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Diesel Exhaust

Risk Assessment

Who is exposed?
Nearly everyone is exposed to diesel exhaust. However, there are certain groups of people who are more exposed than the general population. These include people living in urban and industrialized areas, people living near freeways, children, and workers who operate diesel-powered machinery.

Non-cancerous diesel exhaust effects
The California EPA Office of Environmental Health Hazard Assessment (OEHHA) completed an assessment in 1998 that formed the basis to identify diesel exhaust particles as a “toxic air contaminant.” In 1993, the EPA determined an inhalation reference concentration of 5 micrograms per cubic meter of diesel particulate matter. A reference concentration is an “estimate of inhalation exposure of Diesel exhaust (as measured by diesel particulate matter) to which humans may be exposed throughout their lifetime without being likely to experience adverse non-cancer respiratory effects.” This estimate was derived from dose-response data from rat inhalation studies. California’s OEHHA also adopts this concentration, citing a high confidence from a database of studies that had consistent results and agreement from the World Health Organization.

Carcinogenic properties
The quantitative risk assessment from the OEHHA in 1998 provides a prediction of cancer risk in relation to the mass of diesel particulate matter (which is a surrogate for diesel exhaust because it can be measured). The OEHHA used human epidemiological studies to estimate the risk of lung cancer, since there are several problems with extrapolating human risk from rat studies. One problems is that in most rat studies, the rats are exposed to high amounts of diesel exhaust in a short period of time. In contrast, human exposure to diesel exhaust is generally a low concentration, but over a lifetime. For this reason, results from cohort and case control studies of US railroad workers were used to estimate the risk of lung cancer due to diesel exhaust among the general population. In the OEHHA risk assessment, exposure to diesel exhaust at a concentration of 1.54 micrograms per cubic meter (an average ambient concentration) causes from 200 to 3600 additional cancer cases per 1 million Californians over a 70 year lifetime. Therefore, diesel exhaust “appears to meet the definition of a toxic air contaminant.”

The EPA agrees that diesel exhaust is “likely to be carcinogenic to humans by inhalation.” In their risk assessment, however, the EPA did not give a quantitative estimate of risk of lung cancer due to diesel exhaust exposures. There is some uncertainty “to definitively conclude that diesel exhaust is carcinogenic to humans.” Although rat and mice studies demonstrate mutagenic and chromosomal effects, these studies do not reflect normal human exposure, as previously explained. The EPA decided that the human data from epidemiological studies are too uncertain to derive a quantitative estimate of cancer risk.

In summary, although there are many uncertainties, including gaps in data about human exposure and knowledge of underlying mechanisms of how diesel exhaust causes toxicity in humans and animals, diesel exhaust appears to have carcinogenic effects. For more detailed information and risk assessment documents, refer to the EPA’s risk assessment on diesel exhaust at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=29060

California’s Office of Environmental Health Hazard Assessment at: ftp://ftp.arb.ca.gov/carbis/regact/diesltac/partb.pdf

and the Health Effects Institute at:

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