"We will see a sea of clean-fuel vehicles in California and around the country" if the rules are passed, said Gail Ruderman Feuer of the Natural Resources Defense Council.

Under the plan, believed to be the first of its kind in the nation, every new vehicle purchased by public fleet operators will have to run on alternative fuels - natural gas, methanol, electricity or fuel cells.

The rules are the first phase of a change, to take effect over two years, that could ultimately affect 120,000 public and private vehicles in Southern California, from airport taxis to school buses.

Advocates say it will give a boost to the alternative-fuel industry and force auto makers to build more vehicles operating on such fuel.

Detroit Diesel Corp., which builds about a third of the engines for heavy-duty trucks in the U.S., already manufactures compressed natural gas engines, said John Duerr, manager of regulatory activities.

He said the Southern California plan, while important, would probably not force Detroit Diesel and other manufacturers to change direction, since many are already building engines for alternative fuels.

The regulation will cover government agencies from the city to the federal level, universities and transit agencies in Los Angeles, Orange, Riverside and San Bernardino counties, including 3,700 transit buses and 7,200 garbage trucks, said Bill Kelly, a spokesman for the South Coast Air Quality Management District, which devised the plan.

Only newly purchased vehicles would be affected. Fleets could continue to operate existing diesel buses and trucks.

The rules would also require public agencies in Southern California to buy passenger cars with the lowest emissions on the market, eventually covering 86,000 vehicles.

School buses are not included in this first stage, but the air quality agency is expected to deal with them in August. Other proposals will target heavy-duty public vehicles, street sweepers and taxis and shuttles that serve airports.

California's smog problems led Congress to make the state the only one that can set air-quality standards more stringent than Washington's. But once California sets its stricter standards, other states are free to adopt them, too, and some have done so in recent years.

Environmental groups and the staff of the air-quality agency say the proposal is a step toward getting the state off diesel fuel, which an agency study found is responsible for more than 70% of toxic substances in the air of the smog-plagued Los Angeles basin. One of every 1,000 cancer cases in the region can be attributed to diesel toxins, according to the study.

The higher cost and limited availability of the fuels have some worried, though. And the petroleum industry said there should be a place for cleaner-burning diesel in the new rules.

"The basic question of 'Why can't we meet your standards with our fuel?' is one that begs for an answer," said Scott Macdonald, a spokesman for the oil industry-financed South Coast Clean Air Partnership.

Mr. Macdonald said natural gas has its own environmental problems, producing more greenhouse gases than diesel.

Making the switch to natural gas isn't as difficult as some operators might think, said Alberto Guardado, operations manager for Waste Management's Palm Desert fleet, which has used garbage trucks with compressed natural gas for about four years.

Richard Cromwell III, general manager of Sunline Transit Agency in Thousand Palms, estimates he will save $200,000 a year by converting his 100-vehicle fleet of buses and other vehicles to compressed natural gas. The natural gas buses cost about $350,000, about $50,000 more than their diesel equivalents, he said. But savings kick in because the fuel is cheaper and the buses require less maintenance, he said.

-- (The Wall Street Journal)