Federal scientists want to tighten smog standards, a step that would allow tens of millions of Americans to breathe easier but also would clash with President Bush's plan to wean Americans away from gasoline.
More than half the nation, or nearly 160 million people, breathe illegal levels of smog, mostly in and around major cities in California and the East.
Scientists with the Environmental Protection Agency are due to recommend by Wednesday a range of options for healthier air. Last year, EPA identified hundreds of the nation's most populated counties that were polluting the air with too much smog, and ordered them to clean it up.
What the scientists will recommend has stirred "a great deal of controversy" within EPA and could complicate Bush's push for more ethanol use, said a senior government official speaking on condition of anonymity because the announcement had not yet been made.
In his State of the Union speech, Bush called on Americans to reduce gasoline consumption by 20 percent over 10 years by substituting alternative fuels, mainly smog-causing ethanol, in its place.
Despite Bush's goal, the EPA scientists will recommend allowing less smog, which is produced mainly when tailpipe and smokestack pollutants react with summer heat, the official said.
The EPA administrator, Stephen Johnson, has until mid-June to decide what to do with the recommendation. Last year, more than a dozen states and several environmental groups filed a suit alleging that scientists' recommendations for lower soot levels from smokestacks and exhaust pipes had been ignored. The fine particles contribute to premature deaths and respiratory illness.
Ethanol, a focus of Bush's gasoline-reduction plan, helps cut carbon monoxide in winter. Yet it also can raise smog levels in summer, air pollution experts say. Ethanol releases more nitrogen oxides, a key element of smog, and evaporates more easily than gasoline, adding other air pollutants.
Agency documents show that more ethanol use could raise smog levels less than 1 percent, mainly in parts of the Midwest that don't use cleaner-burning reformulated gasoline to offset the volatility in ethanol, which produces smog.
"If you're a state air pollution official trying to lower the smog, that's not helpful," said A. Blakeman Early, a lobbyist for the American Lung Association, whose legal battle with the EPA over air quality standards forced Wednesday's deadline. "The data we have is pretty thin. We need to look at this question much more carefully."