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Transportation-Related Air Toxics: Case Study Materials Related to US 95 in Nevada



    1.1 Purpose
    1.2 Report Organization

    2.1 Identification of Relevant On-Road Mobile Source Air Toxics
    2.2 Quantification of Cancer Risks from Air Toxics
    2.3 Conclusions Regarding Important Air Toxics

    3.1 On-Road Mobile Sources as a Fraction of Regional-Scale Problems: Brief Citations from Example Literature
    3.1.1 EPA National-Scale Air Toxics Assessments
    3.1.2 California Air Resources Board Emissions and Air Quality Almanac
    3.1.3 SCAQMD MATES-II Report
    3.2 Review of the MATES-II Study
    3.2.1 Fraction of MSAT Emissions from On-Road Mobile Sources
    3.2.2 Monitoring Data and Risk Assessment
    3.2.3 Comparison to Other Recent Southern California Research
    3.2.4 Observations Concerning MATES-II vs. EPA DPM Risk Characterizations
    3.3 Regional Emissions Trends
    3.4 Summary Observations Concerning Mobile Sources and Regional Contributions to Air toxics

    4.1 Background
    4.2 Measurements of Concentrations as a Function of Distance from Roadways
    4.3 MATES-II Data
    4.3.1 Modeling Data
    4.3.2 Discussion of MATES-II Monitoring Results Near Freeways
    4.4 Epidemiological Studies
    4.4.1 Observations Concerning the Denver, CO, Study
    4.4.2 Observations Concerning Additional Epidemiological Literature
    4.4.3 Summary Comments Concerning Epidemiological Literature
    4.5 Overall Conclusions Regarding Localized Air Quality.

    5.1 Applicability of MATES-II Findings to Las Vegas
    5.1.1 Summary of MATES-II Findings
    5.1.2 Criteria to Ascertain the Applicability of MATES-II Findings to Las Vegas
    5.1.3 Consideration of Applicability Criteria
    5.2.1 Sierra Club Comments
    5.2.2 Discussion of Comments

    6.1 Regional Impacts from MSAT
    6.2 Local Impacts from MSAT
    6.3 Transferability of MATES-II Findings to Las Vegas.
    6.4 Fine Particulate Matter (PM2.5)
    6.5 Summary Discussion of Issues Raised by the Sierra Club
    6.5.1 Introduction
    6.5.2 Discussion of Sierra Club Comments




ES-1. Relative contributions of on-road mobile, non-road mobile, and stationary source emissions of DPM, benzene, 1,3-butadiene, formaldehyde, and acetaldehyde in California's South Coast Air Basin, as documented in the SCAQMD's MATES-II study

3-1. Relative contributions of on-road mobile, non-road mobile, and stationary source emissions of DPM, benzene, 1,3-butadiene, formaldehyde, and acetaldehyde in California's South Coast Air Basin, as documented in the SCAQMD's MATES-II study

3-2. Projections for nationwide mobile source emissions of priority MSAT, as identified in the EPA MSAT rulemaking

4-1. Pollutant concentrations as a function of wind speed and distance from the roadway, as predicted by EPA's CAL3QHC2 model, assuming no background concentrations exist

5-1. Relative contributions to Las Vegas air toxics concentrations, estimated by EPA

5-2. Comparison of monitored ambient elemental carbon concentrations for southern California and Las Vegas

5-3. Calendar year 2000 ambient PM2.5 monitoring data for southern California and the Las Vegas area


ES-1. Summary listing of major technical issues raised by the Sierra Club

2-1. EPA's 21 Mobile Source Air Toxics

2-2. Comparison of Unit Risk Factors for priority MSAT

5-1. Comparison of on-road fleet average vehicle emission factors for the South Coast and Las Vegas

