TUESDAY, Feb. 14, 2012 (HealthDay News) -- Exposure to higher levels of air pollution for as little as a week may increase the risk of heart attacks, a new study finds.
Researchers in France conducted a meta-analysis, which combines the results of multiple studies, of 34 recent studies on air pollution and heart attack. The studies were done in the United States, Europe, China, Japan and other developed nations, and represented every continent except Africa, according to study author Dr. Hazrije Mustafic, a public health specialist and cardiologist at the University Paris Descartes.
Major air pollutants included in the analysis included ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide and particulate matter with a diameter of 10 micrometers or less (coarse particulate matter) and 2.5 micrometers or less (fine particulate matter).
With the exception of ozone, the analysis found an increased risk of heart attack in people exposed to higher levels of each pollutant during the week leading up to the heart attack.
The new research comes on the heels of a study released earlier this week that found exposure to air pollution over the long-term may contribute to mental decline in older women.
In the latest study, each unit increase in exposure carbon monoxide was associated with a 4.8 percent increased risk of heart attack. Each 10-unit increase in exposure to nitrogen dioxide was associated with a 1.1 percent increased risk of heart attack.
For sulfur dioxide, coarse particulate matter and fine particulate matter, those rates were 1 percent, 0.6 percent and 2.5 percent, respectively.
"We find that for the main pollutants except for ozone, the association with heart attack was significant," Mustafic said. "It means that when the concentration of air pollutants is higher, the relative risk of myocardial infarction (heart attack) occurrence is higher."
The study is published in the Feb. 15 issue of the Journal of the American Medical Association.
Although the percentage increase in risk is small compared to the increase in heart attack risk posed by other factors such as smoking and high blood pressure, the incremental increase in levels of air pollution exposure is well within the range of what millions of people are typically exposed to, researchers noted.
Air pollution experts typically measure carbon monoxide in units of 1 milligram per meter cubed increments; for the other contaminants, it's 10 micrograms per meter cubed.
Researchers saw the increased risk with each unit increase in exposure. In France alone, the additional risk for each incremental increase in pollution translates into an estimated 20,000 additional heart attacks a year, Mustafic said.
One reason for the lack of an association between ozone and heart attacks may be that studies on the subject did not take into account the fact that ozone tends to rise on warm days, while heart attacks tend to fall during warm weather months, which may have obscured any association, Mustafic noted.
Jennifer Weuve, an assistant professor at the Rush Institute for Healthy Aging in Chicago, said "this is a really nice piece of work that summarizes a huge body of research that's been going on for quite a long time."
And though researchers did not find an association between ozone levels and heart attacks, lots of prior research has found an association between ozone and respiratory disease.
"Although with ozone, the evidence is not as convincing as it is for the other pollutants, it doesn't mean you should count it out," Weuve said. "What the paper also doesn't rule out is the impact of long-term exposure."
The unit increases the researchers used are well within the range of what people are typically exposed to, she noted. "Exposures to these pollutants span from almost zero to 40 or 50 micrograms per cubic meter," she said.
She added: "If we were to kick down everyone's exposure by even one unit, we could prevent quite a few heart attacks."
Over the past several decades, a mountain of evidence has shown that air pollution is associated with respiratory problems, cardiovascular disease and higher deaths.
However, the association between air pollution and short-term risk of heart attacks has been unclear. Some studies have shown an association, while other studies have found either no association or association only for selected pollutants, according to background information in the article.
Although the exact mechanism underlying the heart attack-air pollution link is unknown, prior research has found that air pollution raises levels of inflammatory markers such as C-reactive protein, which are associated with heart attacks.
Other studies have found that air pollution may alter the autonomic nervous system, which controls heart rate. Studies have shown air pollution may lead to a more rapid heart rate and a decreased ability of the heart to respond to changing conditions, according to the study.
The third possible mechanism is that air pollution has been shown to increase the thickness of the blood, which might contribute to the formation of clots and accelerate atherosclerosis.
What's also unknown is what's the cumulative effect on the heart of all six of the most common pollutants. This paper analyzed them individually, but most people are exposed to more than one, Mustafic noted.
In the study published in the Feb. 13 issue of the Archives of Internal Medicine, Weuve and her colleagues at Rush found that women exposed to both fine and coarse particulate matter showed greater declines in memory and thinking skills than women exposed to less as they aged.
The U.S. Environmental Protection Agency has more on air pollution.