“E.P.A. is stepping up to protect Americans from one of the most persistent and widespread pollutants we face,” Lisa P. Jackson, the agency’s administrator, said in a statement. “Smog in the air we breathe poses a very serious health threat, especially to children and individuals suffering from asthma and lung disease. It dirties our air, clouds our cities and drives up our health care costs across the country.”

Smog or ground-level ozone is not emitted by a single source, but is, according to the E.P.A., formed by a reaction of nitrogen oxides, volatile organic compounds, carbon monoxide and methane in the presence of sunlight. The main sources of these pollutants are power plants and factories, fumes from volatile solvents, vehicles emissions and gasoline vapors. Smog is worse in the summer because of heat and sunlight, and can travel hundreds of miles from its source and affect small towns, rural communities and wilderness areas.

The leader of an association of air-quality enforcement agencies welcomed the proposal.

“This is exactly what states and localities have advocated for 30 years,” said S. William Becker, executive director of the National Association of Clean Air Agencies. “This will not be easy to achieve, whichever number the E.P.A. ultimately chooses, but it’s a decision that will ensure that public health is protected with an adequate margin of safety.”

Mr. Becker also said that the projected costs of compliance were likely to be lower than the agency’s estimate. “And the benefits will likely trump the costs many times over,” he said.

The American Petroleum Institute, the oil companies’ chief lobby, criticized the proposal as costly and likely to be ineffective. The group said there was no new scientific basis for changing the standard set at the end of the Bush administration.

“To do so is an obvious politicization of the air-quality standard-setting process that could mean unnecessary energy cost increases, job losses and less domestic oil and natural gas development and energy security,” the group said in a statement issued minutes after the agency’s announcement.

The trade association for electric utilities, the Edison Electric Institute, reacted warily.

“We probably won’t know for a couple of years just what utilities and other emissions sources will be required to do in response to a tighter ozone standard,” said John Kinsman, the institute’s senior director for the environment. “States will have to cast a very wide net when targeting sources for emissions cuts, in part because utilities already have made substantial reductions in ozone-related emissions.”