6-1. Summary listing of major technical issues raised by the Sierra Club

A-1. Comparison of methods to estimate 1982 DPM concentrations in Los Angeles

A-2. Major sources of DPM in MATES-II, and proportions of EC


California Air Resources Board
Carbon Monoxide
Diesel Exhaust Organic Gases
Diesel Particulate Matter
Elemental Carbon
Environmental Impact Statement
U.S. Environmental Protection Agency
Federal Highway Administration
Heavy-Duty Diesel Vehicles
Light-Duty Gasoline Vehicles
Multiple Air Toxics Exposure Study
Mobile Source Air Toxics
National Ambient Air Quality Standard
EPA National Air Toxics Assessment
Nevada Department of Transportation
Organic Carbon
California Office of Environmental Health Hazard Assessment
Polycyclic Aromatic Hydrocarbons
Particulate Matter
Particulate Matter smaller than x mm in diameter
Polycyclic Organic Matter
South Coast Air Quality Management District
Supplemental Environmental Impact Statement
Sonoma Technology, Inc.
Urban Airshed Model
Unit Risk Factor
Vehicle Miles Traveled
Vehicles Per Day


Sonoma Technology, Inc. (STI) prepared this white paper to assist the U.S. Federal Highway Administration (FHWA) in better understanding the contributions of on-road mobile sources to air toxics problems at the regional and local, or "hotspot," scale. The FHWA asked for the assistance in the context of a recently proposed transportation project in the Las Vegas area. The Nevada Department of Transportation (NDOT) proposed roadway widening and improvements along US Route 95 (US 95) and associated roadways in the greater Las Vegas area of Clark County, Nevada (NDOT and FHWA, 1999).

The Sierra Club submitted a US 95 comment letter to FHWA stating that air toxics and fine particulate matter, i.e., PM less than 2.5 microns (mm) in diameter, or PM2.5, were not considered in the US 95 Environmental Impact Statement (EIS) and were a significant enough concern that they should have been addressed in a Supplemental EIS (SEIS) (Spalding, 2002). With respect to air toxics, the Sierra Club cited two published studies as evidence supporting air toxics concerns near freeways: the Multiple Air Toxics Exposure Study (MATES-II) conducted in southern California (SCAQMD, 2000) and a Denver study titled, "Distance-Weighted Traffic Density in Proximity to a Home is a Risk Factor for Leukemia and Other Childhood Cancers" (SCAQMD, 2000). The Sierra Club noted that, based on the reports cited, mobile sources contributed to air toxics problems on both the regional and hotspot scales.

The purpose of this white paper is to provide the FHWA with a brief technical evaluation of the on-road air toxics issues identified by the Sierra Club, with respect to both regional and localized (hot spot) impacts, both in general and as they apply to Las Vegas. This white paper provides answers to three broad questions:

  1. What is the contribution of on-road mobile sources to regional air toxics problems?

  2. What is the relative importance of on-road mobile sources to local, or hot spot, air toxic pollution problems?

  3. What key on-road mobile source air toxics findings are available from the MATES-II study and how transferable are those findings from Los Angeles to Las Vegas?

The white paper also includes discussion of PM2.5 issues. However, the primary objective of the work effort was to give the FHWA a technical framework from which to view mobile source-related air toxics issues, and in particular to place the Sierra Club's comments on US 95 in the context of such a technical framework.

Of the 21 Mobile Source Air Toxics (MSAT) identified by the U.S. Environmental Protection Agency (EPA), EPA identified six as being "likely to present the highest risks to public health and welfare" (61 FR 17257) and identified the need to "focus short-term work on these six": diesel particulate matter and diesel exhaust organic gases (DPM + DEOG), benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and acrolein. (These six MSAT are referred to in this white paper as the "priority MSAT.") The EPA's priority MSAT are consistent with those identified by other agencies such as the California Air Resources Board (CARB) and the South Coast Air Quality Management District (SCAQMD).

