Busing away particulates.

"We have received more complaints about the huge, black billows of smoke from buses than any other issue relating to vehicles," said EPA Administrator William K. Reilly at a press conference last week. To address the problem, his agency has proposed a package of new rules. Beginning with the 1993 model year, bus manufacturers would have to reduce the vehicles' particulate emissions by 60 percent -- to 0.1 gram per brake-horsepower-hour. In 1994 this standard would drop by another 50 percent, to roughly 90 percent of uncontrolled levels. And for the first time, EPA would require installation of particulate controls on existing buses in large cities, which provide roughly 80 percent of the nation's bus service.

Other new controls on heavy-duty, diesel-fueled vehicles are due to kick in with the 1994 model year. Those controls, combined with the rules proposed this month, should reduce particulate emissions from diesel vehicles by roughly 86 percent, according to EPA data. Such vehicles currently contribute 16 percent of all airborne particulates in the United States.

EPA considers diesel particulates -- microscopic particles that can lodge deep within the lung -- "a probable human carcinogen."