..1068 2. Regulation of Fuels……………………………………………………………………………………………………..1071

C. Existing v. New Mobile Sources ………………………………………………………………………………………..1072 D. Issues With Stationary Sources and Their Relationship to Mobile Sources ………………………1073

1. Stationary Sources, Emissions Factors, and Near-Source Exposure …………………………..1076 V. Can the Current CAA Work to Address Near-Source Pollution? …………………………….1078

A. In the Long Run, Greenhouse Gas Emissions Regulation May Ultimately Solve the Problem ……………………………………………………………………………………………………………..1078

1. Mobile Sources …………………………………………………………………………………………………………..1078 2. Th e Power Sector’s Transition to Renewable Fuels Will Help the Stationary Source Problem …………………………………………………………………………………………………………1081

65 UCLA L. Rev. 1038 (2018)

U.C.L.A. Law Review B. In the Nearer Term, CAA Contains Some Potential Regulatory Avenues for Attacking Near-Source Pollution ……………………………………………………………………………………………………….1083

1. Transportation Control Measures ………………………………………………………………………………1083 2. Indirect Source Rules To Control New or Modified Sources ……………………………………..1084 3. Creative Interpretations of Existing CAA Provisions to Tackle Near-Road Pollution …………………………………………………………………………………………………..1085 4. State Solutions to Near-Source Pollution ……………………………………………………………………1086

Conclusion ………………………………………………………………………………………………………………………………….1088


1040 65 UCLA L. REV. 1036 (2018)


What we see and what we can measure are often what we know and understand. In government, they are also frequently what we regulate. Consider the following. Despite huge strides in cleaning up the nation’s air, children who commute to school on conventional diesel-powered school buses can face pollution levels—of black carbon, hydrocarbons, and nitrous oxides—many times higher than allowable background federal pollution standards.1 Much of the pollution enters the inside of the buses from the tailpipe exhaust of the very vehicles in which students ride.2 Similarly, residents of neighborhoods downwind from highways around the country— especially those with significant truck traffic—can experience highly unhealthful air quality several times during the day.3 The time of day is not necessarily intuitive: Early morning air patterns can create air pollution plumes before many people awaken and can enter homes through open windows and vents.4 Even the configuration of highway ramps can alter exposure amounts.5 Residents who live near airports, as well as pedestrians, can also face elevated pollution levels. Living downwind from a small regional airport with only private plane traffic can lead to pollution levels many times higher than nearby neighborhoods outside the planes’ routes.6 Different neighborhoods within the same air basins can experience highly differential risk from exposure to air pollutants, though, again perhaps

1. See JOHN WARGO ET AL., ENV’T. & HUMAN HEALTH, CHILDREN’S EXPOSURE TO DIESEL EXHAUST ON SCHOOL BUSES 41 (2002), (showing exposure levels on buses exceeded 100 ug/m3, significantly above the federal standard of 12.0 ug/m3); Lisa D. Sabin et al., Characterizing the Range of Children’s Air Pollutant Exposure During School Bus Commutes, 15 J. EXPOSURE ANALYSIS & ENVTL. EPIDEMIOLOGY 377, 384–86 (2004). For a table of federal air pollution standards, see U.S. Environmental Protection Agency, NAAQS Table, air-pollutants/naaqs-table.

2. Sabin, supra note 1, at 382. 3. See Wonsik Choi et al., Prevalence of Wide Area Impacts Downwind of Freeways Under

Pre-Sunrise Stable Atmospheric Conditions, 62 ATMOSPHERIC ENV’T 318, 326 (2012). 4. See id. 5. NEALSON WATKINS & RICHARD BALDAUF, EPA, NEAR-ROAD NO2 MONITORING

TECHNICAL ASSISTANCE DOCUMENT 38 (2012), /nearroad/NearRoadTAD.pdf.

6. Wonsik Choi et al., Neighborhood-Scale Air Quality Impacts of Emissions From Motor Vehicles and Aircraft, 80 ATMOSPHERIC ENV’T 310, 316 (2013).

The Clean Air Act’s Blind Spot 1041

counterintuitively, the risk is often largely due to vehicles rather than large industrial facilities.7

Stationary sources, however, can also cause problems for people within close distance of their emissions. Environmental regulators have long relied on engineering estimates—known as emissions factors—to measure the majority of stationary source emissions. Yet evidence about big, industrial sources, like oil refineries, shows that emissions can be orders of magnitude higher than official measures show.8 And epidemiological evidence demonstrates that residents living downwind of coal-fired power plants face increased risk of cardiovascular mortality and disease.9

All of these individual assaults from pollution, from both mobile and stationary sources, can create pollution levels that exceed national air pollution standards or cause health problems. Yet, because of the way pollutants are monitored and measured under the Clean Air Act (CAA), the air district in which the bus, the neighborhood near the freeway or airport, the highway off- ramp, or the oil refinery are located may be considered compliant with national standards.

The scenarios described above share a common characteristic: The areas sampled are relatively small “microclimates” that due to geography, including the built environment and land use choices, create air pollution hotspots. Until relatively recent advances in monitoring and modeling capabilities, the existence of these air pollution hotspots was not well understood. Moreover,

7. See Shih Ying Chang et al., A Modeling Framework for Characterizing Near-Road Air Pollutant Concentration at Community Scales, 538 SCI. TOTAL ENV’T 905, 917 (2015) (showing that roughly half of near-road exposure comes from heavy duty engines); EPA, INTEGRATED SCIENCE ASSESSMENT FOR OXIDES OF NITROGEN—HEALTH CRITERIA, FINAL REPORT (2016) [hereinafter INTEGRATED SCIENCE ASSESSMENT FOR OXIDES OF NITROGEN] (showing that 60 percent of NO2 emissions come from mobile sources and that highest health risks are from near-road exposure); S. COAST AIR QUALITY MGMT. DIST., FINAL REPORT: MULTIPLE AIR TOXICS EXPOSURE STUDY IN THE SOUTH COAST AIR BASIN (MATES-III) (2008), studies/health-studies/mates-iii/mates-iii-final-report (showing that the vast majority of toxic emissions in the Los Angeles basin come from mobile sources).

8. See Daniel Hoyt & Loren H. Raun, Measured and Estimated Benzene and Volatile Organic Carbon (VOC) Emissions at a Major U.S. Refinery/Chemical Plant: Comparison and Prioritization, 65 J. AIR & WASTE MGMT. ASS’N 1020, 1026 (2015); see also Alex Cuclis, Hous. Advanced Research Ctr., Why Emissions Factors Don’t Work at Refineries and What to Do About It, Paper for EPA (Aug. 13–16, 2012),


1042 65 UCLA L. REV. 1036 (2018)

the health effects of exposure to many of the pollutants that contaminate microclimates, while generally known for many years, have come into much sharper focus as our capacity to understand exposure mechanisms has improved.

I argue in this Article that what we now know and can measure about hotspot air pollution has not caught up with how we regulate. Indeed, our current system of air pollution regulation—while enormously successful— may actually mask pollution hotspots, leaving those who are exposed to them unaware of their health risks and in some cases misleading regulators about their existence. The legal literature, too, has largely failed to recognize the implications of our new measurement capabilities. This Article attempts to fill that gap.

My central contention is that microclimate hotspots raise significant challenges to the Clean Air Act’s central regulatory mechanism, the National Ambient Air Quality Standards (NAAQS), for at least two pollutants: fine particulate matter and nitrogen dioxide (NO2). Indeed, I argue that the current system of ambient air regulation contained in the NAAQS can mask bad air quality by providing an imprimatur of clean air for areas of the country that attain the standards even when microclimates within them are unhealthful. Paradoxically, though, using the NAAQS system to tighten the standards to address microclimate pollution creates its own problems. Allowing microclimate pollution to drive setting the NAAQS can distort the regulatory process and divert attention away from tightening diesel and other mobile source emissions regulations, the dominant culprits in creating pollution hotspots, and onto stationary sources. But the current regulatory system also allows stationary sources that emit unhealthful levels of pollutants to continue to do so, as long as overall ambient air pollution limits are met. As a result, tackling microclimate pollution may require new approaches that, while maintaining the NAAQS approach, begin to address its shortfalls.

To date, microclimate pollution has received very little direct regulatory attention under the Clean Air Act. The CAA instead revolves largely— though not exclusively—around the NAAQS system of regulating six common pollutants sufficiently stringently to create healthy background, or ambient, air quality across a large geographic swath.10 The NAAQS are set at

10. I focus in this Article on microclimate pollution caused by the National Ambient Air Quality Standards (NAAQS) pollutants rather than on hazardous air pollutants (HAP). I do so because the regulatory system to regulate NAAQS differs substantially from the system to regulate HAPs, and thus, I argue, raises unique problems. For an interesting article analyzing data about hazardous air pollutants and arguing that, for the most

The Clean Air Act’s Blind Spot 1043

levels designed to protect public health, welfare, and the environment.11 In order to ensure that the NAAQS are met, states and the federal government together regulate emissions from both stationary and mobile sources, with states taking principal responsibility for stationary sources12 and the federal government and California—through a unique statutory role—leading in regulating mobile sources.13

Assuming the NAAQS are sufficiently stringent, attainment with them implies that background outdoor air in attainment areas is healthful.14 Similarly, a NAAQS nonattainment designation signals that an area has unhealthful air quality. Yet the concept of “ambient” as implemented under the CAA is applied to precisely the opposite of microclimates. Instead, attainment is measured for large geographic areas—air districts designated by the Environmental Protection Agency (EPA) after consultation with individual states15—even though those areas may have very different air quality within their jurisdictional borders.16 Furthermore, the quality of the

part, stationary sources do not cause serious risks of exposure at unhealthful levels, see David E. Adelman, The Collective Origins of Toxic Air Pollution: Implications for Greenhouse Gas Trading and Toxic Hotspots, 88 IND. L.J. 273, 277 (2013) (“Industrial sources of air toxins are geographically concentrated, but even where their emissions are the highest they rarely dominate.”). For a discussion about why our monitoring system may underestimate pollutants from certain stationary sources, see supra note 8 and accompanying text. For the statutory text establishing the NAAQS, see 42 U.S.C. §§ 7408, 7409 (2012).

11. The Clean Air Act (CAA) treats hazardous air pollutants—designated in 42 U.S.C. § 4712 of the CAA—differently by regulating individual sources through a technology- based approach.

12. See 42 U.S.C. § 7410 (2012) (setting forth the State Implementation Plan process to implement national standards).

13. See 42 U.S.C. §§ 7521, 7543 (2012). California is given special authority to issue its own mobile sources emissions standards provided the standards are “at least as protective of public health and welfare” as the federal standards. 42 U.S.C. § 7543(b)(2). The state must receive a waiver from EPA in order to implement separate mobile source standards. For an explanation of this process and its implications for federalism, see Ann E. Carlson, Iterative Federalism and Climate Change, 103 NW. U. L. REV. 1097, 1109–28 (2009).

14. For a list of the NAAQS and an explanation of their operation, see EPA, NAAQS Table, (last visited Mar. 22, 2018). The statutory provisions requiring the establishment of NAAQS are included in 42 U.S.C. § 7409 (2012).

15. 42 U.S.C. §§ 7407(b), 7407(c) (2012). 16. To be fair, the setting of the NAAQS has in recent iterations begun to take account of the

risks of exposure to pollutants as a result of proximity to roadways with high vehicle intensity. In setting the most recent NAAQS for nitrogen dioxide (NO2), for example, the standard was revised to include not only an annual background standard but also to include a one-hour standard, recognizing problems of peak exposure during rush hour. See discussion infra notes 114–116.

1044 65 UCLA L. REV. 1036 (2018)

air is designated based on a relatively small number of monitoring stations, combined with complex modeling, designed to measure that quality.17

The NAAQS have been extraordinarily successful in improving the quality of our ambient air, and I do not mean to diminish that accomplishment. I do, however, want to highlight how our system of NAAQS designations and our methods for measuring and modeling background air pollution raise at least two separate problems in what they signal about the relative healthful or unhealthfulness of air quality.

First, the geographic size of air districts can lead to nonattainment areas that are overinclusive in terms of their attainment designation. By relying on readings of monitoring stations in the worst part of a district, a designation of nonattainment may actually overstate how bad the air is in cleaner parts of the district.18 The South Coast Air Quality Management District (SCAQMD) covering Southern California’s notoriously dirty air is a good example. SCAQMD covers 10,743 square miles and has a population of 18 million people. Measurements from the eastern part of the district routinely show levels of pollutants significantly higher than in the western part, including in the City of Los Angeles. In 2016, for example, West Los Angeles experienced no violations of the 2008 eight-hour ozone standard, downtown Los Angeles experienced one violation, and San Bernardino experienced seventy-six

17. The requirements for district monitoring of air pollutants are contained in 40 C.F.R. § 58 (2017).

18. The NAAQS are set based on a particular measurement of a quantity of air averaged over a particular time period. In the case of carbon monoxide, for example, the primary standard (which protects human health) is divided into both a measurement averaged over eight hours (nine parts per million) and over one hour (thirty-five parts per million). Air districts that exceed the standard more than once a year are considered out of attainment. See EPA, NAAQS TABLES [hereinafter NAAQS TABLES], pollutants/naaqs-table [] (last visited Mar. 7, 2018). For ozone, until 2015, the standard was .075 parts per million, averaged over eight hours; it has now been lowered to .070 parts per million. See EPA, TABLE OF HISTORICAL OZONE NATIONAL AMBIENT AIR QUALITY STANDARDS (NAAQS) [hereinafter TABLE OF HISTORICAL OZONE], national-ambient-air-quality-standards-naaqs []. The Trump Administration has indicated that it will delay the implementation of the new .070 standard by one year. See Timothy Cama, EPA Delays Obama Air Pollution Rule by One Year, HILL, (June 6, 2017), environment/336663-epa-delays-obama-air-pollution-rule-deadline-by-one-year?amp=1 []. To determine whether a designated area is out of attainment, EPA takes the annual fourth-highest daily maximum eight-hour concentration and averages it over three years. See NAAQS TABLES, supra note 18. The further out of attainment an area is, the more serious the regulatory consequences.

The Clean Air Act’s Blind Spot 1045

violations.19 Yet the entire district is designated an extreme nonattainment zone for ozone.20 The district boundaries make regulatory sense because of the geographic configuration of the district and air patterns that typically move pollutants from the west to the east. Nevertheless, common perceptions about how bad air quality is in the City of Los Angeles can be both incorrect and misleading.

Ironically, tightening NAAQS to address microclimate pollution or mandating that monitoring stations measure microclimates might make this overinclusiveness problem worse. Take, for example, near-roadway pollution, a phenomenon well-recognized in the scientific literature. Residents living within 300 to 500 meters of major roadways—and millions of people fall within that category—experience elevated levels of air pollutants, particularly during rush hour.21 If air districts were to rely on monitors placed to measure this elevated pollution, many could experience readings that would tip them into nonattainment areas under the NAAQS for at least two pollutants: nitrogen dioxide and fine particulate matter. Or, if EPA tightened a NAAQS in recognition of the deleterious effects of near roadway pollution—as it has done for nitrogen dioxide by adding a new short-term exposure standard22—more air districts would likely be out of attainment.23 The nonattainment designation would, in turn, trigger a series of regulatory requirements for these districts aimed at attempting to bring them back into attainment.24 Yet the most obvious option to increase the likelihood of attainment—tightening various mobile source standards—is outside the regulatory purview of forty-nine states and all local air districts and is instead

19. Cal. Air Res. Bd., Select 8 Summary, [] (selecting for national 2008 eight-hour ozone standard (0.075 ppm) for West Los Angeles/VA Hospital, Los Angeles N. Main and San Bernardino 4th Street).



22. See EPA, AIR QUALITY GUIDE FOR NITROGEN DIOXIDE 2, 23. Environmental Protection Agency (EPA) has yet to designate nonattainment areas for

the short-term exposure standard. See EPA, NITROGEN DIOXIDE DESIGNATIONS: REGULATORY ACTIONS, dioxide-designations-regulatory-actions [] (“EPA has designated all areas of the country as unclassifiable/attainment.”).

24. To be sure, the new one-hour standard for nitrogen dioxide allows measurements averaged over a three-year period rather than finding a violation for a single event. See EPA, NAAQS TABLES, supra note 18.

1046 65 UCLA L. REV. 1036 (2018)

exclusively federal with the exception of California.25 Moreover, most other measures that might significantly control transportation emissions— particularly those measures that interfere in any meaningful way with driving—are not mandatory under the CAA. States have not been willing to impose meaningful transportation or land use controls, and the language of the CAA comes close to actively discouraging transportation control measures. The result is that a nonattainment designation likely causes states to target stationary sources, which are not the principal cause of microclimate pollution, for more stringent pollution control.

But leaving microclimates out of the monitoring system hardly makes them go away. The way in which the NAAQS system operates creates a problem of underinclusiveness that is far more troubling than the overinclusiveness problem. Using a relatively small number of monitors to measure ambient air quality over a large geographic area, by definition, simply ignores many microclimates. Again, to use the Los Angeles basin as an example, the air district uses thirty-eight monitoring stations around its almost 11,000-mile basin to measure air quality, a number higher than required by federal regulation.26 The stations cannot, obviously, measure the quality of the air across the potentially thousands of microclimates that exist within its borders. And, indeed, EPA regulations discourage using microclimate monitoring to establish ambient limits for a number of NAAQS pollutants, including fine particulate matter. The agency has opposed environmental group efforts to require near-road monitors to measure PM 2.5.27

Yet microclimates around the country have air quality significantly more unhealthful than the background measurements of ambient air suggest.

