The study found several constituents, such as elemental carbon, organic carbon, nitrates potassium and some metals were associated with all causes of mortality. A stronger association emerged with cardiovascular disease mortality, and among those older than 65.

Overall, pollution from motor vehicles and other kinds of combustion, like wood-burning, are of particular concern, the researchers noted.

"We can't begin to regulate particles unless we know what sources are toxic," said lead author Bart Ostro, chief of air pollution epidemiology at the Office of Environmental Health Hazard Assessment at the California Environmental Protection Agency.

Ostro and colleagues gathered data on daily mortality for all California residents from state agencies and matched this with data on PM 2.5 components between 2000 and 2003. The researchers collected pollutant data in six California counties, totaling 8.7 million residents, and took readings every third day; in 2003 they took data either every third or every sixth day.

Particulate matter is a mixture of exceedingly small particles, including dust and droplets of liquid in the air, that are created mostly by the burning of fossil fuels or combustion.

The particles can travel into the deepest areas of the lungs when inhaled, and exposure is linked to several health conditions, from aggravated asthma to premature death in people with heart and lung disease, according to the Environmental Protection Agency.

Studies have in recent years targeted PM 2.5, the smallest of the types of particulate matter and the most detrimental to health.

The sources of PM 2.5 pinpointed by Ostro and colleagues included diesel exhaust, wood smoke and secondary pollutants.

People often burn wood in woodstoves in northern California during winter months, Ostro said, and due to the state's hilly topography, much of the pollution settles and concentrates in valleys, exacerbating health effects.

Yet the biggest contributor is still motor vehicles; 80 percent of elemental carbon, for instance, comes from diesel trucks, buses and cars.

Although the makeup of PM 2.5 differs depending on location -- for instance, the East Coast has more sulfates because of its heavy coal-burning -- some core constituents, such as nitrates, elemental carbon and organic carbon, are common everywhere.

By looking closely at PM 2.5 constituents, such as elemental carbon, experts can determine health effects of the pollution better than studying PM 2.5 generically. Knowing how to control diesel particles and elemental carbon can better inform control strategies for pollution, Ostro said.

Then, state or federal agencies could apply targeted solutions to pollution control -- leading to "significant benefits" in public health and the economy, Ostro said.

The research will likely aid California in reaching its ambitious emissions reductions goals. In 2005 the state's Air Resources Board vowed to reduce diesel PM emissions by 75 percent. Gov. Schwarzenegger also recently announced a plan to reduce the amount of carbon, a greenhouse gas, in transportation fuels sold in the state.

Such decisions reflect California's aggressive mission to cut 25 percent of its greenhouse gas emissions by 2020.

In September 2006 the EPA also tightened the 24-hour PM 2.5 standard by nearly 50 percent, from 65 micrograms per cubic meter to 35 micrograms, which is the limit of a person's daily exposure to the pollutant.

The agency decided to preserve the existing PM 2.5 standard of 15 micrograms per cubic meter, originally set in 1997.

Ostro's study did have limitations -- for instance, the sample size of mortality data was relatively small. Collecting the pollution data every third day may have also made the data a bit unstable. More importantly, Ostro said, the researchers found it hard to tell whether a health effect -- e.g., cardiovascular mortality -- was caused by the toxicity of a specific element -- say, organic carbon -- or if something else was to blame.

There could be another kind of unstudied organic material that's acting as a surrogate to bring about the toxic effects, Ostro said.

Indeed, linking exact health effects to sources remains a complicated task, and one that PM researchers struggle with continually, said Philip K. Hopke, a professor of chemical and biological engineering at Clarkson University in Potsdam, N.Y.

PM particles are a very complicated mix of reactive materials, unlike ozone, which is made of a single, well-defined compound. PM particles may have relatively short lifetimes, and they may be hard to sample and take back to a laboratory.

The study has "limited power" and doesn't "tell us anything new," said Hopke, but it does reinforce a direction many epidemiological studies have taken in recent years.

Keith Rodgers, a researcher in the environmental biosciences program at Medical University of South Carolina in Charleston, said the study contributes nicely to the literature, and he found the association with cardiovascular deaths "intriguing and worth further investigation."

Although the exact causes of PM 2.5 health effects remain complex, "we can be confident these effects are real," Ostro said.

Related Links
Environmental Health Perspectives
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