Diesel emissions are the predominant source of cancer risk in Scorecard's assessment of hazardous air pollutants. Inclusion of diesel emissions in EPA's National-Scale Assessment of Air Toxics has totally transformed our scientific understanding of which chemicals and pollution sources are responsible for the largest part of the air toxics problem. Previous analyses (like EPA's Cumulative Exposure Project) have focused only on hazardous air pollutants listed under the federal Clean Air Act and did not include diesel emissions. Now that estimates of diesel particulate concentrations are available from NATA, it is clear that the cancer risks from diesel emissions are about ten times higher than the cancer risks from all other hazardous air pollutants combined. For the U.S. as a whole, the average cancer risk associated with diesel emissions is 580 per million - 80% of the total estimated cancer risk from all hazardous air pollutants (740 per million).

To help validate this important finding, it is valuable to examine other evidence that supports this conclusion. Unfortunately, NATA's exposure estimates for diesel particulate cannot be compared directly against monitoring results. There is currently no technique to directly measure diesel particulates, and this complex mixture has not been the subject of monitoring by most state air toxics networks. Estimates of diesel particulate concentration can be obtained by measuring elemental carbon, but only California regulatory agencies have performed a significant amount of monitoring using elemental carbon as a surrogate for diesel particulates.

The Multiple Air Toxics Exposure Study (MATES-II) is a landmark urban toxics monitoring and evaluation study conducted for California's South Coast Air Quality Management District (SCAQMD). The study represents the most comprehensive air toxics programs ever conducted in an urban environment, including a comprehensive monitoring program, an updated emissions inventory of toxic air contaminants, and a modeling effort to fully characterize health risks from hazardous air pollutants.

Using monitoring data collected from ten stations in the South Coast Air Basin during 1998, the SCAQMD estimates that the average carcinogenic risk in the Basin is about 1,400 per million people, with a range from about 1,120 in a million to about 1,740 in a million among the ten sites. This risk range is consistent with Scorecard's cancer risk range for counties in the air basin: 630 - 1,500 per million.

Using a detailed emissions inventory, the SCAQMD found that mobile sources (e.g., cars, trucks, trains, ships, aircraft, etc.) represent the greatest contributor to estimated cancer risks. About 70% of all risk is attributed to diesel particulate emissions; and about 20% to other toxics associated with mobile sources (including benzene, butadiene, and formaldehyde). This source apportionment is also completely consistent with Scorecard's results based on NATA estimates: In Los Angeles County, 86% of added cancer risks are attributed to mobile source pollutants, and 78% of added cancer risks is attributed to diesel particulate emissions.

The close agreement between MATES and Scorecard's results for Los Angeles is particularly striking because the analyses rely on different exposure assessment methodologies (measured vs. estimated ambient concentrations). MATES also included a modeling component (using different dispersion models and emissions inventories from NATA) which generated results that are consistent with Scorecard's: model results show similar average levels of carcinogenic risk across the South Coast Basin, and also show the strong domination of mobile sources contributing to risk. Both Scorecard and the MATES study rely on the California Air Resource Board's risk assessment value for diesel emissions to estimate potential cancer risks.

The national association of state and local air quality control officers (State and Territorial Air Pollution Program Administrators and the Association of Local Air Pollution Control Officials) conducted a screening-level health risk assessment of diesel emissions in its report Cancer Risk from Diesel Particulate: National and Metropolitan Area Estimates for the United States. The association concluded that diesel emissions may be responsible for 125,000 cancer cases in the U.S. Based on monitoring data for levels of elemental carbon in several areas around the country, the study assumed that the levels of elemental carbon in metropolitan areas in the United States are one-half of the Los Angeles area average (1.65 ug/m3) and levels in non-metropolitan areas are one-tenth of the Los Angeles average (.33 ug/m3). The population of the United States was divided into residents of metropolitan areas (80 percent of the population) and residents of non-metropolitan areas (20 percent) and assigned their respective exposure levels to predict potential cancer incidence.

STAPPA/ALAPCO concluded that the magnitude of the cancer threat from diesel particulate makes an overwhelmingly compelling case for aggressive and timely action by EPA to control onroad and nonroad diesel engines and their fuels.

The U.S. Public Interest Research Group has prepared a report describing the extensive noncancer health risks associated with the particulate matter and ozone pollution caused by diesel engines. In May, 2000, the Environmental Protection Agency proposed new emission standards for heavy duty vehicles. The PIRG report estimates the health benefits the nation could enjoy if the EPA implements this tough new heavy duty rule. EPA has not yet quantified the health benefits that will result from its heavy duty rule, so PIRG used methods from a similar EPA rulemaking on passenger vehicles to estimate health benefits. The report estimates that, beginning in 2030, the heavy duty rule will prevent:
more than 4,000 premature deaths each year,
nearly 3,000 hospital admissions each year,
more than 6,300 total respiratory emergency room visits each year, more than 1,200 of which will be asthma-related
more than 5.6 million restricted activity days each year, and
more than 650,000 work loss days, saving over $67 million, each year.