The major findings of this study are summarized as follows:

The contribution of on-road mobile sources to regional air toxics

The EPA and other agencies such as CARB show that mobile sources (on-road and nonroad) are the predominant contributors to regional-scale air quality problems with respect to the priority MSAT. Based on 1996 monitoring data, the EPA estimated that on-road mobile sources contributed 30% to 65% of the total measured concentrations for the six priority MSAT (U.S. Environmental Protection Agency, 2002b). Figure ES-1 illustrates the extent of on-road and non-road mobile source contributions to the source inventories for the priority MSAT addressed in the MATES-II study. Emissions from on-road mobile sources have been decreasing over time as older, higher-emitting vehicles are replaced with newer vehicles meeting more stringent emission standards, and as cleaner (lower-emissions) fuels have been introduced (U.S. Environmental Protection Agency, 2001).

Figure ES-1

Diesel Particulate
Matter (DPM)

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blue square (legend for fig ES-1) On-Road Mobile Sources
red square (legend for fig ES-1) Non-Road Mobile Sources
yellow square (legend for fig ES-1) Stationary Sources
Relative contributions of on-road mobile, non-road mobile, and stationary source emissions of DPM, benzene, 1,3-butadiene, formaldehyde, and acetaldehyde in California's South Coast Air Basin, as documented in the SCAQMD's MATES-II study (SCAQMD, 2000).

With respect to cancer risk, CARB has identified 10 priority air toxics, which include some of the priority MSAT. Statewide, CARB has identified mobile sources as being responsible for approximately 89% of the excess cancer risk from these 10 air toxics; 70% of the excess cancer risk has been associated with diesel PM (DPM) alone. These findings are consistent with those of the MATES-II study, although statewide data also indicate a larger contribution from non-road mobile sources than in the MATES-II study. Specifically, CARB estimated that 72% of the total DPM emissions are emitted from non-road sources and 26% are emitted from on-road sources, and that non-road sources account for 61% of the total excess cancer risk and on-road sources account for 28% of the total excess cancer risk (California Air Resources Board, 2001).

The relative importance of on-road mobile sources to local, or hot spot, air toxic pollution problems

Both models and experimental data predict that short-term concentrations of air toxics can be elevated for receptors downwind of and very near roadways. The largest concentration gradients are within 100 m. Over longer time periods (several months), MATES-II monitors sited near freeways did not measure concentrations that differed significantly from other concentrations measured in the South Coast Air Basin. However, the MATES-II study was not designed to evaluate how pollutant concentrations change as a function of distance from roads; two of the three monitors were 70 m from roadways (and not persistently directly downwind of the roadways) and the third was 200 m from a roadway.

Epidemiological studies show mixed results concerning health effects and road proximity, and are limited by the fact that (1) they have not been based upon actual exposure measurements and (2) they often have relatively small sample sizes. Furthermore, given the large lifetime risk of cancer in the general population (one death per four people), epidemiological studies may not be sufficiently precise enough to identify and properly attribute relatively small incremental cancer risks.

Key on-road mobile source air toxics findings available from the MATES-II Study, and the transferability of those findings from Los Angeles to Las Vegas

MATES-II estimates that on-road and non-road mobile sources are responsible for approximately 90% of the excess cancer risk from air toxics in the Los Angeles area. The risk estimates are dependent upon the unit risk factors (URFs) assumed for MSAT, and especially the URF for DPM; MATES-II estimates DPM to be responsible for 70% of the total urban-scale air toxics risk. Among the MSAT, benzene and 1,3-butadiene were the most significant contributors to overall risk, behind DPM.

Given the overwhelming importance MATES-II assigns to the cancer risks from DPM, it is important to place DPM risks in the context of current national thinking regarding DPM. There is not yet a scientific consensus concerning the appropriate URF for DPM. As of early 2002, the EPA decided the literature did not support identifying a DPM URF (U.S. Environmental Protection Agency, 2002a); the California Office of Environmental Health Hazard Assessment (OEHHA), however, has identified a DPM URF, and the OEHHA URF was used in the MATES-II study. OEHHA's URF value is within the "exploratory" risk range identified by the EPA (U.S. Environmental Protection Agency, 2002a). However, there are significant uncertainties in setting a DPM URF; these uncertainties could qualitatively affect the conclusion that DPM accounts for the majority of the cancer risk.