25. Even air districts within California are preempted from regulating emissions from mobile sources. The U.S. Supreme Court struck down a South Coast Air Quality Management District (SCAQMD) program that mandated that operators of certain vehicle fleets purchase alternative fuel vehicles on the grounds that the rules were preempted by Section 209(a) of the CAA. See Engine Mfrs. Ass’n v. S. Coast Air Quality Mgmt. Dist., 541 U.S. 246, 252–59 (2004).

26. See RENE M. BERMUDEZ ET AL., S. COAST AIR QUALITY MGMT. DIST., ANNUAL AIR QUALITY MONITORING NETWORK PLAN 2 (2016), plans/air-quality-monitoring-network-plan/annual-air-quality-monitoring-network- plan.pdf?sfvrsn=10. In addition to the thirty-eight multipollutant monitors, SCAQMD also has five monitoring sites specifically for stationary sources of lead. Id.

27. The monitoring regulations are contained in 40 C.F.R. § 52 app. D (2016). A coalition of environmental groups recently sued EPA over its failure to mandate near-road monitors in Southern California to establish ambient limits. See Brief for Petitioner at 1–3, Physicians for Soc. Responsibility v. EPA, No. 12-70016 (9th Cir. filed May 18, 2012) (on file with author). EPA opposed the placement of near-road monitors to measure PM 2.5. See infra note 118 and accompanying text.

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This is true not only in areas of the country that are out of attainment for certain NAAQS pollutants but also in areas of the country considered to comply with Clean Air Act requirements. And it is true in some predictable, measurable ways. To put this more concretely, children who ride buses in central Los Angeles and in rural Connecticut both face in-cabin air pollution significantly dirtier than the outside ambient air.28 People who live in neighborhoods downwind from freeways in Memphis, Las Vegas, Denver, and Detroit can breathe air that is on average significantly dirtier than ambient air in the worst nonattainment area in the country, Southern California.29 And residents of communities near oil refineries and chemical plants in Houston, Texas and Torrance, California may be breathing pollutants at levels that exceed federal standards and yet are represented to be compliant.30 Not surprisingly, these harms disproportionately affect low- income communities of color. National data show that Latinos, African Americans, and Asian/Pacific Islanders are much more likely to live near freeways than whites. Nearly half of near-freeway residents are poor or near- poor.31 Residential communities surrounding refineries share similar demographics.32

The phenomenon I describe in this Article also raises a broader theoretical point, one not confined to air pollution. How we monitor and measure pollution—in water, on the ground, in the air—shapes not only how we regulate pollution but also cultural perceptions about health and safety in ways that do not necessarily comport with the best available scientific information. Our “ways of seeing” these pollution problems are shaped by scientific, technical, and legal practices that grant them a scientific authority that, in turn, shapes—and, in the case of microclimate pollution, limits—our regulatory response and public understanding of the health and safety risks we actually face.33 By using seemingly authoritative emissions factors rather


29. See Traffic-Related Health Impacts, supra note 21, at 10 (summarizing results of studies showing that near-road pollution is elevated and causes health risks).

30. See discussion infra notes 98–100. 31. Tegan K. Boehmer et al., Residential Proximity to Major Highways—United States,

2010, CDC, (2013), [ ] (last visited Mar. 7, 2018).

32. See EPA, FACT SHEET: FINAL PETROLEUM REFINERY SECTOR RISK AND TECHNOLOGY REVIEW AND NEW SOURCE PERFORMANCE STANDARDS 3, production/files/2016-06/documents/2010-0682_factsheet_overview.pdf.

33. See William Boyd, Ways of Seeing in Environmental Law: How Deforestation Became an Object of Climate Governance, 37 ECOLOGY L.Q. 843, 847–50 (2010). For an account of

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than actual measurements of refinery emissions—factors that may systematically understate the risk to residents in adjacent neighborhoods— regulators may simply have been unable to “see” or understand the complaints of residents who can smell the chemicals and regularly experience respiratory problems. By stamping an air district as “in attainment” with all six federal air standards, residents of the district may feel confident that the air they breathe is healthful, even when living close to a heavily trafficked freeway. And by labeling an air district an “extreme non-attainment zone,” residents (and observers) may believe they are being poisoned, even when the air quality for many of them is healthier than the air in parts of attainment zones. In addition to identifying the problem of systematic hotspot air pollution and the role the Clean Air Act plays in its continuation, then, part of my aim in this project is to unmask the role that monitoring, measuring, and labeling air pollution play in our collective understanding of the quality of air we breathe.

I turn in Part I to describing the statutory scheme that establishes the NAAQS and its enormous public health accomplishments since its 1970 passage. I then describe our increasing understanding of the health problems that remain as the result of exposure to three of the NAAQS pollutants that remain ubiquitous: ozone, NO2, and fine particulate matter. I also describe the ways in which near-source exposure to the latter two pollutants are of particular concern, and the sources, both mobile and stationary, that cause this exposure. In Part II, I describe the monitoring system EPA requires of states, including the scale of the area to be monitored for the pollutants that cause near-source health risks. Part II also includes a discussion of three ways in which EPA is attempting to address hotspot pollution under its existing authority. In Part III, I turn to a more systematic discussion of the significant shortcomings of the NAAQS for addressing hotspot pollution despite EPA’s efforts to address it. In Part IV, I describe ways in which EPA programs that are not necessarily designed to address near-source exposure nevertheless can produce significant reductions in the pollutants that cause it. In fact the single most effective way to reduce near road exposure is also one of the central means to reduce greenhouse gas emissions—cleaning up the transportation fleet with a push toward vehicle electrification. As a result, the push for climate change mitigation in the transportation sector has immediate and important health

how changes in our ability to detect toxins in the environment led to a major shift in our “ways of seeing” chemical hazards in the water, on the land, in our food, and in our workplaces, see William Boyd, Genealogies of Risk: Searching for Safety, 1930s–1970s, 39 ECOLOGY L.Q. 895, 944–77 (2012).

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co-benefits that could make such regulation more politically palatable. Since that is a multi-decadal strategy, however, I also evaluate additional ways to curtail hotspots in the shorter term, including through transportation measures and land use controls. I conclude by suggesting that Clean Air Act reform may be necessary to address hotspot pollution meaningfully in the shorter and medium term.


A. The NAAQS Statutory Framework

The central—though not only—means for cleaning up the nation’s air has been through the National Ambient Air Quality Standards.34 Although familiar to many, a recitation of the ways in which the NAAQS system operates helps set the stage for understanding how the system can both produce dramatically cleaner air and mask the deleterious health effects of certain pollutants that concentrate in small geographic areas.

The Environmental Protection Agency sets NAAQS for pollutants that “endanger public health and welfare” and that are emitted by numerous types of sources.35 To date, six pollutants have been designated as NAAQS pollutants: lead, carbon monoxide, nitrogen dioxide, ozone, sulfur dioxide, and particulate matter, which is further broken down into particulate matter, known as PM 10 (for particles that are between 10 and 2.5 microns in size), and fine particulate matter, known as PM 2.5 (for particles smaller than 2.5 microns).36

The process for designating, and then updating, a pollutant regulated under the NAAQS provisions is set forth in Sections 108 and 109 of the CAA. In addition to the requirements that a pollutant “endanger public health and welfare” and come from ubiquitous sources, Section 108 requires the administrator to issue criteria documents that assess the latest scientific evidence about the health and welfare effects of exposure to the pollutant.37 Section 109 requires that the EPA Administrator appoint an independent scientific review committee to assist in the promulgation of NAAQS and that

34. 42 U.S.C. §§ 7408, 7409 (2012). 35. Id. § 7408(a)(1)(A)–(B). 36. See Criteria Air Pollutants, EPA, [

BG4P-VZJU] (last visited April 14, 2018). 37. 42 U.S.C. § 7408(a)(2).

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the NAAQS be reevaluated and revised every five years with the assistance of the committee.38 Based on the evaluation of the scientific data about the health and welfare effects of pollution exposure, NAAQS “shall be ambient air quality standards the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health.”39

Ambient air standards apply to geographic units known as air quality control regions (AQCRs). EPA first designated a number of AQCRs in the 1960s, as the federal government began to expand its role in regulating air pollution; Section 107 of the CAA recognizes the pre–1970 AQCRs and provides authority to EPA to designate air regions for purposes of compliance with the NAAQS.40 As an early EPA document about the designations explains, the regions were to be based on “jurisdictional boundaries, urban- industrial concentrations, and other factors, including atmospheric areas, necessary to provide adequate implementation of air quality standards.”41 States have the primary responsibility for “assuring air quality within the entire geographic area comprising such State” while EPA is responsible for determining whether air districts comply with the NAAQS.42 The EPA Administrator desig- nates districts as either attainment, nonattainment, or unclassifiable depending on whether they meet the standard, do not meet the standard, or lack the requisite information to determine attainment status.43

B. Air Pollution Accomplishments

Since the first NAAQS were promulgated shortly after the passage of the modern CAA in 1970, the progress the United States has made in cleaning up the air is remarkable. Despite the fact that systematic evidence of hotspot pollution exists, these accomplishments deserve highlighting. Over the last forty-five years, emissions of the six pollutants regulated under the NAAQS program have dropped by close to 70 percent.44 EPA estimates that the 1990 amendments to the Clean Air Act will have saved more than 230,000 lives by

38. Id. § 7409(d)(1)–(2). 39. Id. § 7409(b)(1). 40. Id. § 7407. 41. EPA, FEDERAL AIR QUALITY CONTROL REGIONS 1 (1972),

ZyPURL.cgi?Dockey=P10054HI.TXT. 42. § 7407(a)–(b), (d)(1)(B) (2012). 43. § 7407(c)–(d) (2012). 44. See EPA, OVERVIEW OF THE CLEAN AIR ACT AND AIR POLLUTION, [].

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2020, reduced 2.4 million cases of asthma exacerbation, and prevented 17 million sick days.45 For some pollutants, including lead and sulfur dioxide, only a handful of counties across the country are out of attainment with the standard.46 Lead has been almost completely eliminated as an air pollutant, with a 98 percent reduction in airborne lead between 1980 and 2014.47 And for carbon monoxide, the entire country has attained the NAAQS.48 These accomplishments are all the more impressive given U.S. economic49 and population growth50 during that time period.

C. Remaining Health Effects From Pollution Exposure

Significant evidence has mounted over the years, however, that three NAAQS pollutants—PM 2.5, ozone, and nitrogen dioxide (NO2)—continue to cause persistent and ubiquitous health problems and increases in morbidity.51 Two of those pollutants, fine particulate matter and NO2, can be

45. See EPA, BENEFITS AND COSTS OF THE CLEAN AIR ACT, 1990–2020, THE SECOND PROSPECTIVE STUDY, clean-air-act-1990-2020-second-prospective-study [].

46. See EPA, LEAD (2008) DESIGNATED AREA/STATE INFORMATION [hereinafter LEAD 2008 STANDARD NONATTAINMENT AREAS], (last updated Feb. 28, 2018); EPA, SULFUR DIOXIDE (2010) DESIGNATED AREA/STATE INFORMATION [hereinafter SULFUR DIOXIDE 2010 STANDARD NONATTAINMENT], (last updated Feb. 28, 2018). As the 2008 Sulfur Dioxide Integrated Science Assessment explained, “[n]o monitored exceedance of the SO2 annual ambient air quality standard in the lower 48 States of the U.S. has been recorded between 2000 and 2005, according to the EPA Acid Rain Program (ARP) 2005 Progress Report.” EPA, INTEGRATED SCIENCE ASSESSMENT FOR SULFUR OXIDES—HEALTH CRITERIA 2–23 (2008) [hereinafter INTEGRATED SCIENCE ASSESSMENT FOR SULFUR OXIDES].

47. EPA, BASIC INFORMATION ABOUT LEAD AIR POLLUTION, pollution/basic-information-about-lead-air-pollution [] (last visited Mar. 8, 2018).

48. See EPA, CARBON MONOXIDE (1971), DESIGNATED AREA/STATE INFORMATION, (last updated Feb. 28, 2018).

49. The U.S. economy has grown by 80 percent since 1970 as measured by adjusted GDP. See DATA 360, GDP-REAL (ADJUSTED) UNITED STATES, Data_Set_Id=354 [].

50. The U.S. population grew from 205 million in 1970 to 316 million in 2013. See U.S. POPULATION BY YEAR, [ RPAY].

51. This is not to suggest that the other three NAAQS pollutants cause no health problems, but rather that their airborne levels have dropped dramatically enough that exposure levels are causing less concern than for the remaining three. See, e.g., INTEGRATED SCIENCE ASSESSMENT FOR SULFUR OXIDES, supra note 46, at 2–53 (“Because SO2 concentrations have declined markedly over the past few decades, relatively few recent personal exposure studies have focused on SO2.”).

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elevated near the sources from which they are emitted and can cause significant health problems for those breathing in the pollutants. Ozone, by contrast, can be elevated in areas away from the primary sources, and thus the system of NAAQS regulation is better suited for its control. Nevertheless, nitrogen dioxide (along with other NOx and volatile organic compounds) contributes to the formation of ground-level ozone so that better control of NO2 will lead to lower ozone levels.52 And two NAAQS—nitrogen dioxide and sulfur dioxide—can transform in the atmosphere into PM 2.5.53 The ongoing control of all four of these NAAQS, then, is key to reducing the ongoing negative health effects of air pollution.

Since the passage of the contemporary Clean Air Act in 1970, our scientific understanding of air pollution has significantly improved in two important ways relevant to my analysis. First, we have much better data about the specific health effects of particular pollutants at particular exposure levels. Second, we can measure individual exposure to pollutants in much smaller spaces more precisely. Our regulatory structure to control air pollution, however, has not caught up to these improvements in scientific knowledge.

A number of long-term studies have found a strong relationship between exposure to particular pollutants and increased risk of mortality, and all show that the risk of mortality increases with higher levels of exposure to the particular pollutant.54 Perhaps the most striking new finding about the

52. EPA, OZONE POLLUTION [hereinafter OZONE POLLUTION], pollution [] (last visited Mar. 8, 2018); EPA, NITROGEN DIOXIDE BASICS [hereinafter NITROGEN DIOXIDE BASICS], pollution/basic-information-about-no2 [] (last visited Mar. 8, 2017). NO2 is one of seven compounds that make up NOx and the only one of the seven that is separately regulated from ozone. NO2 is the most prevalent of the seven NOx compounds. See EPA, TECHNICAL BULLETIN, NITROGEN OXIDES (NOX): WHY AND HOW THEY ARE CONTROLLED 1 (1999).


54. See Johanna Lepeule et al., Chronic Exposure to Fine Particles and Mortality: An Extended Follow-up of the Harvard Six Cities Study From 1974 to 2009, 120 ENVTL. HEALTH PERSP. 965, 967 (2012) (showing that exposure to fine particulate matter increases the risk of mortality from cardiac disease and lung cancer and that “[e]ach 10- μg/m3 increase in PM 2.5 was associated with a 14% increased risk of all-cause death . . . a 26% increase in cardio-vascular death . . . and a 37% increase in lung- cancer death”); Michelle C. Turner et al., Long Term Ozone Exposure and Mortality in a Large Prospective Study, 193 AM. J. RESPIRATORY & CRITICAL CARE MED. 1134, 1139 (2016) (“We observed significant positive associations between long-term O3 and all- cause, circulatory, and respiratory mortality with 2% , 3%, and 12% increases in risk per 10 ppb, respectively, in this large-scale study . . . .”); Annunziata Faustini et al., Nitrogen

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health effects of air pollution is that populations exposed to two of the most ubiquitous pollutants—ozone and PM 2.5—face increased risk of death even at rates lower than the current stringent federal standards. In a remarkably comprehensive study, researchers looked at 61 million Medicare enrollees from across the country, including in less populated (and less frequently studied) parts of the country.55 They broke the study group down by 1 x 1 km geographic areas. For every increase in average annual exposure of 10 micrograms of PM 2.5 per cubic meter, the mortality rate among Medicare enrollees increased 7.3 percent. Increases of 10 parts per billion of ozone caused less significant, but still real, increases in mortality rates. Surprisingly, for people exposed to levels below the fine particulate matter federal standard, the increase in mortality rate was even larger: For every increase of 10 micrograms per cubic meter of PM 2.5, mortality increased by 13.6 percent.56

We also know that exposure to specific air pollutants causes particular health effects in addition to increased risk of death. In the Environmental Protection Agency’s most recent PM 2.5 Integrated Scientific Assessment— which comprehensively reviews and assesses the evidence of the health effects of pollutant exposure—the agency concluded that short-term exposure to PM 2.5 causes cardiovascular effects, likely causes respiratory ailments, and causes premature mortality. Long-term exposure has the same effects and even more, with evidence suggesting that exposure is related to reproductive harm, developmental problems in children, and cancer.57

Dioxide and Mortality: Review and Meta-Analysis of Long-Term Studies, 44 EUR. RESPIRATORY J. 744, 751 (2014) (showing that exposure to nitrogen dioxide increases overall cardiovascular and respiratory mortality, explaining how the results hold independent of multiple pollutant exposure, including fine particulates, and concluding that “NO2 effects ranging from 3% to 36% per 10 gm-3 have been reported for cardiovascular mortality in European cohorts as well as an effect of 12% for respiratory mortality”).

