Toxicological and epidemiological evidence for health risks from inhaled engine emissions

Information from toxicological and epidemiological studies of the cancer and noncancer health risks from inhaled diesel engine exhaust (DE) and gasoline engine exhaust (GE) was reviewed. The toxicological database is more extensive for DE than for GE. Animal studies have shown that heavy, chronic exposures to both DE and GE can cause lung pathology and associated physiological effects. Inhaled GE has not been shown to be carcinogenic in animals. Chronically inhaled DE at high concentrations is a pulmonary carcinogen in rats, but the response is questionable in mice and negative in Syrian hamsters. The response in rats is probably not attributable to the DE soot-associated organic compounds, as previously assumed, and the usefulness of the rat data for predicting risk in humans is uncertain. Experimental human exposures to DE show that lung inflammatory and other cellular effects can occur after single exposures, and sparse data suggest that occupational exposures might affect respiratory function and symptoms. Epidemiology suggests that heavy occupational exposures to exhaust probably increase the risks for mortality from both lung cancer and noncancer pulmonary disease. The small magnitudes of the increases in these risks make the studies very sensitive to confounding factors and uncertainties of exposure; thus, it may not be possible to resolve exposure-response relationships conclusively by epidemiology. Our present knowledge suggests that heavy occupational exposures to DE and GE are hazardous but does not allow quantitative estimates of risk with a high degree of certainty.