Long-term exposure to diesel engine exhaust (DE) in the air is linked to lung cancer, the US Environmental Protection Agency (EPA) concluded in a report made public on Sept. 3.

"The human evidence from occupational studies is considered strongly supportive of a finding that DE exposure is causally associated with lung cancer, though the evidence is less than that needed to definitively conclude that DE is carcinogenic to humans," the EPA report said.

"Overall, the evidence for a potential cancer hazard to humans resulting from chronic inhalation exposure to [diesel emissions] is persuasive," the report said.

Diesel exhaust comes from a wide variety of sources, including large trucks, earth-moving equipment, farm machinery, and diesel-powered cars, the report noted.

Earlier reports said that diesel truck drivers, mechanics, and people whose occupations expose them constantly to diesel exhaust have higher lung cancer rates.

But the current EPA report focused on those who breathe diesel exhaust in the air around them, whether they work directly with diesel engines or not.

Diesel engine manufacturers issued a statement saying the report is based on exhaust from older engines and that those currently manufactured don't pollute as badly.

But a clarifying statement from the EPA said its conclusions "are applicable to engines currently in use, which include many older engines." It said its conclusions may be re-evaluated as newer diesel engines replace more of the older ones now in use.

The new report provides more support for the EPA to begin enforcing a postponed late-1990s requirement that by 2007 diesel engine manufacturers reduce their products' emissions of soot by 90%, and cut smog-producing nitrogen oxides by 95%.

The regulation also requires oil refiners to remove 97% of the sulfur now in diesel fuel because it gums up engines' pollution control equipment.

Enforcement of the air-cleaning regulation was delayed when oil refiners sued the EPA to stop it, but last May a federal appeals court ruled the EPA could go ahead and the agency said it would do so, according to news reports.

In general, air pollution poses the greatest risk to those with pre-existing breathing problems such as asthma or heart disease, said said Michael Thun, MD, MPH, chief epidemiologist for the American Cancer Society.

About 90% of lung cancer is caused by active smoking, and responsibility for the remaining 10% is split among second-hand smoke, previous exposure to asbestos, exposure to radon gas, and other causes, including diesel exhaust, Thun said.

No one knows what portion of that shared 10% is due to diesel exhaust, but studies of diesel truck drivers, diesel mechanics, and others constantly exposed very directly to diesel exhaust found their risk increased by up to 50%, Thun said.

The risk for those exposed to less diesel exhaust than those directly working around it is likely to be lower, said Thun.

Exercising outdoors at certain times of the day and in places that minimize exposure to any kind of exhaust fumes is obviously desirable, said Thun.

But the bottom line is that lung cancer in non-smokers is a rare disease, said Thun.

"Diesel is an important contributor to particulate air pollution but the risk to individuals is very small for most Americans, and for most Americans, it will not affect their individual risk of lung cancer in any substantive way," said Thun.