The black fumes emitted by diesel-fueled buses and trucks probably cause cancer in humans, according to an Environmental Protection Agency study released yesterday.

Although earlier studies have linked diesel fumes to cancer, the EPA study is the most authoritative confirmation yet of health dangers from the smoke that many city dwellers consider the most irritating and visible form of pollution.

The assessment is expected to fuel debate in the House-Senate conference on Clean Air Act revisions, where provisions approved by each chamber seek to curb emissions of diesel trucks, buses and construction equipment.

Fewer than 2 percent of U.S. passenger cars run on diesel fuel. But most of the hundreds of thousands of heavy trucks and buses sold yearly are equipped with the durable, efficient diesel engines. Up to 20 percent of highway vehicles are diesel.

Although Congress required the EPA in 1977 to control toxic particles emitted by diesel-powered cars, rules proposed for trucks and buses were shelved by the Reagan administration until 1988. The EPA only drafted regulations after losing a federal court suit brought by environmentalists.

The EPA first classified diesel particulates as a carcinogen in 1983 by comparing the compounds of diesel exhaust to other combustion products known to cause cancer.

Diesel fuel exhaust contains tiny particles that absorb sulfuric acids and cancer-causing volatile organic compounds, and burrow deep into the lungs.

"Eventually lungs clear out the particle but leave behind the other substances," said Jeanette Wiltse, deputy director of the EPA's health assessment office.

In yesterday's study by EPA's Office of Health and Environmental Assessment, scientists pulled together more recent laboratory studies on animals and surveys of workers exposed to diesel particulates. The findings are preliminary and subject to review by outside experts.

According to the study, high levels of exposure to the fumes can cause a series of temporary acute symptoms, including headaches, eye irritation, nausea, vomiting, numbness, wheezing, heartburn and chest tightness.

In the long term, cancer is the greatest threat, the report concluded.

Laboratory animals forced to inhale diesel particulate exhaust developed lung tumors, the study said. Application of the pollutant to the skin also produced tumors.

Worker studies showed a significantly increased incidence of liver cancer among heavy equipment operators, lung cancer among truck drivers and transport equipment operators and bladder cancer among railroad workers, according to the EPA survey.

Both houses of Congress have passed bills with tougher provisions for diesel-fueled buses, the most graphic type of air pollution in many inner cities. Both bills would phase in low-polluting fuels to replace diesel, with the House proposing complete conversion by 1996 and the Senate by 1994.

Both houses also seek to curb pollution from so-called "off-highway" vehicles, including construction and farm equipment. Although the House provision would give the EPA discretion to set standards after conducting a study, the Senate calls on the agency to estimate the pollution from off-highway vehicles and draft emission controls for significant sources.