55. See generally Qian Di et al., Air Pollution and Mortality in the Medicare Population, 26 NEW ENG. J. MED. 2513 (2017).

56. Id. at 2513. 57. See ASSESSMENT FOR PARTICULATE MATTER, supra note 53, at 2–9. EPA conducts a

comprehensive Integrated Science Assessment every time it reviews whether to strengthen a NAAQS. For a description of the process, see EPA, LEARN ABOUT THE ISAS, (last visited Mar. 8, 2018). The team conducts an extensive review of existing evidence about the health effects of the pollutant being reviewed and then characterizes the strength of the evidence about particular effects. For some health effects, for example, EPA concludes that there is evidence “sufficient to conclude there is a causal relationship” between exposure to the pollutant and the particular health effect. For others, EPA might find that the evidence is “suggestive of a causal relationship” but not sufficient to infer one. See ASSESSMENT FOR PARTICULATE MATTER, supra note 53, at 1–21. See also Lepeule, supra note 54, at 965 (showing that

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The improvement in our understanding of the health effects of exposure to fine particulate matter since the 1970 passage of the CAA is quite remarkable. The earliest federal standard regulated only “Total Suspended Particles” (TSP) without regard to particle size. As of 1982, when EPA prepared air quality criteria documents for TSP, the agency concluded that “essentially no epidemiological studies” provided sufficient data to connect “respiratory disease or other types of mortality to chronic (annual average) exposures to PM . . .”58 The same document did acknowledge that health effects “might most reasonably and directly [be] attributed to fine- and small coarse-mode particles” but also found very little evidence of health effects for exposure even at levels that far exceed what we now know to be highly unhealthful.59 It was not until 1997 that EPA established a separate NAAQS for fine particulate matter. The new fine particulate standard was set because data from six cities indicated that “[f]ine particles (PM 2.5) showed a consistent and statistically significant relationship to acute mortality” with risks increasing as levels of fine particulates increased.60 By 2012, the standard had been tightened twice, with the most recent standard set based on the evidence described above of substantial health risks at exposure levels far below what was previously understood.61

Our knowledge of the health effects of ozone exposure has increased dramatically as well. We now know that short-term ozone exposure causes respiratory effects and likely causes cardiovascular effects and increases in morbidity, according to EPA’s 2013 Integrated Scientific Assessment.62 Evidence suggests that short-term ozone exposure causes central nervous system effects as well. Additionally, researchers have found associations between short-term exposure and increased hospital visits and increased allergic and asthma-related responses.63 Epidemiological evidence also exists

exposure to fine particulate matter increases the risk of mortality from cardiac disease and lung cancer).


59. Id. at 1-92, 1-99, 1-103. 60. See EPA, AIR QUALITY CRITERIA FOR PARTICULATE MATTER 1-13 (1996). 61. See EPA, PARTICULATE MATTER (PM STANDARDS)—TABLE OF HISTORICAL PM NAAQS, [] (last visited Mar. 7, 2018).

62. See EPA, INTEGRATED SCIENCE ASSESSMENT FOR OZONE AND RELATED PHOTOCHEMICAL OXIDANTS, at 1-5, 2-20 to 2-24 (Feb. 2013) [hereinafter ASSESSMENT FOR OZONE AND RELATED PHOTOCHEMICAL OXIDANTS], deid=247492; see also Turner et al., supra note 54, at 1134, 1139., 193 AM. J. RESPIRATORY & CRITICAL CARE MED. 1134, 1139 (2016).

63. Id.

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showing that short-term exposure is associated with pulmonary inflammation and lung host defenses. Long-term exposure to ozone is connected to similar health outcomes, though the evidence for causation of many of the health outcomes is somewhat weaker for longer term exposure as opposed to shorter term.64 Health effects from ozone exposure appear to be worse for children, older adults, outdoor workers, and individuals with asthma.65 Some evidence—still suggestive—exists showing that individuals who are obese and those of lower socioeconomic status may also suffer worse health outcomes.66 Women, Latinos, and African Americans may as well, though more study is needed.67

The sum total of the scientific knowledge of the health effects of ozone, in 1971, by contrast, was described in the Federal Register notice announcing the final adoption of the first standards: “The revised national primary standard . . . is based on evidence of increased frequency of asthma attacks in some asthmatic subjects on days when estimated hourly average concentrations of photochemical oxidant reached . . . 0.10 ppm.”68

Exposure to NO2 also produces negative health outcomes. In EPA’s most recent Integrated Scientific Assessment of Oxides of Nitrogen, the agency concluded that short-term exposure to NO2 likely causes respiratory ailments and may cause cardiovascular disease. Long-term exposure likely causes respiratory disease and may cause increases in cancer, low birth weight, and cardiovascular effects.69 Vulnerable populations include children, the elderly, and people with asthma.70

64. See id. at 1-5. 65. See id. at 1-8, 8-11 to -15, 8-18 to -23. 66. See id. at 8-26 to -27, 8-30 to -31. 67. See id. at 8-24 to -26, 8-28 to -29. 68. National Primary and Secondary Ambient Air Quality Standards, 36 Fed. Reg. 8186

(Apr. 30, 1971) [hereinafter Ambient Air Quality Standards]; see also Faustini et al., supra note 54, at 751 (exposure to nitrogen dioxide increases overall cardiovascular and respiratory mortality and explaining how the results hold independent of multiple pollutant exposure, including fine particulates). In its most recent Integrated Science Assessment of Oxides of Nitrogen, EPA concluded that evidence of exposure to NO2 is suggestive of, but insufficient to establish definitively, a relationship between short- and long-term exposure and mortality. See INTEGRATED SCIENCE ASSESSMENT FOR OXIDES OF NITROGEN, supra note 7, at lxxxii.

69. For a chart describing EPA’s conclusions about the relationship between NO2 exposure and health effects, see INTEGRATED SCIENCE ASSESSMENT FOR OXIDES OF NITROGEN, supra note 7, at lxxxii.


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Again in contrast to the most recent EPA assessment, in 1971 EPA eliminated a short-term standard for NO2 (while retaining an annual standard) on the grounds that “[n]o adverse effects on public health or welfare have been associated with short-term exposure to nitrogen dioxide at levels which have been observed to occur in the ambient air.”71 In 2013, in a complete turnaround, EPA added to its annual standard a new short-term exposure standard based on the scientific information demonstrating respiratory and other health effects from near-source exposure.72


A. Mobile Sources

Not only do we have a much better understanding of the negative health effects of exposure to ozone, PM 2.5, and NO2, we also know significantly more about where and how human exposure occurs. For some pollutants— ozone being the most important—exposure is not tied to living or working near the sources of ozone pollutants. Ozone is a secondary pollutant, one that occurs as the result of the emission of “ozone precursors”—volatile organic compounds (VOCs) and nitrogen oxides (NOx)—that chemically react with sunlight to cause ground level-ozone.73 Once ozone is formed, those who experience the highest levels of ozone exposure are downwind from the sources, sometimes at significant distances. They are typically suburban and rural residents who live downwind of urban, industrialized areas.74

For particulate matter and NO2 emissions, however, near-source exposure matters a great deal. And significant evidence exists that one source in particular, traffic on major roads and highways, is the largest culprit of near-source pollution.

In a 2010 comprehensive review of studies of exposure to traffic-related air pollution, a high-level panel tasked with evaluating the literature concluded that those living within 300 to 500 meters of a major roadway face

STANDARDS FOR NITROGEN DIOXIDE] (reviewing Integrated Science Assessment evidence of health effects of short-term nitrogen dioxide exposure).

71. See Ambient Air Quality Standards, supra note 68, at 8186. 72. See REVIEW OF THE PRIMARY NATIONAL AMBIENT AIR QUALITY STANDARDS FOR NITROGEN

DIOXIDE, supra note 70, at 1-3. 73. See ASSESSMENT FOR OZONE AND RELATED PHOTOCHEMICAL OXIDANTS, supra note 62, at

1-2. 74. See id.

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the highest health risks from traffic emissions.75 There is significant evidence, the panel concluded, that near-road exposure exacerbates asthma and suggestive evidence that living near a major roadway causes asthma, increases respiratory symptoms that are non-asthma related, impairs lung function, increases cardiovascular mortality, and increases overall mortality from all causes.76

The most recent Integrated Science Assessment for NO2 actually labels NO2 a “traffic-related pollutant,” noting that “recent information shows that motor vehicle emissions are the largest single source of NO2 in the air and that NO2 concentrations tend to be variable within communities, decreasing with increasing distance from roads.”77 NO2 concentrations also vary depending, unsurprisingly, on traffic conditions, with rush hour traffic, areas of traffic delay, and so forth exhibiting higher emissions than areas with freer flowing traffic.78

Near-road PM 2.5 exposure also causes real and significant health effects. Traffic combustion generates significant particulate matter, especially fine and ultrafine particles, and “particulates generated from combustion processes, especially diesel exhaust particulates (DEP), are more potent in posing adverse health effects than those from non-combustion processes.”79 A recent study of two roadways in the United States showed that heavy-duty diesel contributes more than half of fine particulate matter and NOx (including NO2) and thatconcentrations of these pollutants drop dramatically 200 meters away from the roads. Light-duty vehicles are a significant source of other pollutants, including benzene.80

More than 11 million Americans live within 150 meters of a major highway.81 Though this is just under 4 percent of the U.S. population, in urban areas the percentage rises dramatically. In Los Angeles, for example,

75. See HEALTH EFFECTS INST., supra note 21, at 5. 76. See HEALTH EFFECTS INST., supra note 21, at 10. 77. INTEGRATED SCIENCE ASSESSMENT FOR OXIDES OF NITROGEN, supra note 7, at lxxxvii. See

also Evelyn S. Kimbrough et al., Seasonal and Diurnal Analysis of NO2 Concentrations From a Long-Duration Study Conducted in Las Vegas, Nevada, 63 J. AIR & WASTE MGMT. ASS’N 934 (2013) (showing higher concentrations closer to highways).

78. See, e.g., Luther Smith et al., Near-Road Measurements for Nitrogen Dioxide and its Association with Traffic Exposure Zones, 6 ATMOSPHERIC POLLUTION RES. 1082 (2015); Shaibal Mukerjee et al., Comparison of Modeled Traffic Exposure Zones Using On-Road Air Pollution Measurements, 6 ATMOSPHERIC POLLUTION RES. 82 (2015) (showing highest exposure levels for heavily trafficked areas as opposed to traffic signals and bus routes).

79. Xianglu Han & Luke P. Naeher, A Review of Traffic-Related Air Pollution Exposure Assessment Studies in the Developing World, 32 ENV’T INT’L 106, 108 (2006).

80. See generally Shih Ying Chang et al., supra note 5. 81. Boehmer et al., supra note 31, at 46–50.

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more than a third of the population lives within 300 meters of a freeway or major road.82 Of the 11 million Americans living within 150 meters of a major highway, about a quarter are under the age of 18 and just under half are nonwhite.83 Latinos, African Americans, and Asian/Pacific Islanders are much more likely to live next to a highway than whites.84 Close to half of near-highway residents are poor or near-poor.85

Exposure to higher levels of NO2 and particulate matter is not limited to residents who live near roads. Children riding on diesel school buses face some of the most alarming exposure rates to carcinogenic particulate matter. In a study of Connecticut school buses, researchers found some buses with fine particulate measurements five to fifteen times higher than ambient levels outside.86 California researchers found similarly alarming levels inside Los Angeles-area buses.87 Occupants of vehicles in heavily trafficked areas also experience elevated levels of pollutants. Levels of PM can be elevated by as much as 40 percent for PM 10 and by 16 percent to 17 percent for fine particulate matter at trafficked intersections.88 Use of the ventilation system in a car, however, can reduce exposure significantly.89

Mobile sources also play a major role in making ports one of the most significant contributors to air pollution and one of the most polluted microclimates. EPA estimates that about 39 million people live in close proximity to a port and acknowledges that diesel emissions from drayage, longer-trip truck traffic, ocean-going vessels, and so forth produce serious health risks for residents living in close proximity.90 In Los Angeles, emissions from the Los Angeles and Long Beach ports constitute the single largest air pollution source in the region.91

82. See HEALTH EFFECTS INST., supra note 21, at 17. 83. BOEHMER ET AL., supra note 31, at 48 tbl. 84. Id. 85. Id. 86. WARGO, supra note 1, at 5. 87. Lisa D. Sabin et al., Characterizing the Range of Children’s Air Pollutant Exposure During

School Bus Commutes, 15 J. EXPOSURE ANALYSIS & ENVTL. EPIDEMIOLOGY 377 (2005). 88. See Prashant Kumar & Anju Goel, Concentration Dynamics of Coarse and Fine

Particulate Matter at and Around Signalised Traffic Intersections, 18 ENVTL. SCI. PROCESSES & IMPACTS 1220, 1234 (2016).

89. See N. Hudda & S.A. Fruin, Models for Predicting the Ratio of Particulate Pollutant Concentrations Inside Vehicles to Roadways, 47 ENVTL. SCI. & TECH. 11,048, 11,054 (2013).


91. See S. COAST AIR QUALITY MGMT. DISTRICT, CLEAN PORT, /initiatives/clean-port [] (last visited Dec. 25, 2017).

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B. Stationary Sources

There is a surprising dearth of data about near-site exposure to emissions of NAAQS pollutants from stationary sources like power plants, refineries, and chemical plants. Somewhat more data exists for toxic emissions from large stationary sources, but those tend to analyze exposure at the census tract-level, where researchers acknowledge that the size of the census tract is not small enough to provide precise estimates of exposure for those closest to the source.92

One major concern about emissions from refineries and chemical plants—which emit some of the most toxic pollutants along with conventional NAAQS pollutants—is that there may be systematic underreporting errors in emissions measurements based on measuring techniques the plants use with approval from EPA. In a recent evaluation of emissions in the Houston Shipping Channel, home to one of the largest concentrations of chemical and petrochemical refineries in the world, researchers used testing devices that measured emissions of VOCs and benzene at levels far higher than the estimates produced and reported by the facilities themselves.93 VOCs emissions were 41 percent higher than emissions inventories reported, and benzene emissions were 94 percent higher.94 Preliminary results from real-time monitoring of refineries and other sources in the Los Angeles area show similar outcomes, with VOCs emissions three to twelve times higher than emissions inventories report.95

92. See Adelman, supra note 10, at 300 (noting that urban census tracts are on average about two square miles, while rural tracts are much larger). In an extensive analysis of data about toxic releases from stationary sources, culled from EPA data, Adelman concludes that less than 10 percent of toxic air emissions come from large industrial sources, with the exception of Texas, given its high concentration of refineries and chemical facilities. Even in the census tracts packed with large industrial sources, toxic emissions from these facilities comprise just over a quarter of total air toxics emissions and only about 10 percent of the excess cancer risk. Id. at 277.

93. Daniel Hoyt & Loren H. Raun, Measured and Estimated Benzene and Volatile Organic Carbon (VOC) Emissions at a Major U.S. Refinery/Chemical Plant: Comparison and Prioritization, 65 J AIR & WASTE MGMT. ASS’N 1020, 1021 (2015); see also Cuclis, supra note 8., (Aug. 15, 2012), session7/acuclis.pdf.

94. Hoyt, supra note 93, at 1029. 95. Johan Mellqvist et al., Quantification of Gas Emissions From Refineries, Gas Stations, Oil

Wells and Agriculture Using Optical Solar Occultation Flux and Tracer Correlation Methods, Presentation for American Geophysical Union Annual Conference Session (Dec. 12, 2016),, reported in Emily Guerin, Refineries in LA Emit Up to 12 Times More Toxic Chemicals

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A. The Concept of Ambient Air Pollution

The NAAQS system is almost by definition designed not to address near-source pollution. The NAAQS measurements are based on averages, culled from monitoring and models, of the ambient air quality across large geographic areas. They do not, nor are they meant to, measure air quality in small, geographically confined areas.

EPA requires states to monitor background levels of pollution, determine whether those levels are consistent with the NAAQS that has been set, and then issue plans to either stay in attainment with the pollution standard or come into compliance with it.96 The idea is not to limit any particular individual source to a specific amount of pollution but instead to regulate all sources of a particular NAAQS pollutant within an air basin to a level sufficient to produce attainment with the NAAQS.97 As a result, EPA requires monitors to be placed in a location that will measure pollutants at an appropriate scale to measure ambient pollution.98 The number of required monitors provides a good illustration of the ambient nature of the measurements: For metropolitan areas with a population of greater than 10 million, four monitors are required to measure ozone, while three monitors

Than Reported, KPCC (Dec. 29, 2016), area-refineries-emit-up-to-12-times-more-toxic.

96. The monitoring regulations are contained in 40 C.F.R. § 58 (2017). Appendix D provides details about the numbers of monitors required by population for individual pollutants, the types of monitors required, and the ways in which monitoring should proceed. Section 107(a) requires that each state submit “an implementation plan for such State which will specify the manner in which national primary and secondary ambient air quality standards will be achieved and maintained within each air quality control region in such State.” 42 U.S.C. § 7407(a) (2012).

