Pittsburgh had the sixth-worst air pollution of all U.S. metropolitan areas over the last three years, according to a report released this week by the Surface Transportation Policy Project, an environmental coalition.

During the three years from 2000 through 2002, Pittsburgh had 134 days of unhealthy air, based on the U.S. Environmental Protection Agency's daily air quality index, which tracks levels of five primary pollutants.

The Riverside-San Bernardino, Calif., area had the worst air pollution, the report said, with 445 days of unhealthy air during 2000-02. The other cities in the top five also were in California: Fresno, Bakersfield, Los Angeles-Long Beach and Sacramento.

In Pittsburgh, "the quality of air has not improved in the last 10 years," said David Ginns, a transportation specialist with the project who is also associated with Sustainable Pittsburgh, a local public policy group.

Nationwide, nearly half of all Americans -- 133 million people -- are breathing unhealthy air, the report says.

Emissions from cars and trucks are a major contributor to air pollution in Pittsburgh, said Guillermo Cole, spokesman for the Allegheny County Health Department. But local industrial pollution and pollutants that blow in from areas upwind of Pittsburgh also contribute to the unhealthy air here, he said.

Cole said fine dust particles -- known as PM 2.5 because they measure 2.5 micrometers or less in diameter -- are most responsible for Pittsburgh's poor air quality.

"We had known all along that PM 2.5 would be a challenge for our area," Cole said. New standards for emissions of PM 2.5 particles will be implemented nationally, beginning this fall, he noted. Once the new, stricter standards are in place, polluters who violate them will be subject to enforcement actions.

The highest particulate pollution level was recorded in southeastern Allegheny County, in Liberty.

Cole said additional pollution control measures at a regional level will be needed to address the problem.

"The broader picture is that we are driving more," Ginns said. "We need clean fuel and there needs to be more transportation choices such as ... more bicycle paths."

Based on a Federal Highway Administration formula that takes into account the number of miles driven in the city, transportation-related public health costs from air pollution in Pittsburgh were $227 million in 2001, the report said.

The unhealthy air contributes to respiratory diseases like asthma, heart disease and certain cancers, the report said. As of last year, about one out of every 10 adults in Pittsburgh had been diagnosed with asthma at some point, it said.

In a statement issued Tuesday, the Surface Transportation Policy Project, the American Lung Association and several other organizations said efforts to improve air quality are being hampered by some members of Congress who "undermine clean air protections and cut funding for transportation alternatives like transit, rail and buses that reduce traffic and air pollution."