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

2. Relevant Air Toxics and Quantification of Cancer Risks

The term "air toxics" has been used to refer to many different lists of substances. This section identifies the air toxics most pertinent to mobile sources, and the related issue of how cancer risks are estimated. This background material is included here to place concerns about freeway-related emissions in the context of the most important air toxics associated with mobile sources, and to acknowledge the inherent uncertainties involved with estimating excess cancer risk associated with toxics exposures.


The U.S. Environmental Protection Agency (EPA) published a list of 21 Mobile Source Air Toxics (MSAT) in the Federal Register on March 29, 2001 (U.S. Environmental Protection Agency, 2001). The list includes toxics that are emitted from both on-road mobile sources and non-road mobile sources (including non-road vehicles and construction equipment, commercial boats, trains, ships, and aircraft). The EPA noted that this listing was "not itself a determination by EPA that emissions of the compound in fact present a risk to public health or welfare" (U.S. Environmental Protection Agency, 2001; p. 17234), but continued on to say that ". . . .considering the single chemical inhalation health hazards and exposure to the MSAT emissions from on-highway sources, diesel particulate matter and diesel exhaust organic gases (DPM + DEOG), benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and acrolein are likely to present the highest risks to public health and welfare" (U.S. Environmental Protection Agency, 2001; p. 17257). These six pollutants are referred to as "priority MSAT" in this white paper. Table 2-1 lists the 21 EPA MSAT divided into three classes: specific organic compounds, i.e., those that have specific chemical formulas and identities; compounds containing inorganics (commonly referred to as "metals," though not all are technically metals); and categories of pollutants which include an indefinite or unspecified number of chemical species, or are not always consistently defined.

The EPA's findings are similar to findings from other agencies. For example, the California Air Resources Board (CARB) has identified 10 air toxics of concern, five of which are emitted by on-road mobile sources: benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and diesel PM (California Air Resources Board, 2001).

There is overlap among the substances listed in Table 2-1. For example, POM includes a wide range of substances, including polycyclic aromatic hydrocarbons (PAHs) and sometimes the dioxins and furans; the diesel emissions categories include several of the individual MSAT as well. EPA's Health Assessment Document for Diesel Exhaust notes:

  1. Among the gaseous hydrocarbon components of [DEOG] that are individually known to be of toxicologic relevance are the aldehydes (e.g., formaldehyde, acetaldehyde, acrolein), benzene, 1,3-butadiene, and [POM constituents]. . . .A number of toxicologically relevant organic compounds are on the [DPM]. The organics, in general, range from about 20% to 40% of the particle weight, though higher and lower percentages are also reported. Many of the organic compounds present on the particle and in the gases are individually known to have mutagenic and carcinogenic properties. For example, PAHs, nitro-PAHs, and oxidized PAH derivatives are present. . . .[and comprise] about 1% or less of the DPM mass. (U.S. Environmental Protection Agency, 2002a; p. 1-1)

Table 2-1. EPA's 21 Mobile Source Air Toxics (MSAT). (Six "priority MSAT" appear in boldface.)
Specific Organic Compounds Compounds Containing Inorganics Pollutant Categories
a The structure of PCDDs/PCDFs is well-defined, but various methods are used for calculating total PCDDs/PCDFs (i.e., with respect to which isomers are included in the total, and which toxicity weighting factors to use for each isomer, if any).
Methyl tert-Butyl Ether (MTBE)
Arsenic Compounds
Chromium Compounds
Lead Compounds
Mercury Compounds
Manganese Compounds
Nickel Compounds
Diesel Particulate Matter (DPM) + Diesel Exhaust Organic Gases (DEOG)

Polycyclic Organic Matter (POM)

Polycyclic Chlorinated Di-benzop-dioxins PCDDs) and Polycyclic Chlorinated Dibenzo-p-furans (PCDFs) ("Dioxins/Furans")a

In summary, the highest-risk air toxics from on-road vehicles have been identified by the EPA and others as DPM/DEOG, benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and acrolein; the latter five are all components of DEOG.


Although there are health risks from air toxics aside from cancer risks, health-related conclusions regarding air toxics are often quantified only in terms of cancer risk. The MATES-II study assessed only cancer risk, as did the Sierra Club's consultant who drew conclusions about pollutant concentrations resulting from the proposed US 95 modifications (Barry and McCarthy, 2002). For this reason, and the fact that this white paper is largely non-quantitative (i.e., with respect to air pollution in Las Vegas), non-cancer health risks are not addressed in this white paper.

