E.P.A. figures for 2004 to 2006 show there were 166 counties around the country that could have met a standard of 75 parts per billion but not 70. They include populous ones, like Miami-Dade in Florida, King County in Washington, and the Bronx in New York.

In all, 345 counties now violate the standard of 75 parts per billion. Bringing them into compliance would prevent 900 to 1,100 premature deaths a year, according to the agency, and result in 5,600 fewer hospital or emergency room visits. Reaching the standard will cost $8.8 billion a year, according to the agency, but the figure does not take account of health benefits.

Counties that cannot meet the standard face the threat of limits on new highways and industries. Technically the administrator set a standard on ground-level ozone, which is the main ingredient of smog. Ozone is formed by the action of sunlight on two kinds of pollutants, volatile organic compounds, which come from gasoline and various industrial processes, and nitrogen oxides, which are produced in cars and power plants. Ground-level ozone is also a greenhouse gas.

While many groups and elected officials complained, not all the complaints were in the same direction. The Edison Electric Institute, the trade association of the investor-owned utilities, said in a statement that the precise relationship between ozone and human health was still the subject of scientific dispute.

“E.P.A. is promising health benefits that people may never receive, even though they’ll end up paying for them at the pump and through higher energy bills,” said a statement from John Kinsman, the utility group’s senior director for the environment. “Hundreds of counties haven’t been able to meet the current standard set a decade ago, and moving the goal posts again will inflict economic hardship on these areas without speeding air quality improvement.”

But others said the Bush administration was setting its sights too low.

“It will be close to a decade from now before the first ounce of pollution is reduced by any industry,” said S. William Becker, executive director of the National Association of Clean Air Agencies, whose members are state and local officials.

“That decade is an eternity in terms of technological advancement,” Mr. Becker said.

The debate in coming days is likely to center on why Mr. Johnson chose a number outside the range recommended by his 23-member scientific advisory board.

Representative Henry A. Waxman, Democrat of California and head of the House Committee on Oversight and Government Reform, told Mr. Johnson in a letter that the decision was similar to one in 2006 to overrule a recommended standard on fine particles. The decisions “suggest that science is not the primary basis for your decisions,” he wrote, asking for an explanation.