Cardiovascular risks from fine particulate air pollution.

More than a decade ago, prospective epidemiologic studies showed that mortality was increased among people living in communities with elevated concentrations of fine particulate air pollution.1,2 Subsequent research has shown that particulate air pollution is statistically and mechanistically linked to increased cardiovascular disease.3 New data are beginning to shed light on which persons are at heightened risk. In this issue of the Journal, Miller et al.4 report on data from the Women’s Health Initiative (WHI) observational study, which greatly expands our understanding of how fine particulate pollution affects health. Earlier long-term prospective cohort studies showed an association between levels of air pollution consisting of particulate matter of less than 2.5 μm in aerodynamic diameter (PM2.5) and an elevated risk of death from all causes and from cardiovascular disease.1,2,5 The WHI study broadens the scope by finding that nonfatal cardiovascular events are also strongly associated with fine particulate concentrations in the community. Earlier work relied solely on death certificates to define the rate of death from cardiovascular disease. In the WHI study, cardiovascular events and mortality were defined by objective review of medical records. The earlier studies were designed to identify risk factors for respiratory disease1 and cancer2 and therefore had limited ability to adjust for cardiovascular risk factors. The WHI observational study was designed to assess the risk of cardiovascular events and therefore could exclude cardiovascular risk factors as explanations for the observed associations with air pollution. Earlier studies did not include data on the full range of regulated community air pollutants — that is, PM2.5 (and the larger particle fraction, PM10), sulfur dioxide, nitrogen dioxide, carbon monoxide, and ozone. The WHI study considered all of these community air pollutants and found cardiovascular risk associated only with PM2.5 concentrations. Whereas earlier work compared levels of air pollution and rates of death between various cities, the WHI investigators were also able to compare areas within individual cities. Their analysis demonstrated a relationship between increased levels of fine particulate pollution and higher rates of death and complications from cardiovascular and cerebrovascular disease, depending not only on which city a person lived in but also on where in that city she lived. Perhaps most important, the WHI study established a stronger statistical association between fine particulate air pollution and death from coronary heart disease than that found in earlier studies. In the WHI study, Miller et al. found an increased relative risk of 1.76 for death from cardiovascular disease for every increase of 10 μg per cubic meter in the mean concentration of PM2.5. By comparison, a study by the American Cancer Society showed that each increase of 10 μg per cubic meter in the mean PM2.5 concentration was associated with an increased relative risk of 1.12 for death from cardiovascular disease, 1.18 for death from ischemic heart disease (the largest proportion of deaths), and 1.13 for death from arrhythmia, heart failure, or cardiac arrest.5 Samples in previous studies consisted of subjects from the entire population of the cities being investigated. The WHI analysis was restricted to postmenopausal women with no history of cardiovascular health problems. A 22-year follow-up of a cohort of nonsmoking white adults in California showed an increased risk of death from coronary heart disease with rising levels of