"While we know that air pollution is not the dominant cause of atherosclerotic diseases, these results are consistent with findings that air pollution provokes inflammation, accelerates atherosclerosis, and alters cardiac function," said lead author C. Arden Pope III, Ph.D., an epidemiologist at Brigham Young University in Provo, Utah.
"We might be able to reduce the underlying processes of some cardiovascular disease just by reducing the exposure to air pollutants. And possibly, there may be ways to mitigate the impacts of air pollution, such as anti-inflammatory medications or other interventions."
The research, which used data on more than half a million people, also poses a theory on the mechanisms responsible for pollution-related heart deaths -- increased inflammation and nervous system changes that affect heart rhythm.
The researchers found that about 45 percent of all deaths in the study population were due to cardiovascular diseases -- including heart attacks, heart failure and cardiac arrest. Respiratory disease accounted for only 8.2 percent of the deaths.
Pope and his colleagues used risk factor and mortality data collected between 1982 and 1998 by the American Cancer Society as part of the Cancer Prevention Study II (ACS-CPSII). The ongoing study included participants, aged 30 or older, living in U.S. metropolitan areas with available air pollution data.
The ACS-CPSII data were matched with air pollution data from up to 156 cities, mostly gathered by the U.S. Environmental Protection Agency. The analyses focused on microscopic particles in the air that measured less than 2.5 micrometers in diameter called PM2.5, which is also the size range of particles in cigarette smoke.
Over the 16-year period, 22.5 percent of the people taking part in ACS-CPSII died. An analysis of those deaths, published in 2002, confirmed a clear association between air pollution and an increase in death rates.
An increase in PM2.5 of 10ug per cubic meter of air (ug/m3) increased the risk of all cardiovascular diseases plus diabetes by 12 percent. For ischemic heart disease alone (the majority of which will lead to heart attack), the risk was 18 percent higher. For the combined category of arrhythmias (irregular heart rhythms), heart failure and cardiac arrest, the risk was 13 percent higher.
In a sub-analysis, the researchers found that former smokers had a 26 percent greater risk of death due to cardiovascular diseases and diabetes compared to never smokers. Current smokers had a 94 percent higher risk of death. Air pollution conferred additional risk for smokers as well as nonsmokers. For respiratory disease, an increase in PM2.5 conferred twice the risk of death for former smokers compared to never smokers. The risk for current smokers was nearly four times greater. However, for respiratory disease deaths, researchers did not find additional risk from air pollution, Pope said.
"There is no question that active cigarette smoking is a much, much larger risk factor than air pollution," said Pope, an expert in the economic effects of disease. "We don't know precisely how the two relate, but there is some evidence in our study that the mechanisms are similar or complementary."
Co-authors are Richard T. Burnett, Ph.D.; George D. Thurston, Sc. D.; Michael J. Thun, M.D.; Eugenia E. Calle, Ph.D.; Daniel Krewski, Ph.D.; and John J. Godleski, M.D.
The National Institutes of Health funded the study.
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