97. 42 U.S.C. § 7410(a) requires states to demonstrate how they will either maintain or come into compliance with a NAAQS without requiring any individual source to meet a specified emissions limit. The Prevention of Significant Deterioration provisions require new and modified sources to obtain a permit designed to demonstrate that any emissions from their facilities will not throw the air district in which they are operating out of attainment, rather than to control a set amount of emissions. See EPA, PREVENTION OF SIGNIFICANT DETERIORATION BASIC INFORMATION, nsr/prevention-significant-deterioration-basic-information (last visited Mar. 7, 2018).

98. See 40 C.F.R. § 58 app. D (setting forth a number of monitors and placement requirements for various pollutants).

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are required to measure PM 2.5 in cities with a population of more than a million.99

Although the NAAQS system is designed to measure ambient air in large geographical areas, EPA is not insensitive to the issues posed by microclimate pollution and, as the science has become stronger, has taken several steps to try to address it. But the statutory scheme under which the agency regulates was not designed to simultaneously target ambient air pollution caused by a large number of sources and microclimate pollution that causes near-source harm. Moreover, different NAAQS pollutants pose different measurement and monitoring challenges depending on their primary sources.

B. EPA Measures to Address Near-Source Pollution

Despite the limitations of the NAAQS, EPA has attempted to use three separate regulatory mechanisms in order to address near-source pollution. It has redefined the concept of “ambient” by using different background scales for different pollutants, including a “micro” scale. It has established a new NO2 standard directly addressed to near-source pollution. And it has modified some emissions factors for refineries based on evidence that the factors were underestimating certain pollutant emissions. Beginning in 2018, refineries will be required to monitor certain pollutants at their fence-lines.

1. Definition of Scale for Monitoring Purposes

The first mechanism EPA uses to attempt to address microclimate pollution is to require monitoring based on different scales depending on the NAAQS pollutant being addressed.100 The monitoring regulations set forth

99. See 40 C.F.R. § 58 app. D, tbls. D-1 & D-5. 100. I assume in this paper that air districts place their monitors in a manner that is not only

consistent with EPA regulations but that is also designed to measure accurately the ambient pollution levels necessary to establish compliance with the NAAQS. My assumption may be too sanguine. In a forthcoming paper, Corbett Grainger and coauthors evaluate whether air pollution regulators in air districts that are in attainment but only marginally place new monitors designed to measure NO2 and Ozone strategically to avoid kicking their districts into nonattainment. Using remote-sensing and other data to measure actual background levels of the pollutants, Grainger compares the data with collected monitoring data. The authors conclude that these air districts appear to be systematically understating background pollution levels. See generally Corbett Grainger, Andrew Schreiber & Wonjun Chaing, Do Regulators Strategically Avoid Pollution Hot Spots When Siting Monitors? Evidence from Remote Sensing of Monitors (Oct. 2017) (draft on file with author).

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different spatial scales that are designed to be used with different pollutants. There are six scales ranging from smallest (microscale) to largest (national and global scales).101 The regulations explain that “[p]roper siting of a monitor requires specification of the monitoring objective, the types of sites necessary to meet the objective, and then the desired spatial scale of representativeness.”102 States should use neighborhood, urban, and regional spatial scales to measure ozone, for example, given that it is a secondary pollutant that does not cause near-exposure problems.103 In recognition of the health consequences of NO2, by contrast, EPA now requires the placement of one microscale monitoring station near roads in metropolitan areas with a population of greater than 1 million and two for populations greater than 2.5 million.104 NO2 monitoring is not only done at the microscale level, however, but also requires monitoring at the middle and neighborhood scales.105

The choice of the appropriate scale for monitoring PM 2.5 is more complex and also demonstrates one of the reasons why the NAAQS system presents an awkward fit to address microclimate pollution.106 While it is true that near-road exposure to PM 2.5 causes many health problems, PM 2.5 exposure is also a regional pollutant and shows less spatial variability than a pollutant like NO2.107 The composition of PM 2.5 can vary dramatically

101. 40 C.F.R. § 58 app. D at § 1.2. 102. Id. § 1.2(c). 103. Id. § 4.1(c). 104. Id. § 4.3.2(a). Microscale monitoring is also required for carbon monoxide and sulfur

dioxide to accompany the NO2 microscale monitoring. See 40 C.F.R. § 58 app. D at §§ 4.4.4(a), 4.2.3(a).

105. Id. § 4.3.4(a). 106. Interestingly, lead is the one NAAQS pollutant that may be an exception to my argument

that the CAA is an awkward fit for microclimate pollution. Since leaded gasoline was phased out, almost all airborne lead has been eliminated. Some stationary sources that emit lead remain, and they are subject to microclimate monitoring. Very few air districts are out of attainment with the 2008 NAAQS for lead. Those out of attainment have, for the most part, distinct stationary sources of lead that create the nonattainment problem and that, if regulated stringently enough, will bring the district into attainment. See, e.g., Cal. Air Res. Bd., Proposed State Implementation Plan Revision for the Federal Lead Standard 1– 2, 5 (May 11, 2012), (describing the two battery recycling facilities that occasionally cause Los Angeles County to violate the lead NAAQS and describing measures to eliminate violations). The facilities are also required to engage in enhanced source monitoring. Id. at 6. The NAAQS system works well for lead because there are so few sources of lead, such that monitoring and regulation can target them.

107. See, e.g., Kathie L. Dionisio et al., Development and Evaluation of Alternative Approaches for Exposure Assessment of Multiple Air Pollutants in Atlanta, Georgia, 23 J. EXPOSURE

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depending on its sources, with particulate matter from traffic sources comprised of different pollutants than particulate matter from stationary sources.108 As a result, research has demonstrated that PM 2.5 composition varies significantly across the country: Boston, for example, has significantly higher elemental carbon and NO2 in its PM 2.5 than Pittsburgh, while Pittsburgh has significantly higher concentrations of carbon monoxide and sulfur dioxide.109 Cities and regions of the country also exhibit different risk levels for mortality from PM 2.5 exposure; yet a recent analysis that attempted to determine the causes of this difference, as well as the sources of PM 2.5 pollution, concluded that “[w]hile it is clear that each city is impacted by different air pollution source mixtures, it is unclear which sources contribute to the differences in risk estimates between the cities.”110

As a result of the regional nature of PM 2.5 and its different sources, the choice of what scale to monitor is less obvious than that for NO2, which is more uniformly a traffic-related pollutant. EPA therefore recommends monitoring at the neighborhood scale for PM 2.5, though large cities are required to include with their microscale NO2 monitor a PM 2.5 monitor.111 EPA also makes clear that microscale or middle-scale monitoring may be appropriate if it is “considered to represent area-wide air quality.”112

Indeed, EPA has opposed efforts to require air districts to measure and monitor near-source/microclimate pollution for purposes of compliance with the NAAQS. Environmental groups in Southern California recently challenged the South Coast Air Quality Management District’s monitoring plan that EPA had approved for PM 2.5. The plaintiffs argued that the plan should be invalidated for failing to include a near-road monitor. EPA explained its opposition to the plaintiffs’ argument:

SCI. & ENVTL. EPIDEMIOLOGY 581 (2013) (showing less spatial variability across Atlanta for PM 2.5 than for NOx and other traffic-related pollutants).

108. See PM 2.5 ISA (2009) at 2–7. 109. See generally Lisa K. Baxter et al., Examining the Effects of Air Pollution Composition on

Within Region Differences in PM(2.5) Mortality Risk Estimates, 23 J. EXPOSURE SCI. & ENVTL. EPIDEMIOLOGY, 457 (2013).

110. Id. at 463. 111. 40 C.F.R. pt. 58, app. D, 4.6(b), 4.7.1(b)(2) (2017). 112. 40 C.F.R. pt. 58, app. D, § 4.71(b) (2017). The monitoring of SO2 raises similar issues.

As the 2008 Integrated Science Assessment for SO2 explains: If a monitoring site is to be used to determine air quality over a much larger area, such as a neighborhood or city, a monitoring agency should avoid placing a monitor probe, path or inlet near local, minor sources. The plume from the local minor sources should not be allowed to inappropriately influence the air quality data collected.


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As a practical matter, the area of maximum concentration will most often be a very small area in the immediate vicinity of a dominating local source. However, if the monitor is intended to “represent conditions throughout some reasonably homogeneous urban sub-region with dimensions of a few kilometers” as Sections 4.7.1(b) and (c)(3) require, then the location of maximum concentration will usually be elsewhere. If every monitor simply had to be sited at the location where the single highest reading in the Air District was expected, then the concept of Spatial Scale— and the information it conveys as to the risk to people throughout the urban area—would be lost.113

In other words, the highest maximum concentrations may well be close to the heaviest traffic route but measuring those concentrations would not provide the requisite representativeness of ambient air. Because EPA and the environmental groups settled the case, it is unclear whether EPA’s position is correct as a legal matter; however, it is nevertheless helpful for understanding EPA’s attempt to balance highly localized impacts with background air quality.

EPA’s concerns about scale of monitoring may also reflect serious concerns about what monitoring at the microscale would do to the attainment status of many states and air districts. Moreover, such monitoring as a means for measuring NAAQS compliance would raise serious concerns about the regulatory consequences of a nonattainment designation that could result from near-road measurements. I explore those consequences in Part IV, infra. I first turn, however, to the second regulatory requirement EPA has utilized in recognition of near-road pollution, a new standard for NO2.

2. NO2 One-Hour Standard

In 2010, EPA set a new one-hour standard for NO2 directly in response to evidence that NO2 levels are elevated near heavily trafficked roads and highways. As the preamble to the new standard explains:

[E]stimates . . . suggest that on/near roadway NO2 concentrations could be approximately 80% higher on average across locations than concentrations away from roadways . . . . Because monitors in the current network are not sited to measure

113. Brief for Respondent at 31–32, Physicians for Soc. Responsibility v. EPA, No. 12-70016 (9th Cir. filed Oct. 2, 2012) (on file with author).

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peak roadway-associated NO2 concentrations, individuals who spend time on and/or near major roadways could experience NO2 concentrations that are considerably higher than indicated by monitors in the current area-wide NO2 monitoring network.114

The short-term standard is combined with new monitoring requirements for near-road pollution and is meant to recognize the problems of short-term exposure to elevated levels of NO2 pollution and to get nonattainment states to address roadway pollution.115 We do not at this point know whether the new short-term standard will lead to nonattainment status for many air regions; because states are only beginning to implement the new standard and the supporting monitoring network, EPA has yet to designate attainment/nonattainment standards for it.116 If the new standard leads to a significant number of nonattainment designations, states will face a real conundrum about how to respond. This conundrum, raised not just by the new NO2 standard but by any attempt to regulate microclimate pollution caused by traffic, is one of several conundrums the NAAQS provisions of the CAA create for the regulation of microclimate pollution more generally. I address these issues in Part IV, infra. In the next Section, however, I explore a third way in which EPA is attempting to address near-source pollution.

3. Improvements in Stationary Source Monitoring

In response to concerns that refineries have been using emissions factors that consistently underestimate actual emissions, EPA has begun to refine emissions calculations for flaring and some other activities from refineries.117 And beginning in 2018, for the first time refineries will be required to monitor fence-line emissions.118 This is a major advancement in understanding with precision the actual emissions coming from refineries.

114. 75 Fed. Reg. 6471, 6479 (Feb. 9, 2010) (to be codified at 40 C.F.R. pts. 50 & 58). 115. See supra notes 104–105 and accompanying text (describing near-road monitoring

requirements for the NO2 standard). 116. See, e.g., Letter from Jared Blumenfeld, EPA Region 9 Administrator, to Edmund G.

Brown, California Governor (June 29, 2011) (describing EPA decision to designate all areas of the country “unclassifiable” while new monitoring network is implemented).



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Since EPA began requiring the monitoring of emissions, regulators have relied extensively on emissions factors. These factors are developed by EPA as an inexpensive means to represent actual emissions, but they are based on averages of available data rather than actual monitoring.119 EPA and states rely heavily on emissions factors in constructing emissions inventories, making permitting, compliance, and enforcement decisions, and developing emissions reductions strategies.120 And yet a troubling 2006 U.S. EPA Office of Inspector General (OIG) Report evaluating EPA’s use of emissions factors concluded that 62 percent of EPA’s emissions factors were of below average or poor quality.121 To be sure, a number of large industrial facilities do actual monitoring, typically for equipment or facilities, for which a permit is required.122 Nevertheless, the OIG report estimates that EPA relies on emissions factors for 80 percent of its emissions determinations.123

EPA has sometimes taken enforcement actions against facilities whose actual emissions significantly exceed reported emissions based on emissions factors. The OIG report recounts enforcement efforts after the issuance of a critical 1996 OIG report against some refineries, wood products manufacturers, and ethanol producers.124 More recently, the agency—in response to litigation filed by Texas- and Louisiana-based environmental justice groups concerned that emissions data for refineries was inaccurate— evaluated and revised upward VOCs emissions factors for flaring at refineries and several other refinery operations.125 And finally, after many years of

files/2016-06/documents/2010-0682_factsheet_overview.pdf [ ELQX]. As with many EPA reforms, this change is the result of a citizen suit. For the final rule itself, see Petroleum Refinery Sector Risk and Technology Review and New Source Performance Standards, 80 Fed. Red. 75,178, 75,178–354 (Dec. 1, 2015) (to be codified at 40 C.F.R. §§ 60–63), available at 12-01/pdf/2015-26486.pdf.

119. See EPA, AIR EMISSIONS FACTORS AND QUANTIFICATION, BASIC INFORMATION OF AIR EMISSIONS FACTORS AND QUANTIFICATION, quantification/basic-information-air-emissions-factors-and-quantification [https://perma. cc/4YGW-LPX2].


121. See id. at 9. 122. See, e.g., 40 C.F.R. §§ 64.1–64.2 (2017) (specifying circumstances under which

compliance assurance monitoring is required). 123. EPA, OFFICE OF INSPECTOR GENERAL, supra note 120, at 4. 124. See id. at 11–13. 125. EPA entered into a consent decree to evaluate emissions factors in Air Alliance Houston,

Community In-Power and Development Association, Inc., Louisiana Bucket Brigade and Texas Environmental Justice Advocacy Services (“Plaintiffs”). Air Alliance Houston v. Gina McCarthy, No. 1:13-cv-00621-KBJ (D.D.C. filed May 07, 2014). See NEW EMISSIONS FACTORS, supra note 117.

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environmental justice calls for more effective monitoring of refineries, EPA issued a rule in 2016 that will require monitoring at the fence-lines of refineries to determine actual exposure levels beginning in 2018.

The use of emissions factors to estimate emissions from stationary sources makes sense for measuring ambient pollution at a relatively large scale, even if the emissions factors are not completely accurate. If near-source pollution exposure is not a significant health issue, then, as long as ambient concentrations are measured effectively and are within NAAQS limits, the absolute contribution of any individual source is not as important as the cumulative contributions of all the sources in an air basin. But as our scientific understanding about near-source exposure to pollutants like PM 2.5 and NOx has improved, the importance of the accuracy of individual source emissions has increased. Yet our measuring and monitoring of stationary source emissions appear not to have kept up, with the exception of the new refinery monitoring requirements. Nor is the ambient focus of the CAA particularly well-suited to address individual stationary source emissions in order to address near-source exposure.

I turn to the structural problems NAAQS regulations raise for hotspot pollution next.


A. State v. Federal Authority Over Sources and the Problem of Near- Road Exposure

The division of CAA authority between states and the federal government for the regulation of different types of sources poses perhaps the biggest structural barrier to the regulation of near-road pollution. The familiar cooperative federalism structure of the CAA is worth describing here in order to explain this barrier.

Once a state is designated as either in or out of attainment with a particular ambient air standard, it must prepare a state implementation plan (SIP) that demonstrates either how the area will maintain its air quality in order to remain in attainment with the NAAQS or how the area will come into attainment.126 States must submit their SIPs to EPA for approval. Every five years, the CAA requires EPA to review the criteria on which a NAAQS is

126. 42 U.S.C. § 7410(a)(1) (2012).

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based and update the standards consistent with the new information.127 If a standard is tightened, a new series of attainment/nonattainment designations and SIP updating must occur.128

The SIP process is designed to allow states discretion to develop their own regulatory regimes that will either maintain or achieve compliance with the NAAQS. Nevertheless, state discretion is limited in important ways. Most significantly, states have very little power to directly regulate emissions from mobile sources and substantially more control over the regulation of stationary sources, though even that discretion is cabined in important ways.129 Relatively speaking, however, if a state is out of attainment with the NAAQS, it has more power to regulate and tighten up on stationary sources than it does mobile sources.

The establishment of the one-hour NO2 NAAQS brings this distinction between stationary source and mobile source regulatory authority into stark relief: If a state is found to be in nonattainment with the one-hour standard, its options for directly regulating mobile sources are limited. The same would be true if states were required to monitor PM 2.5 at near-road sites, if the monitors produced readings that kicked a state or one of its air districts into nonattainment or a more serious designation.

To be sure, not all options are foreclosed to states. One way to think about how to reduce near-road emissions is to think broadly about ways to reduce human exposure. States could reduce the emissions that are coming out of mobile sources; they could try to channel the emissions away from humans; or they could keep the humans away from the emissions. Below I explore the role of the CAA in reducing mobile source emissions.

