WASHINGTON — The head of the EPA stood firm Thursday against a chorus of congressional criticism over his refusal to allow California and more than a dozen other states to impose greenhouse gas reductions on cars and trucks.

"I am bound by the criteria in the Clean Air Act, not people's opinions," EPA Administrator Stephen L. Johnson testified to the Senate's environmental panel. It was his first congressional appearance since issuing the controversial waiver denial last month.

"The Clean Air Act does not require me to rubberstamp waiver decisions," Johnson said. "It was my conclusion that California didn't meet the criteria, or at least all of the criteria."

Sen. Barbara Boxer, D-Calif., the committee chair, led committee Democrats in assailing Johnson's conclusion.

"You're going against your own agency's mission and you're fulfilling the mission of some special interests," she chided him.

California needs a federal waiver under the Clean Air Act to implement its first-in-the-nation tailpipe law, which would force automakers to cut greenhouse gas emissions by 30 percent in new cars and light trucks by 2016.

If California got the waiver other states could then impose the same rules. Twelve other states have already adopted them — Connecticut, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington — with others preparing to do so.

California and other states sued EPA earlier this month over Johnson's decision.

The EPA chief disputed Democratic suggestions that his decision was made under political pressure from the White House.

"I was not directed by anyone to make the decision, this was my decision," Johnson insisted.

He reiterated his position that it's better to have a single national standard for greenhouse gas emissions than different standards in different states. Congress' newly passed fuel efficiency law, signed by President Bush last month, provides such a national standard, he said.

Environmentalists contend that California's law is much stronger and takes effect much faster than the new federal rules.