Cancer is one of the leading causes of mortality in the United States: approximately one in every four deaths is from cancer, and approximately 40% of people in the United States will develop an invasive cancer sometime during their lifetime (American Cancer Society, 2002). With respect to air pollution (and many other forms of environmental pollution), associated cancer risks are often measured in terms of "excess deaths" due to the environmental exposure, usually per million people. For example, a lifetime risk of 10-5 corresponds to 10 excess deaths per million people; a lifetime risk of 10-4 corresponds to 100 excess deaths per million people. The EPA's risk management policy for stationary sources, as identified in its 1999 Residual Risk Report to Congress (based upon the EPA's 1989 rulemaking for benzene emissions standards), is to "[strive] to provide maximum feasible protection against risks to health from hazardous air pollutants by (1) protecting the greatest number of persons possible to an individual lifetime risk level no higher than approximately 1 in 1 million and (2) limiting to no higher than approximately 1 in 10 thousand the estimated risk that a person living near a plant would have" (U.S. Environmental Protection Agency, 1999, pp. ES-11 and ES-12). The EPA has stated that estimated excess cancer risks for background air pollution even in remote areas of the United States are on the order of 10-5 (Guinnup, 2003). As mentioned previously, the MATES-II study calculated an excess cancer risk of 1,400 per million people, 90% of which was attributed to mobile sources.

Issues surrounding cancer risk assessment and the uncertainties surrounding these assessments are discussed in more detail in Appendix B. However, a common means of assessing cancer risk is to multiply the concentration of each carcinogenic substance by a Unit Risk Factor (URF) for that substance. The risks for each substance are then added to arrive at a total risk. As an example of the extent to which there are differences in opinions regarding the "correct" value for URFs, Table 2-2 illustrates differences between URFs assigned by the EPA, and those assigned by the California Office of Environmental Health Hazard Assessment (OEHHA).

Table 2-2. Comparison of Unit Risk Factors (expressed in (mg/m3)-1) for priority MSAT.
Pollutant OEHHAa EPAb Difference between OEHHA and EPA
a Data are from California Office of Environmental Health Hazard Assessment,, as of November 18, 2002.
b Unless noted otherwise, data are from EPA Integrated Risk Information System (IRIS),, as of November 18, 2002.
c Carcinogenic risks not quantified.
d "The [diesel exhaust] human exposure-response data are considered too uncertain to derive a confident quantitative estimate of cancer unit risk, and with the chronic rat inhalation studies not being predictive for environmental levels of exposure, EPA has not developed a quantitative estimate of cancer unit risk." (U.S. Environmental Protection Agency, 2002a, pp. 1-5).
Acetaldehyde 2.7 x 10-6 2.2 x 10-6 20%
Acrolein N/Ac N/Ac N/A
Benzene 2.9 x 10-5 2.2 x 10-6 to 7.8 x 10-6 factor of approx. 4-13
1,3-Butadiene 1.7 x 10-4 3.0 x 10-5 factor of approx. 6
Formaldehyde 6.0 x 10-6 1.3 x 10-5 factor of approx. 2
DPM 3.0 x 10-4 Insufficient datad N/A

It should be noted that the differences presented in Table 2-2 are not necessarily representative of the uncertainties in these values, or the cancer risks derived from them. OEHHA and EPA often reference the same studies when evaluating cancer risks, and therefore the differences are a reflection only of the differences in opinion regarding which URFs should be used. Uncertainties associated with the accuracy of the risk calculations are actually higher, as they depend upon not only these differences of opinion, but also any uncertainties associated with the data used to develop the URF, or the URF approach in general.


Of the 21 MSAT identified by the EPA, the EPA stated that diesel particulate matter and diesel exhaust organic gases (DPM + DEOG), benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and acrolein were likely to present the most significant risks. This is consistent with priorities identified by other agencies such as CARB. DEOG includes the other five priority MSAT pollutants, and there is at least some evidence that the chemical composition of DPM varies with drive cycle and engine technology (as discussed in Appendix A).

The quantitative cancer risk associated with the concentration of a given substance is typically calculated as the product of the concentration and a URF. There are several caveats that should be taken into account when using this approach, and it is not uncommon for different authorities to identify different URFs for a given substance; in some cases, agency differences are substantial (e.g., an order of magnitude). Conclusions regarding both (1) the total excess cancer risk due to concentrations of several pollutants, and (2) the relative importance of any one of those pollutants, are highly dependent upon the URFs chosen.

Updated: 3/17/2015
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