B. Directly Reducing Emissions From Mobile Sources

1. Tailpipe Emissions From New Mobile Sources

Mobile source emissions are the cause of most near-road emissions, comprise 60 percent of NO2 emissions, and make up a large share of PM 2.5. Diesel emissions from heavy trucks and other diesel engines are of particular concern, with some studies showing that half of near-road particulate matter

127. 42 U.S.C. § 7409(d)(1) (2012). 128. 42 U.S.C. § 7410(a)(2)(H)(i) (2012). 129. Requirements for technology-based standards for ozone attainment zones are one

example of this, as the stringency of the requirement ratchets up the further an air district is out of attainment with the NAAQS.

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comes from heavy-duty diesel engines.130 In the long term, the most direct way of cleaning up these emissions is to prevent them from coming out of tailpipes in the first place.

To be sure, federal and California regulations have been remarkably successful in reducing tailpipe emissions for new passenger automobiles, achieving reductions by as much as 99 percent compared with cars manufactured in the 1960s.131 EPA has also issued a series of regulations over the years to clean up heavy-duty diesel engines and to require the use of low- sulfur diesel fuel. The most significant of these regulations required new standards for model year engines 2004 and later and standards mandating engine modifications to allow for the use of low-sulfur diesel fuel.132 EPA has also recently paired with the National Highway Transportation and Safety Administration to tighten fuel economy standards for passenger, medium, and heavy duty vehicles, which I explore in more detail in Part V, infra.

Nevertheless, even with stringent emissions standards, as I have already detailed, mobile sources continue to emit huge percentages of many NAAQs pollutants (as much as 60 percent of NO2 pollution and more than a quarter of PM 2.5) and continue to cause near-road exposure problems. As a result, a state facing nonattainment for the new NO2 short-term exposure standard,

130. Shih Ying Chang et al., supra note 5, at 916. 131. See EPA, History of Reducing Air Pollution From Transportation in the United States, pollution-transportation (last visited March 26, 2018).


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or a state forced to use near-road monitoring to demonstrate PM 2.5 attainment status for the twenty-four-hour standard, would most likely want to rely on more stringent tailpipe emissions in its SIP to demonstrate how it would come into attainment.

The problem with a strategy that relies on regulating tailpipe emissions, however, is that states have very little authority to do so. Section 209 of the CAA preempts all states from regulating emissions from mobile sources, with the exception of California, which can set standards so long as they are at least as protective of public health and welfare as the federal standards.133 States may choose to follow either the federal standards or the California standards, but may not impose their own.134 States are also prohibited from regulating emissions from nonroad vehicles, again with the exception of California, with a similar option to choose to follow the federal standards or the California standards.135 As a result, a state or air district seeking to reduce near-road emissions can either follow the federal standards or California standards but cannot regulate any class of motor vehicles more stringently than those two standards in an attempt to come into attainment. A state’s SIP for nonattainment for near-road sources, then, could not easily rely on the most direct mechanism for addressing the problem. At best, a state that does not already follow the California standards could choose to do so.

Even following California standards does little, however, to address heavy-duty vehicle emissions. Because trucks often enter a state from other states in order to transport cargo, in-state heavy-duty regulation is likely to be insufficient to solve the problem. Even California, which has its own authority to regulate heavy-duty emissions and an economy far larger than most states, faces this problem. Two of California’s air districts, along with a number of air districts from across the country, recently petitioned EPA to regulate NOx emissions from heavy-duty engines more stringently in order to assist them in coming into attainment with the new ozone and PM standards.136 As EPA noted in its response, between 40 and 67 percent of NOx emissions across the country come from mobile sources. In Southern California, this number rises to 88 percent, with the largest source category

133. 42 U.S.C. § 7543 (a)–(b) (2012). 134. Id. § 7507. 135. Id. § 7543(e). 136. See EPA, MEMORANDUM IN RESPONSE TO PETITION FOR RULEMAKING TO ADOPT ULTRA-

LOW NO(X) STANDARDS FOR ON-HIGHWAY HEAVY-DUTY TRUCKS AND ENGINES 5 (2016) [hereinafter MEMORANDUM IN RESPONSE TO PETITION], production/files/2016-12/documents/nox-memorandum-nox-petition-response-2016- 12-20.pdf [].

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being diesel trucks.137 Without federal regulatory action, many districts will not be able to comply with new standards for ozone and PM.138 This problem is even more acute for near-road exposure, which is entirely caused by mobile sources, with heavy-duty trucks playing a key role.

2. Regulation of Fuels

Unlike tailpipe emissions, states do have authority to regulate fuels with permission from EPA, subject to some significant limitations, and could attempt to do so to reduce near-road exposure. State authority here, however, is limited in important ways. Under the 1990 Amendments to the CAA, states gained authority to require the use of new fuels under certain conditions with EPA approval. EPA could approve a state fuel requirement, but only if it found that the regulation of fuel is necessary to achieve the NAAQS and that “no other measures . . . would bring about timely attainment . . . ,” unless the state could show that other measures “are technically possible to implement, but are unreasonable or impracticable.”139 This authority led EPA to approve seven different fuels used in twelve different states around the country.140 The proliferation of these so-called “boutique” fuels led to opposition from industry groups and Congress responded in the Energy Policy Act of 2005. The Act restricted state authority by allowing the use of only those seven fuels already approved by EPA as of the time of the amendments.141 Thus, states can require less polluting fuels, but can only use those already on the boutique fuels list and only with EPA approval.

California, once again, has its own authority to regulate fuels, but unlike the mobile source emissions authority, other states cannot adopt California fuels absent a showing to EPA that adoption of California fuel is necessary for NAAQS attainment.142 Moreover, for diesel fuel, some of the same interstate

137. See id. at 5. 138. Although EPA indicated in its responsive memorandum to the air district petition that

it would initiate rule-making proceedings, see id. at 20, given the change in EPA leadership such a regulatory move now seems unlikely.

139. 42 U.S.C. § 7545(c)(4)(C). 140. See EPA, GASOLINE STANDARDS, STATE FUELS,

state-fuels (last visited Mar. 7, 2018) (listing approved fuels). 141. Energy Policy Act of 2005, Pub. L. No. 109-58, § 1541(b), 119 Stat. 1107 (2005). 142. See Motor Vehicle Mfrs. Ass’n v. N.Y. State Dep’t of Envt’l Conservation, 810 F. Supp.

1331, 1343 (N.D.N.Y. 1993) modified, 831 F. Supp. 57 (N.D.N.Y. 1993) aff’d in part, rev’d in part, 17 F.3d 521 (2d Cir. 1994) (describing approval requirements).

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issues that arise with the regulation of heavy-duty truck emissions also arise with fuels, with cross-border trucks creating significant emissions.

C. Existing v. New Mobile Sources

Just as the CAA requires much more stringent regulation of new stationary sources as opposed to existing ones, it has also long focused much more attention on new vehicles than existing ones. Yet older vehicles of all categories are significantly dirtier than new ones and are the major cause of near-road pollution. A recent and sophisticated study of passenger vehicle emissions showed that 90 percent of NOx emissions came from just 25 percent of automobiles and the top 5 percent of emitters contributed 40 percent of carbon monoxide and black carbon.143 California estimates that 70 percent of air toxics come from diesel engines.144

The CAA does require some states to address pollution from older passenger vehicles in relatively limited ways. In recognition of the contribution of existing mobile sources to ambient air pollution, the 1990 CAA amendments required ozone and carbon monoxide nonattainment areas to establish inspection and maintenance programs for existing vehicles.145 The programs have until very recently been rife with fraud and largely ineffectual, and they also raise complex distributional questions about how to pay for repairs and upgrades to older cars.146 And they have been aimed at nonattainment areas rather than near-road exposure.

EPA has done even less to address the largest existing mobile source problem: emissions from heavy-duty vehicles. In 1993, EPA required large cities (greater than 750,000 in population) to retrofit older urban buses, if they were

143. See J. M. Wang et al., Plume-Based Analysis of Vehicle Fleet Air Pollutant Emissions and the Contributions From High Emitters, 8 ATMOSPHERIC MEASUREMENT TECH. 3263, [3271] (2015) (studying vehicular emissions in Ontario, Canada). Canadian emissions regulations follow U.S. 2007 Tier 2 standards. See id. at [3271].

144. Cal. Air Res. Bd., Overview: Diesel Exhaust and Health, diesel/diesel-health.htm (last visited March 26, 2018).

145. See, e.g., 42 U.S.C. § 7511a(a)(2)(B) (2012) (requiring inspection and maintenance programs for marginal ozone non-attainment zones).

146. See Amihai Glazer et al., Clean on Paper, Dirty on the Road: Troubles With California’s Smog Check, 29 J. TRANSPORT ECON. & POL’Y 85, 85–86 (1995) (showing large discrepancies between inspection failure rates in Inspection & Maintenance facilities compared with failure rates in roadside audits, lower emissions reductions than predicted, fraud in testing facilities).

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being replaced or rebuilt.147 EPA also has a number of voluntary incentive programs to retrofit heavy-duty diesel engines and, since 2005 with the adoption of the Diesel Emission Reduction Act, has provided some funding for states to retrofit old engines in school buses and other heavy duty vehicles.148 The Trump Administration has, however, proposed reducing this funding by 83 percent in the 2017–2018 budget.149 The result is that long-lived heavy-duty engines remain on the road for decades.150 Moreover, EPA recently proposed rolling back emissions standards for “glider kits,” (which is essentially a new chassis with an old engine) even though evidence shows that they emit vastly more particulate matter and NOx than new engines.151

D. Issues With Stationary Sources and Their Relationship to Mobile Sources

The regulation of stationary sources under the CAA raises its own issues in how states can address near-road exposure. The first, the application of New Source Review, has already arisen with respect to the NO2 short-term standard. The second, the propensity to regulate stationary sources in response to a problem caused largely by mobile sources, could occur with efforts either to implement the NO2 standard or in response to monitoring of PM 2.5 at roadside. This propensity is even more problematic because of the distinction the CAA makes between new and existing sources. Finally, the likelihood that stationary source-targeting will occur is compounded by the Clean Air Act’s requirement that SIPs contain enforceable emissions limitations requirements.

147. See 40 C.F.R. § 85.1401 (2016); United States: Urban Bus Retrofit Rebuild (UBRR) Program, DIESELNET, (last visited Mar. 7, 2018) (describing program).

148. See EPA, CLEAN DIESEL AND DERA FUNDING, [] (last visited Mar. 7, 2018) (describing programs and funding levels).

149. See Lauren Tyler, Trump’s Proposed Budget Cuts Funding Available for Clean Transportation, NGT NEWS (May 23, 2017), budget-cuts-funding-available-for-clean-transportation [].

150. See CAL. ENVTL. PROT. AGENCY AIR RES. BD., DRAFT, SUPPORTING INFORMATION FOR TECHNOLOGY ASSESSMENTS: TRUCK AND BUS SECTOR DESCRIPTION, at VI-2 (2016) (describing shelf-life and durability of heavy duty engines).

151. EPA, PROPOSED RULE FOR REPEAL OF EMISSIONS REQUIREMENTS FOR GLIDER VEHICLES, GLIDER ENGINES AND GLIDER KITS; see also, Heavy-Duty Truck “Glider Kit” Rule, ENVTL. L. HARV., rule/ [](last visited March 28, 2018).

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Before detailing problems with stationary source regulation and microclimate pollution, it is worth noting that EPA has made significant strides in reducing pollutants from these sources, including from existing, rather than stationary, sources. Three programs are particularly noteworthy. The first, the Acid Rain Trading Program, targeted the electric power sector through the country’s first large-scale cap-and-trade program. It cut sulfur dioxide from power plants by 50 percent compared to 1980 levels and led to a significant decline in NOx pollution.152 The second, a series of cap-and-trade programs EPA established to tackle cross-state ozone pollution, have significantly reduced NOx and sulfur dioxide as well.153 Importantly, these reductions have also led to significant reductions in particulate matter.154 And finally, a recently adopted rule aimed at toxic pollutants emitted from the power sector, known as the MATS rule,155 will result in major reductions not only in toxic pollutants like mercury but in particulate matter as well. In fact, the vast majority of the health benefits that will result from the MATS rule will come from reductions in fine particulates, including NO2 and sulfur dioxide particles.156

Despite these successful regulatory efforts, problems remain with stationary sources and near-source exposure. The first issue has already arisen with the regulation of NO2 and the new near-road standard. The CAA requires any new or modified stationary source to obtain a permit before beginning operation, whether in an attainment or nonattainment area, if the source will emit a specified number of tons per year of a regulated pollutant.157 Although

152. See U.S. EPA, CLEAN AIR MARKETS, ACID RAIN PROGRAM, acid-rain-program [] (describing pollution reductions).

153. See U.S. EPA, CLEAN AIR MARKETS, INTERSTATE AIR POLLUTION TRANSPORT, [ L5E7] (describing and linking to programs).

154. See id. 155. 40 C.F.R. pt. 63 (2017). The MATS rule was successfully challenged on the grounds that

EPA issued it without considering the costs of the rule in Michigan v. EPA, 135 S. Ct. 2699 (2015). EPA subsequently reconsidered the rule in light of the Court’s decision and found that cost considerations did not change its determination that the rule is appropriate. 81 Fed. Reg. 24,420 (April 25, 2016). The rule is now being put into effect.

156. See U.S. EPA, REGULATORY IMPACT ANALYSIS FOR THE FINAL MERCURY AND AIR TOXICS STANDARDS, at ES-1 (Dec. 2011), matsriafinal.pdf (outlining massive co-benefits from reduction of PM 2.5).

157. For an explanation of these New Source Review requirements, see U.S. EPA, NEW SOURCE REVIEW (NSR) PERMITTING, LEARN ABOUT NEW SOURCE REVIEW, [ 9HDT]. The PSD (attainment) provisions are contained in 42 U.S.C. §§ 7475(a). The provisions for new sources in non-attainment areas are contained in 42 U.S.C. § 7503 (2012).

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the NO2 one-hour standard was explicitly motivated by the health problems associated with exposure to near-road pollution, EPA issued a guidance memorandum shortly after the standard was adopted, making clear (appropriately given the statutory requirements) that the stationary source permitting provisions were triggered by the new standard.158

Shortly after the issuance of the initial guidance memorandum, it became clear that a relatively large number of new and modified stationary sources were having trouble getting permitted. These sources apparently found, based on modeling, that they would be in violation of the standard.159 The sources included emergency electric generating units, pump stations, power plants, paper mills, and refineries.160 In order to reduce the number of stationary sources having difficulty getting permits given the modeling results, EPA has issued a series of guidance memoranda to “facilitate the permitting of new and modified PSD [Prevention of Significant Deterioration] major stationary sources.”161 Presumably, even with this guidance, seemingly designed to demonstrate that the sources will not need permits, some sources will continue to violate the standard and will need to install the “best available control technology” (BACT) as a result.162

There are at least two problems with the fact that new and modified stationary sources may be required to get permits and install BACT technology as a result of the new one-hour NO2 standard. First, the new standard was motivated by near-road pollution, not by stationary source emissions. Thus, requiring permits of new and modified stationary sources will do nothing to solve the problem, nor will it help bring air districts that are out of compliance because of near-road exposure into attainment with the one-hour standard. Moreover, to the degree that stationary sources are causing near-source exposure problems, exposure levels are likely to be far worse at existing stationary sources, like refineries and power plants. Yet existing sources are





162. 42 U.S.C. § 7475(a)(4) (2012).

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largely exempt from NAAQS regulation under the CAA, unless they are modified in a manner that triggers New Source Review163 or unless they are in nonattainment areas.164 The existing/new source division and problem are hardly unique to the one-hour NO2 standard. However, it is worth pointing out that if the aim of the new standard is to reduce near-term exposure to pollutants at dangerous levels, targeting new stationary sources, as the CAA does, is not likely to solve the problem.165

The second problem with the structure of the CAA for regulating near- source pollution is that, if states are found to be out of attainment or if near-road monitors are used to measure PM 2.5 and kick a district out of attainment or further out of attainment, then the CAA pushes states to clamp down further on stationary as opposed to mobile sources. This structural incentive exists, again, because states have much less power over mobile source emissions than they do over stationary sources, even when the pollution problem is caused by mobile source emissions. States must impose “reasonably available control measures” on existing sources in the area if out of attainment with a NAAQS.166

Finally, the SIP provisions of the CAA contain a strong directive to states to include “enforceable emission limitations” and other direct control measures to demonstrate maintenance or attainment with an air standard.167 As a result, states are likely to look to stationary sources, on which they can impose enforceable emissions limits, to reduce NO2 or PM 2.5 emissions.

1. Stationary Sources, Emissions Factors, and Near-Source Exposure

As I described above, EPA and states rely extensively on emissions factors for stationary sources to estimate emissions. For ambient concentrations of pollutants that do not create near-source problems (SO2 is an example),168 emissions factors work reasonably well as long as background monitoring is accurate and effective. For a pollutant like PM 2.5, however, with known near-

163. See 42 § 7479(1) (2012) for a definition of new source. 164. See 42 U.S.C. § 7502(c)(1) (2012) (requiring “reasonably available control measures,”

including “from existing sources”). 165. See, e.g., Dialogue, Grandfathering Coal: Power Plant Regulation Under the Clean Air

Act, 46 ENVTL. L. REP. 10541, 10542 (2016) (discussion of RICHARD REVESZ & JACK LIENKE, STRUGGLING FOR AIR: POWER PLANS AND THE “WAR ON COAL,” with Revesz calling the grandfathering provisions of old stationary sources the “tragic flaw” of the Clean Air Act).

