Air Pollution Linked to Increased Cardiovascular Events and Death in Postmenopausal Women

SEATTLE, Jan. 31 -- Fine-particulate air pollution over time significantly increased the risk of first heart attack or stroke, as well as cardiac mortality, in postmenopausal women, researchers here reported.

Moreover, the risk increased as pollution worsened so that for every 10 g per cubic meter increase in fine-particulate matter the risk of a cardiovascular event rose by 24% and the risk of death jumped by 76%, found Kristin A. Miller, M.S., of the University of Washington, and colleagues, in an observational study.

The risk of cerebrovascular events increased by 35% as air pollution increased (HR 1.35, 95% CI, 1.08-1.68) they reported in the Feb. 1 issue of the New England Journal of Medicine. The findings emerged from a study of 65,893 postmenopausal women who participated in the Women's Health Initiative.

The researchers examined the effect of variations in city-to-city air pollution levels as well as neighborhood variations in air pollution levels within select cities.

The women, who were enrolled from 1994 through 1998, lived in 36 U.S. Metropolitan Statistical Areas. The participants were ages 50 to 79 at baseline and had no history of cardiovascular disease.

The women were followed for a median of six years and only women who lived within 30 miles of an Environmental Protection Agency monitor that recorded levels of particulate matter of 2.5 g or less were included in the analysis.

Events were determined by responses to annual questionnaires and review of medical records. First cardiovascular event was defined as myocardial infarction, coronary revascularization, stroke, and death from coronary disease or cerebrovascular disease.

Among the findings:

• A total of 1,816 women had one or more cardiovascular events.
• Although the within-city estimates tended to be larger than between city differences, these differences were not statistically significant.
• The association between the particulate matter pollution level and cardiovascular events was stronger with increasing obesity as measured by BMI (P=0.02) and waist-to-hip ratio (P=0.05).
• Neither education nor household income significantly modified the relationship between air pollution and cardiovascular risk.

But women have a number of gender-specific risks. These are smaller coronaries that are likely to "harbor more diffuse atherosclerosis than in men," more microvascular dysfunction, and a marked increase in diabetes, obesity, hypertension, and inactivity following menopause. All these muddy the association between air pollution and cardiovascular risks, wrote editorialists Douglas W. Dockery, Sc.D., and Peter H. Stone, M.D., of Harvard Medical School.

As a result, "sex may not define susceptibility to air pollution but, rather, may be an indicator of underlying cardiac substrate that puts women at increased risk," Drs. Dockery and Stone said.

That said, they acknowledged that there "is evidence that inhalation of particulate air pollution creates and exacerbates both pulmonary and systemic inflammation and oxidative stress, leading to direct vascular injury, atherosclerosis, and autonomic dysfunction."

Moreover, they cited a number of studies that linked increased levels of fine particulate air pollution with a greater build-up of atherosclerotic plaque, as well as increased fibrinogen, plasma viscosity, platelet activation, and endothelins.

Overall, they said the public health burden of "cardiovascular disease attributable to air pollution is large, the evidence suggests that individual risks are modest."

The challenge, the editorialist concluded, will be to use data from the Women's Health Initiative to "identify intrinsic and acquired individual factors that can lead to increased adverse cardiovascular responses to air pollution" so that it will be possible to "offer focused interventions to persons who are at greatest risk and thereby ameliorate at least some of the patient-specific damages of air pollution."