Even if the DPM URFs were reduced by an order of magnitude, a possibility given the range of risks discussed by the U.S. Environmental Protection Agency (2002a), the total excess cancer risk would decrease by approximately 65%, but mobile sources (on-road and non-road) would still be responsible for over 70% of the total MATES-II air toxics risks.

Several qualitative conclusions can be drawn from MATES-II that are likely to be applicable to Las Vegas:

Quantitatively, it was not possible to transfer results from MATES-II to Las Vegas with information readily available during this study. Air toxics concentrations are a function of the quantity of emissions being emitted, the types of sources emitting them, their dispersion throughout the airshed (which depends upon local meteorology and topography), and physical transformations and chemical reactions in the atmosphere. Las Vegas has no ambient air toxics monitoring network. Limited monitoring data for elemental carbon (EC, a surrogate for DPM) indicate that DPM concentrations may be lower in Las Vegas than in southern California. PM2.5 concentrations are also lower in Las Vegas than in southern California. Within the scope of this work, we were not able to determine whether there is a single factor responsible for the majority of these differences, such as differences in emissions source strengths or differences in meteorology, or whether several different factors are contributing.

PM2.5 information

PM2.5 mass concentrations in Las Vegas are lower than in southern California, an indication that some combination of PM2.5 emission inventories and meteorological variables are different for the two areas. Based on current estimates of emissions factors (e.g., Kirchstetter et al., 1999b), it is estimated that a large fraction of mobile source PM2.5 emissions originate from diesel vehicles (i.e., heavy-duty diesels), even though these vehicles are less numerous than light-duty passenger vehicles. Emissions from on-road vehicles may create problematic PM2.5 concentrations near roads (e.g., within 100 m of the road).

Information related to technical issues raised by the Sierra Club

Within the context of air toxics and PM2.5 problems, the Sierra Club and its consultants raised a variety of concerns related to US 95. The Sierra Club's concerns can be categorized under eight broad topics; these are listed in Table ES-1.

Table ES-1. Summary of major technical issues raised by the Sierra Club.
  1. Due to the evidence demonstrating the relationship between road proximity and risk, the FHWA should evaluate the potential risks associated with air toxics and fine particulate matter from completion of US 95 improvements. The 21 MSAT identified by the EPA should be addressed in the evaluation.

  2. US 95 will result in higher traffic densities, which in turn will produce increased emissions and pollutant concentrations.

  3. Existing National Ambient Air Quality Standard (NAAQS) and NAAQS-associated air quality control programs are not sufficient to provide public health protection against exposure to air toxics.

  4. There is evidence to indicate that health risk increases with proximity to roads and traffic, and especially heavy-duty diesel vehicle traffic.

  5. The MATES-II study in southern California offers an opportunity to approximate air toxics-related health risks from roads.

  6. MATES-II modeling analyses underestimate the pollutant concentrations likely to occur in close proximity to roads.

  7. The risks estimated by the MATES-II study exceed federal health risk guidelines for excess cancer risk.

  8. Future conditions once the US 95 highway project is complete remain uncertain. Although congestion may decrease and speeds increase, it is unclear whether congestion relief will prove beneficial with respect to lowering air toxics emissions. In addition, there will likely be increased traffic density over time, resulting in increased emissions. The implementation of various mobile source controls may lower emissions, but those benefits may be many years distant (e.g., heavy-duty diesel vehicle standards are not scheduled to take effect until 2007).

  1. With respect to issue #1, currently available data indicate that MSAT evaluations should focus on the six priority MSAT, rather than all 21 MSAT identified by the EPA, since (a) the majority of the risk from the MSAT is associated with the six priority MSAT, and (b) mobile sources are the predominant source of the six priority MSAT. CARB also identifies only five priority air toxics that are emitted by mobile sources, and all five are included among the EPA's six priority MSAT (California Air Resources Board, 2002). The extent to which evaluations can quantitatively be conducted may be compromised by known inadequacies in the available emissions estimation and dispersion modeling tools. A more complete evaluation of the appropriateness of existing tools is a research need.

