A new analysis of the health effects of diesel exhaust exposure may be the basis for additional restrictions on emissions from diesel cars and trucks, the Environmental Protection Agency said in a draft document obtained by BNA April 6.
The draft health assessment, still to be reviewed by EPA's Clean Air Scientific Advisory Committee, found that diesel exhaust as a whole is a possible, or even a "likely," human carcinogen, depending on the parameters used in gauging the hazard.
The health assessment pulls together "key data relating to toxicity," assesses the scientific findings, then provides "a risk characterization describing the human health hazards of [diesel exhaust] exposure."
The draft document was prepared by the Washington office of the National Center for Environmental Assessment, a risk assessing program in EPA's Office of Research and Development. It was prepared for EPA's Office of Mobile Sources, which has the authority to regulate diesel engines and fuels.
OMS officials were unavailable for comment April 7. However, further controls on diesel engines and fuels is under consideration, the draft assessment indicated. California and European countries also are mulling additional controls. The draft health assessment is expected to provide a solid scientific foundation for EPA efforts, the National Center for Environmental Assessment said in the draft.
Diesel engines are a significant source of particulate matter and nitrogen oxide, two pollutants regulated under the Clean Air Act for their effects on human health. Diesel engines "contribute significantly to the national" emission inventory of extra fine particles (smaller than 2.5 microns in diameter), the draft assessment said.
Definite Health Hazard
Taken together, these and other facts are enough for EPA to label diesel exhaust as a human health hazard, the draft assessment indicated. Even when scientific uncertainty and debate over various pieces of the evidence are taken into account, "Overall, EPA considers that the human evidence for [diesel exhaust] gives a clear signal about the likely presence of a human hazard," the draft said.
On the question of whether diesel exhaust causes cancer, however, answers are more difficult to obtain.
Diesel exhaust definitively causes lung cancer in rats, the assessment said, as long as those rats are subjected to extended exposures at high concentrations. However, this response "to date has been clearly demonstrated only in rats."
Lung cancer in humans is "suggested" by the results of a number of epidemiological studies, the draft assessment said, but uncertainties exist.
"Because of the uncertainties created by limited exposure data and the possibility of exposure to other agents," the draft said, "the evidence for carcinogenicity of diesel engine emissions in humans is considered limited under the [current] EPA Cancer Assessment Guidelines."
On the other hand, if the carcinogenicity of diesel exhaust is assessed using the parameters of EPA's Proposed Guidelines for Carcinogen Risk Assessment, diesel exhaust "is considered to be a likely human carcinogen," the report said. Furthermore, when compared to other agents similarly classified as "likely" human carcinogens, diesel exhaust ranks at the "upper end" of the list, the draft assessment said.
Using Proposed Cancer Guidelines
The draft cited at least 11 epidemiological studies that tried to determine whether diesel exhaust causes lung cancer. In six of those studies, "increased incidence of lung cancer was observed" among people exposed to diesel exhaust. In four studies, no increase was observed. But each of these latter four studies "had several methodological limitations," the report said. Mainly, sample sizes were small, follow-ups were short, or there was a "lack of adjustment for confounding factors."
An upcoming report by the National Toxicology Program will list three diesel exhaust constituents for the first time as "reasonably anticipated to be a human carcinogen." They are 1,6-dinitropyrene (CAS No. 42397-64-8), 1,8-dinitropyrene (CAS No. 42397-65-9), and 1-nitropyrene (CAS No. 5522-43-0) (20 DEN A-5, 1/30/98). The National Toxicology Program assesses chemicals for a number of federal agencies, including EPA.
The draft health assessment by the National Center for Environmental Assessment is different from the NTP report in that it looks at diesel exhaust as a whole, and not at individual constituents.
Officials in California are undertaking a similar, though not identical, assessment of diesel exhaust. The California Air Resources Board may decide officially in June whether to list diesel exhaust as a carcinogen and further regulate emissions (19 DEN A-3, 1/29/98).
Diesel engines, already used widely in heavy-duty trucks, buses, and off-road equipment, are likely to be used in other vehicles in coming years, EPA said in an introduction to the draft assessment. "New technologies, such as hybrid vehicles powered by advanced diesel engines, and the potential for increased use of light-duty diesel engines in the sport utility market could fuel this expansion," EPA said.
Diesel Use Expanding
At the same time, the United States and Europe, along with California, are deciding how further to regulate diesel engines and diesel fuel. A final version of the health assessment should enable decisions "based on sound scientific foundations," the draft said.
The document, dated Feb. 1, was not made available until April.
The draft health assessment is available on the World Wide Web at www.epa.gov/ncea/pdfs/. Individual chapters from the report are marked "27-Mar-98." The assessment also can be obtained from the National Technical Information Service by calling (703) 487-4650.
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