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

1. Introduction


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) and FHWA 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 project Environmental Impact Statement (EIS) noted that Clark County is classified as nonattainment with respect to the National Ambient Air Quality Standard (NAAQS) for carbon monoxide (CO) and particulate matter smaller than 10 microns (PM10), and discussed impacts and mitigation for these pollutants.

The Sierra Club submitted a letter to the 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 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" (Pearson et al., 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. With respect to regional air quality, one of the MATES-II study conclusions was:

  1. The carcinogenic risk in the [South Coast Air] Basin is about 1,400 per million people [based on the average of the pollutant concentrations at the ten fixed monitoring sites]. Mobile sources. . . .represent the greatest contributor. About 70% of all risk is attributed to diesel particulate emissions; about 20% to other toxics associated with mobile sources. . . .[and] about 10% of all risk is attributed to stationary sources (SCAQMD, 2000, p. ES-3).

The Denver study focused on hot spots, i.e., health risks in the immediate vicinity of roadways, as opposed to regional air pollution. It concluded that, ". . . .results are suggestive of an association between proximal high traffic streets with traffic counts = 20,000 [vehicles per day] and childhood cancer, including leukemia" (Pearson et al., 2000, p. 175).

Given the Sierra Club's concerns about road use and air toxics, the organization sponsored three studies to further explore potential air quality impacts related to the US 95 expansion project. The reports from these studies discussed the toxicology associated with roadway traffic air pollution in general (Environmental Health & Engineering, Inc., 2001), applied the results from MATES-II to estimate air pollutant concentrations in the Las Vegas area after the proposed US 95 modifications (High, 2001), and determined the associated cancer risk associated with those concentrations (Barry and McCarthy, 2002).

For PM2.5, the Sierra Club did not cite roadway-specific studies or additional reports they commissioned, but noted that motor vehicles emit PM2.5, that exposure to PM2.5 endangers public health, and argued that a US 95 SEIS was needed to determine the frequency of daily exposures above 16-20 mg/m3 and the effects of highway emissions on those exposures (the PM2.5 NAAQS is 65 mg/m3 on a 24-hour average basis and 15 mg/m3 on an annual average basis).


This white paper provides 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. Given the limited scope of the project, this effort does not constitute a comprehensive literature review, nor is it a detailed point-by-point analysis of each of the many technical issues raised by Sierra Club correspondence and studies. Rather, the goal for this work effort was to help the FHWA answer 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?

Some discussion of PM2.5 issues has also been included, with respect to the Sierra Club's comments for that pollutant. 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. Accordingly, the FHWA also asked STI to identify whether, for the most important issues raised by the Sierra Club regarding US 95, there were either technical justifications for the issues or whether there were technical concerns or uncertainties that suggested an interpretation of the issues raised that was different from that presented by the Sierra Club.

Regulatory, legal, and political aspects of the Sierra Club's comments, for example clarifying legally when an SEIS should be required and when it should not, were not addressed in this study.


This white paper is organized into sections that address various aspects of the questions being raised. Section 2 provides background information regarding air toxics, with respect to both how this category is defined and how quantitative cancer risk assessments are typically conducted. Section 3 provides background information on air toxics at the regional scale, and includes a summary of the MATES-II study and its conclusions regarding regional air toxics and transportation impacts. Section 4 addresses the issue of localized air toxics impacts. Section 5 includes comments related to issues raised by the Sierra Club, including the transferability of the MATES-II findings to Las Vegas and information regarding PM2.5. Overall results and conclusions, including a summary of the comments from the Sierra Club and appropriate technical considerations, are included in Section 6. Section 7 provides references cited in this white paper. Appendix A provides additional technical information concerning the assessment of diesel particulate matter (DPM) concentrations and composition of DPM; Appendix B provides additional information with respect to the assessment of health risks from DPM; and Appendix C provides details concerning the mobile source emission factor model runs completed to compare vehicle emission factors in southern California and Las Vegas.

Readers of the entire report will note that some findings are restated in several report sections. The duplication is a function of the report's three main functions. Some sections (2, 3, 4) of the report focus on discussing air toxics in general; some sections (5) focus on discussing MATES-II and its applicability to Las Vegas; and the last portion of the report (section 6) organizes material to address each of the Sierra Club's main issues. The result is repetition of points raised. For example, MATES-II findings are discussed as one of the resources used to explain air toxics issues generally; MATES-II is discussed again when we consider its applicability to Las Vegas; and finally MATES-II issues are reintroduced when we discuss each of the issues raised by the Sierra Club. The repetition is not desirable, but was left intact in the final white paper version to reflect the interests of readers who might want to focus on only one or two report sections.

Updated: 3/17/2015
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