The EPA approved regulation amendments adopted by the California Air Resources Board in 2003 that allow manufacturers to produce fuel cells as an alternative to the battery-powered cars and light trucks previously required by the state.

Since then 10 other states -- Connecticut, Maine, Massachusetts, New York, New Jersey, Oregon, Pennsylvania, Rhode Island, Vermont and Washington -- have adopted the same rules.

"This waiver simply reflects the prominence of fuel cells," said John Millett, an EPA spokesman. "Fuel cells have really taken off."

California initially adopted its regulations in 1990, requiring by 2003 that 10 percent of the new cars sold in the state by major manufacturers be zero-emission vehicles.

The rules have been modified several times since then. Currently, they call for 2 percent of the six biggest automakers' new cars to be zero-emission vehicles, 2 percent to be gasoline-electric hybrids and 6 percent to be super-low-polluting gasoline-powered vehicles known as PZEV's.

The hybrid and PZEV requirements, which some smaller companies must also meet, kicked in in 2005.

The 2 percent requirement for fuel cell or battery-powered cars starts in 2009 with a ramp up period that will require the industry to market at least 2,500 of the vehicles nationwide over the first three years of the program and larger numbers in subsequent years, said Jerry Martin, an ARB spokesman.

California officials expect credits earned by the car companies by producing zero-emission vehicles before 2009 to fill out the rest of the 2 percent requirement during the first few years of the program, Martin added.

The programs allows manufacturers to produce either battery-powered or fuel-cell vehicles, which use hydrogen and oxygen to run an electric motor. But Martins said he doubts that companies will opt for battery-powered cars.

"Battery technology has been moving forward and batteries are still zero-emission technology, but the car companies have made it very clear that fuel cells are the technology of the future," he said.

A spokesman for General Motors, Dave Barthmuss, said fuel cell vehicles were "very viable to be a portion of any automakers' compliance strategy."

"A lot of milestones are being met and a lot of progress is really being made" in developing the vehicles, he added.

General Motors plans to put 100 fuel-cell vehicles on the road next year as a demonstration project, he said.

But Barthmuss also said the ARB should be willing to review its requirements so they stay in line with the "pace of technology."

Jennifer Moore, a spokeswoman for Ford Motor Co., said there is still a lot of uncertainty about fuel-cell vehicles and that her company agrees with a U.S. Department of Energy forecast that predicts the vehicles won't be available in large numbers before 2015.

"There are a lot of challenges that remain ahead for fuel-cell vehicles, everything from infrastructure to cost to range," she said. "In terms of when they're going to be commercially viable, it's pretty difficult to say at this point."