166. 42 U.S.C. § 7502(c)(1) (2012). 167. Id. § 7410(a)(2)(A). 168. See INTEGRATED SCIENCE ASSESSMENT FOR SULFUR OXIDES (2017), at 1-8 to 1-9

(describing exposure).

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source consequences, emissions factors may create a significant problem by masking exposure to emissions from stationary sources and consequent health effects for those who live near them.

The PM 2.5 standard was adopted in 1997, but it was not until 2008 that states submitted SIPs that either demonstrated how they would maintain attainment status or come into attainment.169 The attainment designations themselves, along with the SIPs, were based largely on emissions factors for stationary and mobile sources.170 Indeed, emissions inventories for PM 2.5 continue to rely extensively on emissions factors to estimate emissions from fuel combustion, refineries, and so forth.171

The 2006 OIG report about emissions factors cautioned EPA about the importance of developing emissions factors based on good data.172 It is difficult to determine, however, whether EPA heeded this advice. A 2004 report commissioned by the U.S. Department of Energy cautioned that:

There are few existing data regarding emissions and characteristics of fine aerosols from oil, gas and power generation industry combustion sources, and the information that is available is generally outdated and/or incomplete. Traditional stationary source air emission sampling methods tend to underestimate or overestimate the contribution of the source to ambient aerosols because they do not properly account for primary aerosol formation, which occurs after the gases leave the stack.173

If emissions factors are systematically overestimating PM 2.5 from these sources, there should not be a problem with near-source exposure. Not only are the sources producing fewer emissions than emissions factors estimate, but regulated parties are also likely to challenge the application of emissions estimates that subject them to more stringent regulation than they would experience with accurate estimates.

But if emissions factors systematically underestimate emissions from stationary sources, like refineries and power plants, our current system of regulation is unlikely to detect these problems. This is likely to be particularly

169. See U.S. EPA, EPA CAN IMPROVE EMISSIONS FACTORS DEVELOPMENT AND MANAGEMENT, supra note 120, at 14 (describing role of emissions factors in PM 2.5 process).

170. See id. at 4–6. 171. See U.S. EPA, EPA’S REP. ON THE ENVIRONMENT, PARTICULATE MATTER EMISSIONS, [] (last visited Dec. 28, 2017).



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true in attainment areas where existing stationary sources are subject to virtually no regulation. These sources are not required to monitor their emissions if they are not otherwise subject to regulation. Moreover, monitoring requirements for PM 2.5 do not require microscale monitoring and thus are unlikely to detect near-source emissions from a stationary source.174

Even in nonattainment areas, the use of emissions factors may underestimate near-source emissions from stationary sources and our regulatory apparatus to regulate NAAQS is not well-suited to respond. The monitoring to determine nonattainment, again, is not likely to detect these emissions since the monitoring is at a broader scale. Moreover, existing stationary sources enjoy much more favorable treatment than new sources and thus would not be required to install control technology as effective as that required of new sources.175


A. In the Long Run, Greenhouse Gas Emissions Regulation May Ultimately Solve the Problem

1. Mobile Sources

Ironically, the problem of microclimate pollution may ultimately be resolved because of a pollution problem at the opposite end of the planetary scale, climate change. Solutions aimed at mitigating climate change will, however, take multiple decades, and, in the meantime, communities that live near highly polluted sources will continue to breathe unhealthful air.

Although greenhouse gas (GHG) emissions, in adding to heat-trapping gases that envelop the planet, create the exact opposite problem of pollution hotspots in their global scale, a large percentage of GHG emissions come from the same sources that create near-source pollution.176 The transportation and electricity sectors are collectively responsible for more than half of GHG emissions in the United States. When industrial sources are included, the total

174. See discussion of monitoring requirements at notes 106–112, supra. 175. See discussion of existing new sources at notes 163–164, supra. 176. See U.S. EPA, INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS, [] (last visited Aug. 14, 2017).

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rises to nearly 80 percent.177 Many observers suggest that the most effective long-run strategy to reduce emissions from the transportation sector is to electrify the vehicle fleet.178 The electricity used to charge vehicles would come largely from renewable sources, combined with battery storage and other non- carbon mechanisms, eventually making fossil fuel power plants and oil refineries unnecessary or much less prevalent.179 The major hotspot sources would, in other words, be clean. And the Clean Air Act in its current form could be used to achieve this transition.

How the CAA came to cover greenhouse gases is a well-known story, beginning with the U.S. Supreme Court’s holding in Massachusetts v. EPA that greenhouse gases are pollutants under the Act and that EPA must determine whether they endanger public health and welfare.180 Based on that authority, EPA under President Obama made the endangerment finding181 and then issued two rounds of regulatory requirements focused on vehicles, including passenger, medium, and heavy duty.182 The details of the standards are less important than the fact of them: The National Highway Transportation and Safety Authority worked with EPA for the first time to issue combined fuel economy standards and GHG regulations to bolster fuel economy from all categories of vehicles.183 The standards were designed to be the first step on the road to much deeper decarbonization by mid-century.184

177. See id. 178. See, e.g., MIT & SLOAN AUTO. LAB. ENG’G SYS. DIV., ON THE ROAD TOWARD 2050:


179. See SUSTAINABLE DEV. SOL. NETWORK, PATHWAYS TO DEEP DECARBONIZATION IN THE UNITED STATES, at vii (U.S. 2050, vol. 1, Technical Report Executive Summary (2015)).

180. 549 U.S. 497 (2007). For an account of how the CAA has been used to address GHG regulations, see generally Ann Carlson, An Ode to the Clean Air Act, 30 J. LAND USE & ENVTL. L. 119 (2014).

181. See U.S. EPA, ENDANGERMENT AND CAUSE OR CONTRIBUTE FINDINGS FOR GREENHOUSE GASES UNDER THE SECTION 202(A) OF THE CLEAN AIR ACT, ghgemissions/endangerment-and-cause-or-contribute-findings-greenhouse-gases- under-section-202a-clean [] (last visited Aug. 18, 2017).

182. For an explanation of all of the regulatory programs, see JAMES E. MCCARTHY & BRENT D. YACOBUCCI, CONG. RESEARCH SERV., R40506, CARS, TRUCKS AND CLIMATE CHANGE: GREENHOUSE GASES FROM MOBILE SOURCES 3–17 (2016). See also U.S. EPA, REGULATIONS FOR GREENHOUSE GAS EMISSIONS FROM PASSENGER CARS AND TRUCKS, gas-emissions-passenger-cars-and [] (last visited Aug. 18, 2017).

183. See MCCARTHY & YACOBUCCI, supra note 182, at 3. 184. See U.S. Cover Note, INDC and Accompanying Information (Mar. 31, 2015), America%20First/U.S.A.%20First%20NDC%20Submission.pdf (describing U.S. climate

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At least some of the new regulatory requirements are currently under review by the Trump Administration, and passenger vehicle standards for model years 2021–2025 and standards for glider kits (which combine new trailers with refurbished engines) are likely to be loosened or eliminated.185 As of this writing, EPA has just issued a revision of a required midterm review stating that the 2022–2025 model year standards are not economically or technologically feasible.186 The expectation is that the administration will roll back those standards in some way.187 But because of the special CAA authority California has to regulate mobile sources, the state has already recommitted to maintaining the current standards, has been granted a waiver by EPA to do so, and is investing massive resources into electrifying its vehicle fleet.188 The state also has a regulatory mandate that 22 percent of vehicles be zero-emission by 2025.189 Twelve states follow California’s vehicle standards, covering more than a third of the country’s vehicles.190 Whether EPA will attempt to revoke the California waiver is unclear as of press time.191

commitment as part of a long-term strategy to cut U.S. emissions by 80 percent by 2050).

185. See U.S. EPA, EPA ANNOUNCES INTENT TO REVISIT PROVISIONS OF PHASE 2 HEAVY-DUTY RULES (Aug. 17, 2017) [hereinafter EPA ANNOUNCES INTENT TO REVISIT PROVISIONS], heavy-duty-rules []; U.S. EPA, EPA TO REEXAMINE EMISSIONS STANDARDS FOR CARS AND LIGHT DUTY TRUCKS—MODEL YEARS 2022-2025 (Mar. 15, 2017) [hereinafter EPA to Reexamine Emissions Standards], duty-trucks-model-years-2022-2025 []; U.S. EPA, EPA, DOT OPEN COMMENT PERIOD ON RECONSIDERATION OF GHG STANDARDS FOR CARS AND TRUCKS (Aug. 10, 2017) [hereinafter EPA, DOT OPEN COMMENT PERIOD], ghg-standards-cars-and-light-trucks [] (adding Model Year 2021 standards for reconsideration).

186. EPA Mid-Term Evaluation of Greenhouse Gas Emissions Standards for Model Year 2022–2025 Light-Duty Vehicles, 83 Fed. Reg. 16,077 (April 13, 2018).

187. See Hiroko Tabuchi, Calling Car Pollution Standards ‘Too High,’ E.P.A. Sets Up Fight With California, N.Y. Times (April 2, 2018), climate/trump-auto-emissions-rules.html.

188. See Press Release, CARB Finds Vehicle Standards are Achievable and Cost-Effective, Cal. Air Res. Bd. (Mar. 24, 2017), [] (describing state re-affirmation of vehicle standards); GOVERNOR’S INTERAGENCY WORKING GRP. ON ZERO-EMISSION VEHICLES, 2016 ZEV ACTION PLAN: AN UPDATED ROADMAP TOWARD 1.5 MILLION ZERO-EMISSION VEHICLES ON CALIFORNIA ROADWAYS BY 2025 (2016) (outlining state programs).

189. CAL. CODE REGS. Tit 13, § 1962.2(b)(1)(a) (2017). 190. See CARB Finds Vehicle Standards Are Achievable and Cost-Effective, supra note 188. 191. See Tabuchi, supra note 187.

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The current regulatory environment at the federal level is, to say the least, dispiriting with respect to the regulation of greenhouse gases. Moreover, technological challenges remain in electrifying or moving to zero-emissions engines, particularly with respect to the electrification of heavy-duty engines for long-distance travel at reasonable cost.192 Before long-distance trucks are fully electrified, we may be more likely to see hybrid trucks and improved diesel efficiency,193 particularly if EPA and/or California use their authority to require additional improvements. Nevertheless, over the long haul, EPA and California collectively have the tools to address the biggest culprit in near- source exposure. The effort will take decades, and the problem of existing engine pollution will remain long after regulations are adopted for new engines. But the problem of climate change may ultimately provide us with the solution to near-road pollution. In fact, the significant and immediate health benefits that would result from dramatically eliminating or reducing entirely the biggest hotspot problem, near-road pollution, could be a more effective and persuasive way, rather than relying on arguments about climate change, to convince the public and its representatives that we should transition away from a fossil-fuel dominated transportation sector.

2. The Power Sector’s Transition to Renewable Fuels Will Help the Stationary Source Problem

The Clean Air Act can also assist with the transition of the electricity sector from fossil fuels to renewable energy sources, and market forces in the sector, as well as state policies, are also helping.194 As with GHG regulations for mobile

192. See, e.g., Peter Fairley, Can Tractor-Trailers Go Electric Along With Cars? MIT TECH. REV. (July 29, 2015), go-electric-along-with-cars [].

193. See id. 194. Twenty-nine states currently have Renewable Portfolio Standards that require their

utilities to procure a certain percentage of energy from renewable sources. See Jocelyn Durkay, State Renewable Portfolio Standards and Goals, NAT’L CONF. ST. LEGISLATURES (Aug. 1, 2017), []. 2016 saw record renewable energy installations around the world, fueled in large measure by dropping prices. See Damian Carrington, ‘Spectacular’ Drop in Renewable Energy Costs Leads to Record Global Boost, GUARDIAN, (June 6, 2017, 6:30 PM), spectacular-drop-in-renewable-energy-costs-leads-to-record-global-boost [ 8GKQ-V74D].

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sources, however, the Trump Administration is rolling back CAA regulations for the power sector.195

Once EPA made its finding in 2009 that greenhouse gases endanger public health and welfare, the finding set off a cascade of regulatory activity focused on different sources of greenhouse gases.196 The Obama Administration’s Clean Power Plan (CPP) included regulations for new and existing electric generating units.197 The CPP would, if implemented, cut GHG from the power sector by more than 30 percent below 2005 levels.198 Importantly, the cuts were based in part on assumptions that the power sector could increase its reliance on renewable, as opposed to fossil, fuels.199 The regulations would, in other words, begin to transition away from many of the stationary sources that can cause near-source pollution. Though its implementation is highly unlikely given that the Trump Administration has begun proceedings to withdraw the rule, the point is that the CAA in its current form contains mechanisms to reduce greenhouse gas emissions from stationary sources—mechanisms that could, in turn, reduce particulate and NO2 pollution that cause human health problems.200 Indeed, EPA estimated that the CPP would prevent between 1400 and 3200 premature deaths and 1700 heart attacks, largely from reductions in PM 2.5 and ozone.201

195. Review of the Clean Power Plan, 82 Fed. Reg. 16,329 (proposed Apr. 4, 2017) (to be codified at 40 C.F.R. pt. 60) (notice of review of Clean Power Plan (CPP)).

196. For an explanation of this regulatory cascade, see Carlson, supra note 180. 197. For a snapshot of EPA’s website containing extensive materials about the CPP,

including an in-depth description and regulatory materials, see U.S. EPA, OVERVIEW OF THE CLEAN POWER PLAN, CUTTING CARBON POLLUTION FROM POWER PLANTS, power-plan_.html []. The EPA materials about the CPP are no longer available from EPA itself. Id. Instead, EPA Administrator Scott Pruitt has launched a review of the CPP and has made clear that he intends to withdraw the rule. See Review of the Clean Power Plan, 82 Fed. Reg. at 16,329.


199. Id. 200. The legal status of the CPP was, and remains, uncertain. Numerous regulated parties

and a coalition of states led by now EPA Administrator Scott Pruitt, who was the Attorney General of Oklahoma, challenged the plan as outside the scope of EPA’s statutory authority. This challenge is pending in the U.S. Court of Appeal for the D.C. Circuit, and the case is currently under abeyance.

201. See EPA, REGULATORY IMPACT ANALYSIS FOR THE CLEAN POWER PLAN FINAL RULE, 4–31 (2015). A number of policies and sharp drops in the price of solar and wind technology have led to large increases in wind and solar installations over the past decade. Researchers estimate that this shift from conventional to renewable energy sources has resulted in a reduction in premature deaths of between 3000 and 12,700, and an increase in cumulative economic benefits of between $35 billion and more than $200 billion.

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B. In the Nearer Term, CAA Contains Some Potential Regulatory Avenues for Attacking Near-Source Pollution

1. Transportation Control Measures

One potential method for addressing near-source roadway pollution is through transportation control measures, such as the management and flow of traffic, the provision of alternative and cleaner forms of transportation, and the dedication of carpool lanes. While these approaches to reducing near-road exposure are indirect, as compared to direct emissions controls on mobile sources, they can in the short run help reduce emissions levels on heavily trafficked roads.202

Though the CAA, is in many ways, prescriptive in requiring states to implement provisions that can be quite directive, the Act’s treatment of transportation control measures is striking for—the most part—failing to require any significant adoption of such measures.203 States are not required to include transportation control measures in SIPs except in serious and extreme ozone nonattainment and serious nonattainment carbon monoxide zones (of which there are none) and for transportation conformity.204 Even in these nonattainment areas, EPA has taken a remarkably narrow approach to what is required.

The Clean Air Act requires serious and severe ozone nonattainment areas “to offset any growth in emissions from growth in vehicle miles traveled . . . and to attain reduction in motor vehicle emissions as necessary . . . .”205 In 2004, California submitted a SIP revision for the South Coast Air Quality Management plan to attain the ozone standard, a revision that EPA approved

Dev Millstein et al., The Climate and Air-Quality Benefits of Wind and Solar Power in the United States, 2 NATURE ENERGY 1, 1 (2017).

202. See Multi-Pollutant Emissions Benefits of Transportation Strategies-FHWA, U.S. DEP’T TRANSP. FED. HIGHWAY ADMIN. (Nov. 14, 2006), air_quality/conformity/research/mpe_benefits [].

203. See Transportation Control Measures, U.S. DEP’T TRANSP. FED. HIGHWAY SAFETY ADMIN., ontrol_measures [] (describing transportation control measures).

204. In nonattainment zones for several of the criteria pollutants, states must ensure that new transportation projects do not interfere with federal air quality goals. For an explanation of this process, see Fed. Highway Admin., Air Quality, Transportation Conformity, (last visited March 28, 2018).

205. 42 U.S.C. § 7511a(d)(1)(A) (2012).