  2. With respect to issue #2, according to the EIS, future traffic densities (expressed in vehicle miles traveled, or VMT) would be higher than existing traffic densities regardless of whether US 95 modifications are completed ("build case") or not ("no-build case"). In addition, emissions are a product of VMT and an emissions factor (grams of pollutant per VMT). Because emissions factors depend upon both calendar year (i.e., each year, the on-road vehicle fleet has a higher percentage of new vehicles subject to more stringent emissions standards) and speed, it is not possible to categorically state that because traffic densities will be higher in the future, emissions and ambient concentrations will also be higher in the future. For example, for CO, although the EIS shows that "build" scenario concentrations are often higher than "no-build" concentrations, the highest concentration predicted corresponds to an intersection evaluated for the "no-build" scenario.

  3. With respect to issue #3, we agree that the NAAQS do not directly address air toxics. However, section 112 of the Clean Air Act addresses stationary sources of air toxics, section 202(l) addresses the issue of MSAT in general, and section 211(k) regulates a subset of MSAT in reformulated gasolines used in certain ozone nonattainment areas. The EPA also predicts future MSAT emission reductions from on-road mobile sources in part due to volatile organic compound (VOC) and PM controls (U.S. Environmental Protection Agency, 2001). (VOC and PM are controlled as part of the ozone and PM NAAQS and through implementation of more stringent on-road vehicle emissions standards.)

  4. Although some published studies have indicated that health risks increase with proximity to roads and traffic, others have not found these correlations; many of the findings are from studies that do not measure exposures and have relatively low numbers of subjects. The possibility exists that there is a correlation between road proximity and health risk, but that finding is not conclusive.

  5. At the regional scale, the MATES-II study approximates air toxics-related health risks from roads, but also cautions that results are highly dependent upon the unit risk factors assumed, particularly for diesel particulate matter, for which uncertainties are an order of magnitude or more. At the microscale, MATES-II was not designed to effectively assess changes in pollutant concentrations with varying distance from roadways.

  6. Field measurements show that for short time periods (e.g., less than 24-hours), on-road emissions result in pollutant concentrations that are highest within 100 m of roadways. MATES-II modeling was not on the 100-m scale; the modeling grid divided the region into 2-km x 2-km squares, and predicted an average concentration for each square. It is therefore likely that the MATES-II modeling underpredicted concentrations within 100 m of roads. The MATES-II monitoring effort was not designed to evaluate pollutant concentration differences as a function of proximity to roads. The limitations of the MATES-II modeling have important implications for the transferability of the MATES-II methodology to Las Vegas. MATES-II style modeling will not address project-specific impacts for the Las Vegas US 95 project. The three MATES-II monitors that were placed to study roadway effects were at least 70 m away from the roads and prevailing winds were not from the directions of the closest point on the roadways.

  7. The EPA generally acts to reduce cancer risks greater than 10-4 and considers risks less than 10-6 to be acceptable. Even in remote areas of the United States, the EPA has estimated that risks from background levels of air pollution are 10-5 (Guinnup, 2003). We agree that the risks estimated by MATES-II are greater than 10-4, regardless of the value assumed for the DPM URF (i.e., estimated excess cancer risk associated with the other air toxics was estimated at approximately 400 in 1 million, or 4 x 10-4).

  8. Overall, we would agree that there is uncertainty regarding future conditions, whether the US 95 highway project is completed or not. However, fleet average g/mi emission rates (expressed in grams per mile, or g/mi) are decreasing, even before the new heavy-duty diesel vehicle standards are implemented in 2007. Although it is predicted that more vehicle miles will be traveled in the future, improved traffic flow and increased travel speeds from roadway improvements may decrease per-vehicle emissions per mile depending upon actual speeds driven and fleet mix.

Updated: 07/06/2011
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