Smog and soot levels have dropped significantly in Southern California over the last decade, but the Los Angeles region still has the highest levels of ozone nationwide, violating federal health standards an average of 137 days a year.
The city ranks second in the country, behind Bakersfield, Calif., for the highest year-round levels of toxic particles or soot, and fourth in the nation for the number of short-term spikes in soot pollution.
The rankings, part of the annual "State of the Air" report by the American Lung Association, are based on federal and state data, which show that more than 90 percent of Californians live in counties with unhealthful air.
Unlike parts of the East and Midwest, where coal-fired power plants are a primary source of toxic pollution, Southern California's chemical stew is the product of tailpipe emissions from cars and diesel pollution from trucks, trains and ships linked to the ports of Los Angeles and Long Beach. Oil refineries, manufacturing plants and residential wood burning also are significant contributors.
"There has been tremendous progress in California," said Jane Warner, president and chief executive of the association's California branch. But the level of air pollution in the state remains "a critical public health issue," she added. "It is not just a nuisance that burns your eyes or stings your throat."
Air pollution aggravates asthma, heart and lung disease and diabetes and can have a severe effect on children, stunting lung growth. Diesel emissions have been linked to cancer. According to the state Air Resources Board, 9,200 Californians die prematurely each year because of dirty air.
Over the last decade, the average number of high-ozone days has dropped 28 percent in the South Coast basin, which includes Orange County and the urban portions of Los Angeles, Riverside and San Bernardino counties. In the Central Valley, Bakersfield, Fresno and Sacramento also experienced their lowest ozone levels since the association's first report was published in 2000.
Nationally, 15 of the 25 most ozone-polluted metropolitan areas showed their lowest levels in a decade, and particle pollution dropped in 25 of the 27 most sooty cities.
Lung association officials acknowledged that some of the drop registered in this year's report, which averages results from 2007 through 2009, may be the result of the economic downturn. Southern California ports experienced a steep drop in tonnage in 2007 and 2008.
But Janice Nolen, the association's assistant vice president for national policy, noted that new emission-control equipment has been installed at power plants and new engine standards have been approved for diesel trucks, along with a program to replace the dirtiest diesel trucks with newer models. "With those changes, we expect much of the reductions in emissions are permanent," Nolen said.
Even with the recession, some areas grew more polluted. Bakersfield and Hanford, Calif., each had worse average year-round soot levels in 2007-09 than in the previous three-year period. Truck and farm equipment emissions, along with winter wood-burning, were major factors, but severe wildfires, which blanketed the state with smoke from burning trees, also played a part.
If Southern California is ever to have consistently clean air, "we need to take dramatic new steps," said Bonnie Holmes-Gen, the association's senior policy director in California. "At this point, anything that's easy has already been done. ... We need to transition away from petroleum fuel to plug-in electric vehicles and redesign cities around public transit, biking and walking."
Nationally, the lung association said more than 154 million people - over half the population - live in areas with dirty air.
The report comes as Republicans in Congress are seeking sharp cuts in the Environmental Protection Agency budget; a rollback of proposed limits on mercury, arsenic and other toxic emissions from coal-fired power plants; and an easing of proposed rules to toughen ozone and soot standards.
"These are perilous times," said Charles D. Connor, president and chief executive of the association. "Despite tremendous gains, the Clean Air Act is under attack from the polluters lobby."
Under the George W. Bush administration, the EPA ignored its science advisers, setting health standards for ozone and particulates that were successfully challenged in court. The agency is expected to issue stricter standards this year that will throw even more cities and counties out of compliance, and increase pressure for further pollution controls.
Explore further: Miscanthus-based ethanol boasts bigger environmental benefits, higher profits