19 November 1999

The US Environmental Protection Agency published a revised draft assessment of the possible health hazards from human exposure to diesel engine exhaust emissions. The document, dated November 1999, updates two earlier drafts (1998 and 1994) that were reviewed by the EPA's Science Advisory Board (SAB) Clean Air Scientific Advisory Committee (CASAC). The current draft assessment focuses on health hazard identification and dose-response analysis of diesel particulate matter (PM) for the purpose of characterizing the risk of diesel exhaust exposure.

Two classes of possible health effects are discussed in the report: nonmalignant respiratory effects and carcinogenic effects. The EPA confirmed its earlier findings on the non-cancer effects of ambient PM, such as increased mortality and morbidity, and upheld its recommended chronic exposure level without appreciable hazard (Reference Concentration RfC) for diesel particulates of 5 µg/m3.

A number of conclusions has been drawn on the cancer effects of diesel exhaust:

  • Diesel exhaust is "highly likely" to be carcinogenic. Epidemiological studies suggest that lung cancer increases about 33-47% in occupational exposures to diesel exhaust. The EPA believes the cancer hazard is also applicable to ambient (i.e. environmental) exposures, even though none of the studies examined by the EPA show that lung cancer hazard is present at environmental levels of exposure.
  • Animal (rat) cancer studies are not clear for human hazard prediction and unsuitable for environmental exposure risk estimate. Quantitative statements on human risk cancer should be based on human epidemiological studies. However, the currently available data, due to a number of uncertainties, is deemed unsuitable for quantitative risk assessment.
  • As a result, the current EPA assessment does not adopt or recommend any specific cancer unit risk estimate for diesel exhaust.

The above position of the EPA on quantitative cancer risk assessment is in agreement with recent findings by the HEI, and in contradiction to the findings of the California Air Resources Board, which has established unit risk estimates for cancer from diesel exhaust particulates.

This assessment will undergo another review by the CASAC in December 1999, and, following a consideration of comments, a final document will be issued. The final document will have a significant impact on future diesel emission regulations in the USA.

Download the report

Source: US EPA
Contact: Dr. William Pepelko, 202-564-3309, pepelko.william@epa.gov