There is no conclusive evidence that breathing diesel exhaust causes cancer or other diseases in humans, even though diesel materials painted on or fed to rats may cause cancer in them, the National Academy of Sciences reported yesterday.
A special health effects panel of the academy's National Research Council added that the amount of cancer-causing material, found in diesel fumes is "within the same order of magnitude" as that found in ordinary gasoline engine exhaust.
At the request of the Environmental Protection Agency and the departments of Energy and Transportation, the panel reviewed literature and unpublished research on the diesel emissions controversy. With fuel-saving, high mileage diesels expected to power one in every four new cars sold by the turn of the century, regulators had become alarmed at research showing diesel chemical exposure caused birth defects, cancer, lung disease and other problems in laboratory animals.
"In many cases, the conditions under which humans are exposed to diesel exhaust are generally different from those used in laboratory animal experiments," the panel noted. Concentrated extracts of diesel exhaust chemicals, the particles in the exhaust or just the gases may be painted on the animals' skins, injected or fed to them. But human beings breathe a diluted mix of diesel fumes, air and other pollutants, it said.
Studies of workers regularly breathing diesel exhaust found no excess cancers, "even though some of the individual components [lot of the fumes] are known to exert adverse effects," the report said. However, the report was also critical of the methods used in many of the studies.
Similarly, research does not show that diesel exhaust causes birth defects or lung disease, although massive short-term doses can cause nausea and dizziness, according to the panel.
The group called for more research into diesel effects, concluding only that carcinogens are present in the exhaust, that apparently they are not highly active, and that the degree of cancer-causing activity is greatly affected by the engine design.
The EPA set tighter standards in February for the level of particulate matter, or soot, that diesel cars and light trucks are permitted to produce. David Hawkins, EPA's assistant administrator for air quality, said yesterday those rules would not be affected by the academy findings. The rules, he said, were not related to concern over cancer but to concern that the extremely fine particles of soot embeded themselves in the deepest recesses of the lungs.