Southern Californians are among those at highest risk of death due to air pollution, according to recent U.S. Environmental Protection Agency research published in the journal Risk Analysis.

The study, published last month, was conducted to “provide insight to the size and location of public health risks associated with recent levels of fine particles and ozone, allowing decision-makers to better target air quality policies,” the federal agency said in a statement responding to California Watch inquiries.

“While overall levels of fine particles and ozone have declined significantly in the past two decades, these two pollutants still pose a burden to public health,” the EPA statement said.

The study examined air pollution exposure based on 2005 air quality levels and projected there could be between 130,000 and 360,000 premature deaths among adults in coming years. The 2005 data was the best available for analyzing fine particulates and ozone, the EPA said. Among vulnerable populations like children, the EPA also estimates that fine particulate matter and ozone results in millions of cases of respiratory symptoms, asthma and school absences, as well as hundreds of thousands of cases of acute bronchitis and emergency room visits.

The analysis also found that Southern Californians and residents of the industrial Midwest experience the highest exposure to fine particulate matter, which has been found to exacerbate respiratory illnesses and increase heart attacks, according to the Centers for Disease Control and Prevention.

Among the most populated areas of the country, Los Angeles had the highest estimated rate of deaths attributable to air pollution, at nearly 10 percent; San Jose had the lowest at 3.5 percent.

The Bay Area Air Quality Management District conducted a similar risk assessment last year and found that about 1,700 premature deaths can be attributed to fine particulate matter in the Bay Area each year, which is about 3.8 percent of all deaths.

Particulate matter is made up of extremely small particles and liquid droplets that are 2.5 micrometers in diameter or smaller – which means they have a width 30 times smaller than a human hair. Common sources of fine particulate matter, often referred to as PM 2.5, are forest fires and emissions from power plants, industrial sources and cars. Unhealthy forms of ozone are created when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight; ozone is typically linked to byproducts from industrial facilities and electric utilities, car exhaust, gas vapors and chemical solvents.

Local air districts in Southern California and the Bay Area have attempted to limit fine particulate matter and ozone emissions through Spare the Air days by regulating wood burning and offering financial incentives to businesses to phase out the use of diesel engines.

Public health advocates say that the EPA study illustrates the importance of improving air quality and that these types of studies on the risks of air pollution have been used to determine federal regulations and inform local clean air plans.

“One of the hardest things to explain to the public is that while the air is cleaner, we continue to find that we have underestimated the health effects of breathing in air pollution,” said Joe Lyou, president and CEO of the Coalition for Clean Air and a governing board member of the South Coast Air Quality Management District. “Yes, we have made significant accomplishments, but we still have a long way to go. The public needs to understand that this is a life-and-death situation.”

The EPA’s research on air pollution and mortality have, however, been the subject of political and scientific debate.

James Enstrom, a researcher with UCLA’s School of Public Health, argues that while there is a connection between air quality and health effects, the EPA study fails to acknowledge regional nuances when it comes to the real risks of premature deaths.

“The question is whether there is enough epidemiological evidence to conclude that air pollution kills people,” Enstrom said. “Every piece of evidence for the state of California as a whole shows that there’s no effect (on mortality). There’s some effect in the Los Angeles basin, but that’s not a fair representation of absolute risk.”

Enstrom, who in the past has received research funding from industries opposed to stricter air quality regulations, said the costs of these regulations are “only justified if it’s killing people.” “The other morbidities associated with (air pollution) are lung problems, hospitalizations, asthma, and those don’t amount to enough to affect the cost-benefit ratios,” he said.

In a November letter to the Office of Management and Budget, U.S. Reps. Andy Harris, R-Md., and Paul Broun, R-Ga., both physicians, also challenged the agency’s “troubling scientific and economic accounting practices” that “appear designed to provide political cover for a more stringent regulatory agenda rather than to objectively inform policy decisions.”

But Dan Farber, a UC Berkeley law professor and co-director of the university’s Center for Law, Energy & the Environment, said the debates over the EPA’s air quality findings are ultimately political.

“There is strong industry opposition to these regulations and strong opposition from groups who are ideologically opposed to regulation in general,” Farber wrote in an e-mail. “EPA’s most important role in terms of economic impact and public health relates to air pollution. So it’s not surprising that this is the area where EPA is being attacked.”