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in 2009.206 The plan contained no transportation control measures, even though SCAQMD is an extreme nonattainment zone for ozone.207 California took the position that no transportation control measures were necessary, because, even though vehicle miles traveled (VMTs) would increase, overall emissions would decline as a result of other regulatory efforts. EPA agreed.208 Plaintiff environmental groups challenged EPA’s approval, and the court held that EPA’s interpretation of the statutory language requiring transportation control measures was erroneous. Instead, the court found that the text requires transportation control measures if VMTs have grown over a previous baseline year to offset any emissions growth associated with that growth, not merely if overall emissions increase.209

Despite EPA’s narrow interpretation, transportation control measures could be used if air districts are designated as out of attainment of the new NO2 one-hour standard. Similarly, were EPA to require near-road monitoring for PM 2.5 or establish a new PM 2.5 standard based on near-road pollution, transportation control measures could play a role in reducing exposure. Given that the standard is directed at near-road pollution and that air districts cannot directly regulate mobile source emissions, transportation control measures are, in fact, likely to be a primary means for controlling exposure. Whether air districts and EPA will have the political will to impose such measures is a different matter.210

2. Indirect Source Rules To Control New or Modified Sources

The CAA contains another provision that can be used to control mobile source emissions from new or modified sources that are stationary, but the provision is entirely permissive. Section 110(a)(5) authorizes—but does not require—states to adopt indirect source review to include in State Implementation Plans for new or modified “indirect sources” that will attract emissions from mobile sources.211 Indirect sources are defined to include “a facility, building, structure, installation, real property, road, or highway

206. Ass’n of Irritated Residents v. EPA, 686 F.3d 668, 672–73 (D.C. Cir. 2011). 207. Id. at 673. 208. Id. 209. Id. at 680–81. 210. See, e.g., U.S. GOV’T ACCOUNTING OFFICE, GAO/RCED-93-169, URBAN TRANSPORTATION:

REDUCING VEHICLE EMISSIONS WITH TRANSPORTATION CONTROL MEASURES 3 (1993) (describing some TCMs as “politically painful”).

211. 42 U.S.C. § 7410(a)(5)(A) (2012).

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which attracts, or may attract, mobile sources of pollution.”212 States using this provision can regulate emissions from the indirect source in order to maintain or come into compliance with a NAAQS even when the cause of those emissions is mobile sources (construction equipment, for example).213 States do need, however, to ensure that they do not impose direct emissions controls on mobile sources that increase indirect source emissions or they risk running afoul of the mobile source preemption provisions.214

The indirect source provision appears to be underutilized but has been used by some air districts, including California’s Central Valley, to impose limitations on mobile source emissions on construction sites. Like the transportation control measures described above, the indirect source rule applies only to new or modified sources, not existing ones, so cannot alone solve the problem of pollution hot spots. Nevertheless it is another underutilized mechanism that can address hot spots that arise from new development and construction.

3. Creative Interpretations of Existing CAA Provisions to Tackle Near-Road Pollution

EPA might also use its existing regulatory authority creatively to attempt to require states to address near-source and, particularly, near-road pollution.215 To begin with, the NAAQS provisions themselves require EPA to establish ambient standards in a way that is sufficient to “protect the public health” with “an adequate margin of safety.”216 This language should provide the agency with sufficient authority to establish mechanisms to regulate near- source pollution and arguably could provide a basis for a citizen suit against the agency for failing to do so. One obvious possibility is to create a near-source standard for PM 2.5 similar to the new NO2 standard. The agency could also explore more unusual regulatory measures, such as establishing air districts for a particular pollutant (PM 2.5 is the obvious one) that encompass only the areas near trafficked roadways or determining that highways are stationary sources.

212. Id. § 7410(a)(5)(C). 213. Id. § 7410(a)(5)(D). 214. See Nat’l Ass’n of Home Builders v. San Joaquin Valley Unified Air Pollution Control

Dist., 627 F.3d 730, 734–35 (9th Cir. 2010) (discussing tension between CAA mobile source preemption provision and indirect source provision and upholding San Joaquin Air District’s rule).

215. These suggestions are preliminary in nature and offered merely to suggest that the expansive nature of and language in the CAA might be deployed to address hotspot pollution.

216. 42 U.S.C. § 7409(b)(1) (2012).

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These are preliminary suggestions and raise many complexities, but they highlight the fact that the CAA is a remarkably flexible and expansive statute that can evolve as new pollution problems come to light.217

4. State Solutions to Near-Source Pollution

Finally, EPA might look to a number of solutions that some states, particularly California, have adopted in an attempt to address near-source pollution. None of these fully fix the problem in the way that a fully electrified vehicle fleet would, but they can nevertheless reduce exposure levels and consequent health effects of breathing in elevated levels of pollutants and serve as transition measures, as the vehicle fleet turns over and moves toward zero emissions. And of course, other than preemption provisions in the Clean Air Act for new engine emissions standards, nothing prohibits states from attempting to address the near-source problems voluntarily.

Most importantly, California now requires the retrofitting of existing heavy-duty diesel engines and the eventual replacement of old diesel trucks with new technology.218 Some of this regulatory activity stems from California’s special authority to regulate emissions under the Clean Air Act219 but the requirement that older heavy-duty trucks install engine filters could be done by other states.220 Other states can also choose to follow new California emissions standards.221 Though California does not control emissions from all trucks that enter the state, it is still regulating a substantial portion of truck traffic that contributes to near-road emissions.

California’s two largest ports, Los Angeles and Long Beach, have also adopted extensive regulatory programs to address port emissions, primarily from mobile sources with diesel engines, including heavy trucks, drayage, and even ocean vessels.222 The mayors of the two cities that house the ports just

217. For an evaluation of many of the ways in which the CAA has proved adaptable, flexible, and durable, see THE FUTURE OF LONG-TERM ENERGY POLICY: LESSONS FROM THE CLEAN AIR ACT (Dallas Burtraw & Ann E. Carlson eds., forthcoming 2018).

218. Truck and Bus Regulation: On-Road Heavy-Duty Diesel Vehicles (In-Use) Regulation, CAL. AIR RES. BD., [ 4FE3-AMS2] (last reviewed Dec. 14, 2017).

219. See 42 U.S.C. § 7543(b) (2012). 220. The preemption of state emissions standards for motor vehicles applies to new vehicles.

See id. § 7543(a). 221. Id. § 7507. 222. See San Pedro Bay Ports Clean Air Action Plan (CAAP), PORT L.A., [].

The Clean Air Act’s Blind Spot 1087

committed to zero emissions from cargo-handling equipment by 2030 and from drayage trucks by 2035.223

California is also embarking on a new monitoring program to obtain better data about near-source exposure in disadvantaged communities and to install control measures on sources that are contributing to elevated near- source levels.224

Finally, California and some of its localities have restricted some land uses near freeways, including elementary schools,225 and have required air filters in housing located near major highways.226 Nevertheless, there are no barriers to the construction of new housing near freeways and a recent study showed that Los Angeles County has 169 child care centers—which, unlike schools, can be built near freeways—located within 500 feet of a freeway.227

EPA could look to some of these state solutions, particularly those involving the retrofitting of old heavy-duty engines, as measures available to other states with near-road pollution problems. To do so, however, EPA would need to take other steps outlined above to identify and require the regulation of near-source pollution in order to shoehorn the problem of near- road exposure into the existing statutory structure.

None of these regulatory measures is a magic bullet to solve the microclimate pollution problem. One lesson from the many years of Clean Air Act implementation, however, is that the process of cleaning the air has involved relying on numerous statutory provisions, adopting many iterations of regulations, and engaging in sustained effort to identify and regulate new pollution problems.228 A legislative solution to the hotspot pollution problem

223. See Joint Declaration, Eric Garcetti & Robert Garcia, Creating a Zero Emissions Goods Movement Future: A Joint Declaration of the Mayors of the Cities of Los Angeles and Long Beach, Long-Beach-mayors-on.html.

224. See Cal. Air Res. Bd., Community Air Protection Program, AB 617 (last viewed March 28, 2018).

225. CAL. PUB. RES. CODE § 21151.8(a)(1)(D) (2009). 226. See Tony Barboza, L.A. Requires Air Filters to Protect Residents Near Freeways. Are They

Doing Their Job?, L.A. TIMES (July 9, 2017, 5:00 AM), lanow/la-me-ln-freeway-pollution-filters-20170709-story.html [ JR6R] (suggesting that the requirement is not well-enforced and the systems often not well-maintained).

227. See Deepa Fernandes with Aaron Mendelson, Polluted Preschools: 169 LA Childcare Centers Are Too Close to Freeways, 89.3 KPCC, (Mar. 29, 2016), news/2016/ 03/29/58878/pollution-near-preschools-is-impacting-nearly-10-0 [ W87W].

228. For an in-depth analysis of the multiple iterations of passenger vehicle regulations in California and at the federal level, see generally Ann E. Carlson, Iterative Federalism and

1088 65 UCLA L. REV. 1036 (2018)

might well be preferable, but, in the current legislative climate, reform is highly unlikely.


The very successful ambient structure of the Clean Air Act may, ironically, also help to mask a ubiquitous and harmful form of pollution: hotspots of emissions in microclimates. Indeed, by requiring air districts around the country to measure, monitor, and regulate pollutants on a large, background scale, the CAA may actually lead residents to misunderstand the health risks they face from the air they breathe. In some instances, background ambient air may actually be cleaner than the labels some air districts receive when out of attainment—this is the case for large parts of Los Angeles that have cleaner ambient air than the air measured by monitors in the dirtiest parts of the basin. But much more troubling, the imprimatur of clean air in many districts may also lead residents to believe the air they breathe is of high quality even when living or playing or working in a microclimate with predictably unhealthful air. These measurements, monitors, and labels may even lull regulators into believing their jobs are done when many of their residents—often the lowest income and disproportionately of color—face unhealthful conditions during many parts of the day. The microclimates that remain largely (though not entirely) unregulated are, in my view, the blindspot of the CAA. This Article is an effort to make us see what the ambient focus of the Act has kept hidden.

Climate Change, 103 NW. U. L. REV. 1097 (2009); see also THE FUTURE OF LONG-TERM ENERGY POLICY: LESSONS FROM THE CLEAN AIR ACT, supra note 217.

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1238 volume 120 | number 9 | September 2012 • Environmental Health Perspectives


In a recent published commentary (Mauderly et al. 2010), a group of scientists represent- ing academia, government, and industry groups posed the question, “Is the air pollu- tion health research community prepared to support a multi pollutant air quality manage- ment framework?” In agreement with several other contemporary reviews, editorials, and opinion papers on the subject (e.g., Dominici et al. 2010; Greenbaum and Shaikh 2010; Vedal and Kaufman 2011), the authors con- cluded that, although significant data gaps limit our current understanding of health effects resulting from exposure to air pol- lutant mixtures, much can be gained in the near future through an increased emphasis on multi pollutant issues across the spectrum from basic scientific research through imple- mentation of air quality control strategies. Although single-pollutant approaches to air pollution research, health assessments, and setting standards for ambient air quality have been successful in reducing air pollution over the past few decades, there is a clear need for parallel efforts within both the scientific and the regulatory and policy communities to advance methods for evaluating and manag- ing the effects of air pollution using a multi- pollutant approach.

This review derives from a multipollutant science and risk analysis public workshop that was held on 22–24 February 2011 in Chapel Hill, North Carolina, with the purpose of providing a brief overview of the state of the science and identifying data gaps related to addressing the health consequences of air pollu tion in a multipollutant context, includ- ing realistic steps and targets to advancing scientific and policy decisions. Co-organized and sponsored by the U.S. Environmental Protection Agency (EPA) and the Health Effects Institute (HEI), this workshop was designed to facilitate open discussions among expert scientists; these discussions are now playing a key role in multi pollutant research planning within the U.S. EPA’s Office of Research and Development and are also helping to guide the development of the U.S. EPA’s framework for conducting multipollutant science and risk assessments.

Evaluating the health impacts of multi- pollutant exposures has been identified as a priority research area in the U.S. EPA’s integrated, cross-disciplinary research planning (U.S. EPA 2012), including the establishment of four university-based Clean Air Research Centers (CLARCs) to study exposures to air pollution mixtures and their associated

health effects. As additional evidence of its commitment to this new thinking, scientists within the U.S. EPA’s National Center for Environmental Assessment (NCEA), which is responsible for evaluating and synthesizing the scientific information related to the effects of exposure to criteria air pollutants as a part of the National Ambient Air Quality Standards (NAAQS) review process, are currently developing plans for conducting a formal multi pollutant science assessment (MSA) of the health effects of exposure to air pollutant mixtures. As an initial step in the development of this proposed human health MSA, the U.S. EPA is preparing a framework describing the purpose and scope of the MSA, along with plans for conducting multipollutant analyses using existing data and information that will provide scientific support to the development of the MSA. The MSA is intended to serve as a companion document to single-pollutant Integrated Science Assessments (ISAs) of the criteria air pollutants (i.e., particulate matter, ozone, nitrogen oxides, sulfur oxides, lead, and carbon monoxide), and allow for a more effective evaluation of both the health effects of air pollutant mixtures, as well as the effects of single pollutants in a multi- pollutant context. This approach is consistent with the recommendations from the 2004 National Research Council (NRC) Report,

Address correspondence to L.W. Stanek, National Center for Environmental Assessment, U.S. Environmental Protection Agency, 109 T.W. Alexander Dr., Mail Code B243-01, Research Triangle Park, NC 27711 USA. Telephone: (919) 541-7792. Fax: (919) 541-2985. E-mail: stanek.

We gratefully acknowledge M. Patel and A. Vette for their thoughtful reviews of this manuscript. The work reported here was performed by the U.S. Environmental Protection Agency (EPA) and the Health Effects Institute (HEI), Boston, Massachusetts. The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of the U.S. EPA, HEI, or HEI’s sponsors.

K.W. and D.G. are employed by HEI. HEI receives about half of its core funds from the U.S. EPA and half from the worldwide motor-vehicle industry, although other public and private organizations peri- odically support special projects or certain research programs. The other authors declare they have no actual or potential competing financial interests.

Received 9 January 2012; accepted 29 May 2012.

Practical Advancement of Multipollutant Scientific and Risk Assessment Approaches for Ambient Air Pollution Douglas O. Johns,1 Lindsay Wichers Stanek,1 Katherine Walker,2 Souad Benromdhane,3 Bryan Hubbell,3 Mary Ross,1 Robert B. Devlin,4 Daniel L. Costa,5 and Daniel S. Greenbaum2

1National Center for Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA; 2Health Effects Institute, Boston, Massachusetts, USA; 3Office of Air Quality Planning and Standards, 4National Health and Environmental Effects Research Laboratory, Office of Research and Development, and 5Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA

Objectives: The U.S. Environmental Protection Agency is working toward gaining a better understanding of the human health impacts of exposure to complex air pollutant mixtures and the key features that drive the toxicity of these mixtures, which can then be used for future scientific and risk assessments.

Data sOurces: A public workshop was held in Chapel Hill, North Carolina, 22–24 February 2011, to discuss scientific issues and data gaps related to adopting multipollutant science and risk assess- ment approaches, with a particular focus on the criteria air pollutants. Expert panelists in the fields of epidemiology, toxicology, and atmospheric and exposure sciences led open discussions to encour- age workshop participants to think broadly about available and emerging scientific evidence related to multipollutant approaches to evaluating the health effects of air pollution.

synthesis: Although there is clearly a need for novel research and analytical approaches to better characterize the health effects of multipollutant exposures, much progress can be made by using existing scientific information and statistical methods to evaluate the effects of single pollutants in a multipollutant context. This work will have a direct impact on the development of a multipollutant science assessment and a conceptual framework for conducting multipollutant risk assessments.

cOnclusiOns: Transitioning to a multi pollutant paradigm can be aided through the adoption of a framework for multi pollutant science and risk assessment that encompasses well-studied and ubi- quitous air pollutants. Successfully advancing methods for conducting these assessments will require collaborative and parallel efforts between the scientific and environmental regulatory and policy communities.

Key wOrDs: air pollution, exposure, human health, multipollutant, risk assessment. Environ Health Perspect 120:1238–1242 (2012). [Online 29 May 2012]

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Air Quality Management in the United States (NRC 2004):

Although the committee does not believe that the science has evolved to a sufficient extent to permit development of multipollutant NAAQS [National Ambient Air Quality Standards], it would be scien tifically prudent to begin to review and develop NAAQS for related pollutants in parallel and simultaneously.

It is anticipated that the development of the MSA may eventually inform multi- pollutant risk assessment; however, conducting such a risk assessment has been one of the main challenges in multipollutant science and policy (Brook et al. 2009). Technologies for exposure assessment, knowledge of exposure– response relationships, as well as the ability to communicate risk to different stake holders are key challenges for multipollutant risk assessment. A conceptual framework that will address risk assessment in a multi pollutant context is currently under consideration by staff within the U.S. EPA’s Office of Air and Radiation (OAR). However, the introduction of this conceptual framework is not an indicator that the use of multi pollutant risk assessment is imminent. Instead, this conceptual scheme would probe the current technology, modeling tools, and the data available to help design building blocks for the implementation of a multi pollutant risk assessment. Figure 1 outlines the essential elements in air pollution risk assessment, beginning with emission sources, pollutant transport and transformation, and control. In addition to source-specific emissions, global emissions (including transboundary transport and transformation) impact regional air quality and thus contribute to overall risk. The middle section of this figure identifies traditional pillars of risk assessment that are heavily reliant on research and data collection through monitoring, modeling, and experimental evaluation of health impacts in support of assessing risk. At the base of Figure 1, the different approaches that are used to characterize adverse biological responses are provided: concentration–response curves that are obtained through observational data such as epidemiologic studies; exposure–response functions based on modeling or personal exposure studies; and dose–response curves derived from controlled human exposure studies or observational studies using bio- markers of exposure. The weight of evidence for interpreting risk results requires recognition of these different approaches that depend on the properties of the pollutant of interest and the data available.

