LOS ANGELES - California has the worst rate of soot pollution in the United States, according to a report released Thursday by an environmental group.

The Riverside-San Bernardino-Ontario area ranked as the nation's top offender for soot pollution among large metropolitan areas. Pittsburgh ranked second, followed by the Los Angeles-Orange County area. Atlanta ranked fourth.

Soot pollution can lead to serious respiratory and cardiovascular problems, including heart attacks, strokes and asthma attacks, according to the American Lung Association.

The report blames California's power plants and large numbers of diesel trucks, cars and ships for air pollution. Although the state has tightened environmental regulations, a growing population may be contributing to more pollution.

The metropolitan areas were ranked by micrograms of pollution per cubic meter, or soot in the air. Soot particles are often released from fossil fuel combustion and emissions from coal-fired power plants or factories.