Air pollution may trigger heart attack in at-risk patients

provided by Beth Israel Deaconess Medical Center

rief exposure to elevated levels of fine particulate air pollution as little as two hours may temporarily increase the risk of heart attack, particularly among people already at risk for heart disease, according to a study led by researchers at Beth Israel Deaconess Medical Center. The Boston-based epidemiological study is the first of its kind to document the link between short-term exposure to air pollution and heart attack. The findings are published in the June 12 Circulation, the research journal of the American Heart Association.

    “Our findings suggest that people who have heart disease or an elevated risk of heart attack would be well-served to avoid being outdoors for extensive periods of time when the air quality is poor,” says Murray Mittleman, MD, PhD, director of cardiovascular epidemiology at Beth Israel Deaconess. “This is especially the case on the hot, hazy days of summer when the problem is much more prominent.”

    The researchers conducted medical history interviews with 722 people shortly after they were admitted to Boston area hospitals for a heart attack. Forty seven percent of the participants were between 50 and 69 years of age and 63 percent were male. Nearly one third had a previous heart attack, 41 percent were hypertensive, 19 percent had diabetes mellitus, 34 percent were obese, and 32 percent were current smokers.

    The researchers also concurrently measured for concentrations of several air pollutants at several Boston area sites, including fine particulates (less than 2.5 microns in mass), carbon black, ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide as well as measurements of relative humidity and temperature.

    An analysis of the data showed that the study participants had nearly a 50 percent increase in heart attack risk in the two hours following exposure to high levels of fine particles. The researchers also observed a delayed response associated with an increased risk 24 hours after exposure to high levels of fine particles. Positive associations were identified between concentration levels of other pollutants and the onset of heart attacks, but the findings were not statistically significant.

    Auto emissions and the by-products of urban industry are responsible for the bulk of fine particulate air pollutants in the United States.

    Previous studies have shown that long-term exposure to air particulates can initiate a chain of events that trigger a heart attack in individuals with cardiovascular disease or cardiovascular risk factors. The presence of the tiny, invisible pollutants inside the lung's air sacs, the alveoli, can cause an inflammatory response beginning within the lungs and triggering a systemic inflammatory response that can be detected as high levels of C-reactive protein, which has been associated with an increase in heart attack risk. Exposure to high levels of air pollution also turns on the clotting system, increasing the likelihood of clot formation and restricted blood flow. In elderly people, air pollution has been linked to accelerated heart rates and diminished heart rate variability.

    This study, which is part of the Determinants of Myocardial Infarction Onset-Study, utilized a novel case-crossover design to eliminate the confounding effects of chronic conditions, using each study participant as his or her own case control. Air pollution measurements in the hours before the patients' heart attack symptoms began were compared with measurements taken during control periods in the days before. Other immediate triggers of heart attack discovered by the eight-year multi-center study include cocaine, exercise, sex, and certain psychological states.

    Mittleman, who also is an assistant professor at Harvard Medical School and Harvard School of Public Health, says that more research is needed to determine how a person's exposure to high levels of fine particulate air pollution can lead to a heart attack. Understanding that triggering mechanism, he believes, could spur the development of new drugs that could protect individuals during peak exposure to air pollution, such as they would experience during rush hour traffic or outside on a hot, hazy summer day.

    The paper's other authors are Annette Peters, PhD, Harvard School of Public Health and the GSF-National Research Center for Environment and Health in Nueherberg, Germany, Douglas W. Dockery, ScD, Harvard School of Public Health, and James E. Muller, MD, Massachusetts General Hospital.

    The study was supported by grants from the National Heart, Lung and Blood Institute and the American Heart Association.

    Beth Israel Deaconess Medical Center is a major patient care, research and teaching affiliate of Harvard Medical School and a founding member of CareGroup Healthcare System. Beth Israel Deaconess is the third largest recipient of National Institutes of Health research funding among independent US teaching hospitals.