The complexities involved in the develop- ment of multi pollutant science and risk assessments are considerable, and will require scientists from various disciplines and orga- nizations to work more collaboratively going forward. For the purpose of this review,

“multipollutant” is generally defined as the criteria air pollutants along with some prior- ity air toxics. Although this approach does not include all ambient air pollutants to which populations are exposed, it does repre- sent a relatively manageable set of pollutants. Further, the health impacts of exposures to the criteria air pollutants have been extensively studied, thus making them an ideal start- ing place when considering a multi pollutant approach to evaluating the health effects of exposure to air pollution.

Evaluating the Human Health Effects of Multipollutant Exposures Human exposures. Characterizing exposure to multi pollutant mixtures requires an advanced understanding of the sources of air pollutants, the chemical transformations and interactions between multiple pollutants, and informa- tion on the correlations in space and time between their individual concentrations. The most relevant consideration for risk assess- ment is how different mixtures contribute to the overall exposures of populations, which are governed by the time spent in contact with each mixture and its constituent com- ponent pollutants. Assessing multi pollutant exposure will be aided by the ability to distin- guish which mixtures or parts of mixtures are most closely associated with particular health outcomes, rather than assessing exposure to all possible mixtures. Once the most relevant mixtures are identified, it is also possible to

study the origins of those mixtures including transport and transformation of emissions from those sources to exposure environments where populations receive significant expo- sures. Although the complexity of the mixture can be considerable, recognizing and appreci- ating this complexity may, in the end, allow assessments to be conducted with information going beyond a finite set of what seem to be the so-called “most relevant” mixtures.

Several innovative, albeit ambitious, ideas have been proposed to better characterize expo- sures to air pollutant mixtures. Approaches involving the use of existing data may cen- ter around grouping air pollutants based on sources, micro environments, or chemical and physical properties (e.g., Suh et al. 2011). In developing these groupings, it is essential that atmospheric and exposure scientists work col- laboratively with health scientists to consider approaches informed by pollutant-specific toxicological pathways. Source apportionment techniques may be useful in characterizing the contributions to personal exposure from particular emission sources. However, specific source categories can be difficult to identify given multiple sources of pollutants, their dif- fering spatial distribution relative to monitors, and the formation of secondary air pollutants (Brinkman et al. 2009). In addition, the com- position and concentrations of some source emissions change over time due to technologi- cal and regulatory factors (e.g., changes in die- sel engines), as well as to economic influences (e.g., changes in the prices of fuels). Although

Figure 1. Conceptual scheme for multipollutant risk assessment. Abbreviations: CO, carbon monoxide; NOx, nitrogen oxides; Pb, lead; PM, particulate matter; O3, ozone; SOx, sulfur oxides.

Source emissions Mixtures

Global emissions Transboundary

transport Management actions Multipollutant context

Air quality (Monitored, modeled)

Exposure assessment (Modeling, measurement)

Health impacts (Experimental data)


PM, O3, NOx O3, SOx CO,Pb

Transformation and control

Research and data collection

Multipollutant risk characterization

(Weight of evidence)

Interpretation of results

Exposure–response Dose–response

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many sources and exposure environments may be important, existing and supplementary monitoring evidence suggests that an initial subset of key source categories, including ports, roadways, industrial centers, and biomass burn- ing can form the basis for studies in the nearer term, while others may be identified in future source apportionment studies.

It is clear that neither modeling nor addi- tional monitoring alone is likely to solve the problem of characterizing multi pollutant exposures. For example, while grid modeling techniques have the potential to address uncer- tainties related to spatial variability of air pol- lutant mixtures, in many cases these models do not provide adequate temporal resolution, require detailed source and meteorological data inputs, and are difficult to evaluate (Marmur et al. 2006). Similarly, efforts to reduce com- plexity by use of a measured chemical marker species to represent a given source or micro- environment have clear advantages, but are likely overly simplistic due to differences in monitor location and frequency of data collec- tion that vary by pollutant (termed spatial and temporal misalignment, respectively) (Gryparis et al. 2009). However, one potentially promis- ing approach to reduce uncertainties associated with spatial and temporal misalignment is the integration of monitoring data, as well as satel- lite imagery, with modeling techniques that provide interpolation of atmospheric concen- trations, potentially resulting in a more com- plete characterization of spatial and temporal variations of individual pollutant and com- bined pollutant exposures (Liu et al. 2005; Villeneuve et al. 2011).

With respect to monitoring data, trade- offs must inevitably be made between quan- tity and quality of the data. For example, the development of inexpensive personal air quality sensors has considerable appeal as an approach to gathering personal expo- sure data, but equal concern exists about the analytical sensitivity and accuracy of these monitors. However, if sensors are deployed widely, patterns of pollutants and variations in time and space may be invaluable in model development despite short comings in accu- racy. Recognizing the limitations of personal monitoring and uncertainties associated with the use of central-site monitors to represent personal exposure to ambient air pollutants, the implementation of an exposure supersite program may be warranted. At such a site, the spatial and temporal variability of the con- centrations of multiple pollutants would be characterized in multiple micro environments using similar sampling methods. Although each of these approaches has merit, adequately characterizing exposures to air pollutant mix- tures to support multi pollutant risk assess- ments may best be accomplished through integration of central site monitors, widely

deployed sensors, satellite measurements, and refined atmospheric chemistry and expo- sure modeling that can draw across the rela- tive strengths of each approach, to allow for focused validation and tool develop ment.

Health studies. The development of a MSA for health effects will entail extensive integration of findings from observational, experimental, and exposure studies. For observational studies, moving toward a multi- pollutant focus will require drawing upon many established epidemiologic approaches. Single-pollutant models can be used to evalu- ate one pollutant as an indicator of a group of pollutants, and under certain conditions, traditional two-pollutant models can provide insight into combined effects through sum- ming model coefficients or partial derivatives (Bateson et al. 2007). Epidemiologic studies that rely on source apportionment methods to characterize exposure are another way to gain insight into the relationship between health effects and a group of correlated pollutants.

Generally, there are three broad approaches to evaluating interactions or other types of effect modification in epidemiologic studies— traditional regression models, dimension reduction techniques, and Bayesian hierarchical methods (Billionnet et al. 2012). All have their merits, which include ease of use for the former two approaches and flexibility for the latter approach. Future approaches may borrow and build on newer techniques from the genomics and other “omics” communities, applied in a multi pollutant context, including clustering methods and random forest approaches (Breiman 2001; Siroux et al. 2011).

Monitoring networks will need to provide enhanced capabilities to evaluate temporal and spatial scales. Spatial considerations include improved resolution of exposure and a better understanding of the representativeness of central-site monitors. Spatial misalignment of exposure data is an issue when evaluating multi pollutant effects because the ability to estimate exposures in different locations for different pollutants varies. Observational studies already consider some temporal patterns of health outcomes; for example, daily concentrations (or < 24 hr) are necessary for assessing acute effects and long-term concentrations (including seasonal variation) are required for assessing more chronic effects. Moving forward, the development of statistical methods that allow for clearer separation of the impacts of temporal and spatial variation may enable the resolution of multi pollutant effects (Bell et al. 2007; Peng and Bell 2010). These types of methods may also be valuable in understanding why changes are observed in the magnitude of risks associated with individual pollutants over time. We can hypothesize that these changes may be due to changing levels and mixes of pollutants, potentially due to

new regulations on diesel engine and power plant emissions, but additional research is needed to test these hypotheses.

Experimental studies with animals or intentional environmental exposures involving humans will likely continue to provide evi- dence of biological plausibility for the impact of mixtures as they do for single pollutants. Because they are hypothesis driven, they can be used to directly identify individual causal agents and their contributions to an overall effect. Although information on effects occur- ring at ambient levels is most relevant when considering human health risks, differences in respiratory tract deposition and biological response between animals and humans may justify the use of exposure concentrations that exceed ambient concentrations in ani- mal toxicology studies (Brown et al. 2005). Various experimental approaches are cur- rently available to evaluate multi pollutant effects. One such approach involves animal and human exposures to “real world” mix- tures of air pollutants under controlled condi- tions or environ ments that simulate ambient conditions, such as in photochemical cham- bers (Lemos et al. 2011; Sexton et al. 2004) or a traffic tunnel (Kooter et al. 2006). One limitation of this type of study is its inability to precisely control or manipulate the expo- sure concentrations or pollutant mixture. The potential strength of this study design resides in the spatial and temporal contrasts for exposure and response. Another approach is laboratory-generated mixture studies where a few pre-selected individual pollutants are combined to create an exposure atmosphere, with the most informative studies being those that have an air control as well as exposures to individual pollutants (Mauderly and Samet 2009). Although the mixtures being studied do not reflect the aging and transformation that occur under ambient conditions, they do provide relevant information on the nature of the interactions among different chemicals.

A mode-of-action (MOA) framework describing a sequence of key events leading to an end point or clinical outcome represents a possible unifying theme when considering multiple pollutants and the common toxicity pathways through which they act (Ankley et al. 2010). Such a framework should make it possible to characterize and illustrate the interconnectedness of key event pathways and the upstream responses that produce those events. The focus of the framework should be on commonalities between air pollutants, such as robust toxicity pathways informed by well established measurements of phenotypic changes and biomarkers linking exposure and effect. This approach also has the capabil- ity of evaluating the nature of interactions between pollutants and identifying potentially susceptible populations.

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Environmental Health Perspectives • volume 120 | number 9 | September 2012 1241

Thus, two possible approaches to address pollutant interactions are currently being developed: the common occurrence approach where a related group of pollutants from a particular source or resulting atmospheric mixture are associated with a health end point, and the MOA approach that groups specific chemicals in the atmosphere based on their effects with the same health end point. Limitations in toxicological approaches and epidemiologic data and statistical methods will require cross-disciplinary hypothesis development, research planning, and execution of this research (Dominici et al. 2008).

Synthesis and Discussion The historical focus on single pollutants has brought us a long way—a substantial data base of exposure monitoring data; a large and grow- ing body of evidence on different facets of the MOA for individual pollutants; a diverse toolbox of toxicological, clinical, and epide- miologic study designs; statistical and other data analysis methods; and air quality models that we can rely on in the short term. But it is also clear that we have a ways to go—whether it is toward the adaptation of existing data and tools or toward the development of new ones. In short, the transition to a deliberative multi pollutant approach will require substan- tial research effort and time. The U.S. EPA’s current efforts to develop a MSA, along with plans for designing a framework for conduct- ing multi pollutant risk assessments, provide an important structure and context for assessing the strengths and limitations of the existing science related to the health effects of expo- sures to air pollutant mixtures. In addition, the current and planned U.S. EPA and U.S. EPA- funded multi pollutant scientific research is

clearly vital in making a successful transition from single to multi pollutant approaches to air quality evaluation and management.

The U.S. EPA began using a multi- pollutant approach in a limited context in the recent evaluation of the secondary standards for nitrogen oxides and sulfur oxides. The development of a U.S. EPA policy assessment for nitrogen oxides and sulfur oxides resulted in consideration of a multi pollutant NAAQS to protect against the combined effects of nitrogen and sulfur oxides on aquatic acidi- fication (EPA 2011). OAR also has recently conducted a pilot study that demon strated how multi pollutant approaches to imple- menting primary (human health–based) air pollution standards could result in greater health benefits and more cost-effective imple- mentation, which may also help to achieve greater reductions in air toxics when com- pared with single-pollutant approaches (Fann et al. 2011). The adoption of a multi pollutant policy for the review of ambient air pollutants would potentially result in increased efficien- cies, bene fits, and cost savings throughout the process beginning with the initial evaluation of the scientific evidence. More important, increasing the emphasis on multi pollutant approaches may allow for a better under- standing of the types of air pollutant mixtures most likely to result in adverse health effects, which could, in turn, facilitate the identifica- tion of control strategies to minimize expo- sures to these mixtures.

As the science, risk assessment, and risk management communities advance toward adopting multi pollutant approaches, it is important to set realistic targets for progress amidst a very large number of possible direc- tions for improvement. We should aim to

take concrete steps in that direction in at least four major areas (Figure 2):

Exposure—seeking to increase the scien-• tific base of health studies in which multiple pollutants are measured simultaneously in ambient air Toxicology—seeking enhanced measure-• ment and analysis techniques for source and ambient mixture (e.g., concentrated ambient particles) exposure, and better understanding of common MOAs (key events) as a useful way to group and assess pollutants Epidemiology and statistics—seeking • enhanced techniques, especially statistical analysis techniques, that enable the charac- terization of associations between multiple pollutants (i.e., more than two at a time), sources, and health outcomes Modeling and risk assessment—seeking the • development and testing of multi pollutant approaches to estimating population expo- sure and risk.

Enhanced science in each of these areas can inform the three major components of air quality management—science assessment, risk assessment, and risk management (as in Figure 2). Additional scientific questions may arise leading to new scientific developments, further supporting progress toward under- standing effects of joint exposures to mul- tiple pollutants (e.g., the criteria pollutants and major air toxics) and, if the data permit, identifying individual pollutants and sources within the mixture that may be dispropor- tionately responsible for adverse effects on human health.


Ankley GT, Bennett RS, Erickson RJ, Hoff DJ, Hornung MW, Johnson RD, et al. 2010. Adverse outcome pathways: a conceptual framework to support ecotoxicology research and risk assessment. Environ Toxicol Chem 29:730–741.

Bateson TF, Coull BA, Hubbell B, Ito K. Jerrett M, Lumley T, et al. 2007. Panel discussion review: session three—issues involved in interpretation of epidemiologic analyses— statistical modeling. J Expo Sci Environ Epidemiol 17(suppl 2):S90–S96.

Bell ML, Dominici F, Ebisu K, Zeger SL, Samet JM. 2007. Spatial and temporal variation in PM2.5 chemical composition in the United States for health effects studies. Environ Health Perspect 115:989–995.

Billionnet C, Sherrill D, Annesi-Maesano I, GERIE study. 2012. Estimating the health effects of exposure to multipollutant mixture. Ann Epidemiol 22:126–141.

Breiman L. 2001. Random forests. Mach Learn 45:5–32. Brinkman GL, Milford JB, Schauer JJ, Shafer MM, Hannigan MP.

2009. Source identification of personal exposure to fine particulate matter using organic tracers. Atmos Environ 43:1972–1981.

Brook JR, Demerjian KL, Hidy G, Molina LT, Pennell WI, Scheffe R. 2009. New directions: results-oriented multipollutant air quality management. Atmos Environ 43:2091–2093.

Brown JS, Wilson WE, Grant LD. 2005. Dosimetric comparisons of particle deposition and retention in rats and humans. Inhal Toxicol 17:355–385.

Dominici F, Peng RD, Barr CD, Bell ML. 2010. Protecting human health from air pollution: shifting from a single-pollutant to a multipollutant approach. Epidemiology 21:187–194.

Dominici F, Wang C, Crainiceanu C, Parmigiani G. 2008. Model selection and health effect estimation in environmental epidemiology. Epidemiology 19:558–560.Figure 2. Schema showing continuous enhancement of multipollutant science.

Continuously enhanced multipollutant science:

• Exposure • Toxicology • Epidemiology • Risk models

Science assessment:

Multipollutant science assessment in context of single pollutant NAAQS

Risk assessment:

Multipollutant exposure and risk analysis

Risk management:

Multipollutant source controls and

multipollutant air quality management plans

Enhanced science input to MSA; new tools for data

synthesis and assessment

Enhanced input to source control decisions;

multipollutant priorities for AQ plans

Enhanced exposure and risk models

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Fann N, Roman HA, Fulcher CM, Gentile MA, Hubbell BJ, Wesson K, et al. 2011. Maximizing health benefits and mini- mizing inequality: incorporating local-scale data in the design and evaluation of air quality policies. Risk Anal 31:908–922.

Greenbaum D, Shaikh R. 2010. First steps toward multipollutant science for air quality decisions. Epidemiology 21:195–197.

Gryparis A, Paciorek CJ, Zeka A, Schwartz J, Coull BA. 2009. Measurement error caused by spatial misalignment in environmental epidemiology. Biostatistics 10:258–274.

Kooter IM, Boere AJ, Fokkens PH, Leseman DL, Dormans JA, Cassee FR. 2006. Response of spontaneously hypertensive rats to inhalation of fine and ultrafine particles from traffic: experimental controlled study. Part Fibre Toxicol 15:3–7.

Lemos M, Diaz EA, Gupta T, Kang CM, Ruiz P, Coull BA, et al. 2011. Cardiac and pulmonary oxidative stress in rats exposed to realistic emissions of source aerosols. Inhal Toxicol 23(suppl 2):75–83.

Liu Y, Sarnat JA, Kilaru V, Jacob DJ, Koutrakis P. 2005. Estimating ground-level PM2.5 in the eastern United States using satellite remote sensing. Environ Sci Technol 39:3269–3278.

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