The EPA report concludes that uncertainties remain about long-term health effects of exposure to diesel exhausts. It said, however, that studies involving both tests on animals and occupational exposure suggest strong evidence of a cancer risk to humans.
"Overall, the evidence for a potential cancer hazard to humans resulting from chronic inhalation exposure to (diesel emissions) is persuasive," said the health impact report released by the EPA.
The report mirrors conclusions made previously in documents from various world health agencies and studies in California and is particularly significant because the EPA is the federal agency that regulates diesel emissions under the Clean Air Act.
Some environmentalists have expressed worries recently that the Bush administration might have been backing away from a Clinton-era regulation that would establish tougher requirements on emissions from large trucks and a separate rule that virtually would eliminate sulfur from diesel fuel.
EPA Administrator Christie Whitman repeatedly has promised to go ahead with the tougher diesel rules. Last month, with White House approval, the EPA rebuffed attempts by some diesel engine manufacturers to postpone the requirements, approving new penalties against manufacturers who fail to meet an October deadline for making cleaner-burning truck engines.
The engine rule does not effect emissions from trucks already on the road, although the separate regulation cutting the amount of sulfur in diesel fuel is expected to produce pollution reductions.
Allen Schaeffer, executive director of the industry group Diesel Technology Forum, said the EPA's report "focused on the past," whereas "the future is clean diesel: Diesel trucks and buses built today are more than eight times cleaner than just a dozen years ago."
In an interview with CBS News Radio, Schaeffer also noted that in measuring air pollution, diesel engines account for 7 percent of the overall quantity of fine particles in the air.
"In the period from 1990 to 2000," Schaeffer added, "levels of particulates in the atmosphere from diesel engines of all kinds dropped by 37 percent."
The EPA's 651-page diesel health assessment report had been awaited by environmentalists, health advocates and state air quality regulators who have been pushing for diesel emission reductions.
The report reiterated that environmental exposure to diesel exhausts poses "a chronic respiratory hazard to humans" in the long term including increased asthma and other respiratory problems. In some urban areas diesel exhausts account for as much as a quarter of the airborne microscopic soot, the report said.
As for cancer, the report noted occupational health studies and tests on animals that showed diesel emissions to be a carcinogen, a cancer-causing substance. While there remain uncertainties, the report continued, "it is reasonable to presume that the hazard extends to environmental exposure levels" as well.
"The overall evidence for potential human health effects of diesel exhausts is persuasive," the report said.
"This assessment concludes that (diesel exhaust) is likely to be carcinogenic to humans by inhalation, and that this hazard applies to environmental exposure ... based on the totality of evidence from human, animal and other supporting studies," said the report.
Environmentalists welcomed the study as clear evidence that pollution needs to be curtailed not only from large trucks but also from off-road diesel-powered vehicles. EPA spokeswoman Steffanie Bell said the agency expects to publish a rule early next year dealing with those diesel exhaust sources, which include farm tractors and construction equipment.
Emily Figdor of the U.S. Public Interest Research Group, a private environmental organization, said: "To reduce the public's exposure to harmful diesel emissions, the Bush administration should ... fully implement clean air standards for diesel trucks and buses and should pass equivalent standards for diesel construction and farm equipment."
Figdor noted that the report is surfacing just as children across the country are returning to schools, many in diesel-powered buses. "Children riding buses back to school ... need stronger protection against the health impacts of diesel exhaust," she said.