Washington ? Counties in 31 states are flunking air-quality standards, drawing a federal warning to clean up industrial plants, put new restrictions on cars and take other action to make their air less polluted.

Nearly 500 counties, mostly in California and the eastern third of the country, were cited Thursday as having too much smog-causing pollution in violation of the federal clean air law.

The Environmental Protection Agency told state and local officials to develop new pollution controls to reduce ground-level ozone, a precursor of smog. Some 159 million people, about half the U.S. population, live in areas singled out by the government for contributing to unhealthy air.

Acting under court order, the EPA identified all or parts of 474 counties that either have air that is too dirty or have pollution that causes neighboring counties to fail the air quality test.

Despite having some of the toughest air pollution requirements, California still has the worst air, the EPA said.

The Los Angeles basin was designated as having severe air pollution, the only one in the category. The area has until 2021 to come into compliance with the federal standard.

Three California regions — Riverside County, San Joaquin Valley, Sacramento — were listed with serious pollution, the fourth-worst designation, and given until 2013 to curtail the pollution.

Other areas with marginal or moderate pollution problems have until either 2007 to 2010 to comply. Areas that continue to violate the standard could lose federal highway dollars.