Women in polluted areas at higher risk of cardiovascular disease.

Women living in areas with higher levels of air pollution have a greater risk of developing cardiovascular disease and subsequently dying from cardiovascular causes, according to a University of Washington (UW) study that was reported in the February 1 issue of the New England Journal of Medicine. The study was one of the largest of its kind, involving more than 65,000 Women's Health Initiative Observational Study participants, 50 to 79 years of age, living in 36 cities across the United States.

UW researchers studied women who did not initially have cardiovascular disease, following them for up to nine years to see who went on to have a heart attack, stroke, or coronary bypass surgery, or died from cardiovascular causes. They linked this health information with the average outdoor air pollution levels near each woman's home and found that higher pollution levels posed a significant hazard--much higher than previously thought--for development of cardiovascular disease.

The researchers studied levels of fine particulate matter, which are airborne particles of soot or dust less than 2.5 microns in diameter. Typically the particles are invisible to the human eye once they're in the atmosphere, although they may be visible in dense clouds as they come out of a tailpipe, smokestack, or chimney and are responsible for urban haze.

"These soot particles, which are typically created by fossil-fuel combustion in vehicles and power plants, can contain a complex mix of chemicals," explained Dr. Joel Kaufman, professor of environmental and occupational health sciences, epidemiology, and medicine at UW and leader of the study. "The tiny particles--and the pollutant gases that travel along with them--cause harmful effects once they are breathed in."

Cities in the study had average levels of fine particulate matter ranging from about 4 to nearly 20 micrograms per cubic meter. The researchers found that each 10-unit increase in fine particulate matter level was linked to a 76 percent increase in the risk of death from cardiovascular disease after known risk factors such as blood pressure, cholesterol, and smoking were taken into account. Higher long-term average levels of fine particulate matter also led to a higher overall risk of cardiovascular-disease events, including stroke and heart attack.

The researchers also found that differences in particulate matter levels within cities, as well as differences between cities, translate to a higher or lower risk of cardiovascular disease and related death.

Previous studies have found apparent links between airborne particulate matter and cardiovascular disease, but this study was the first to look specifically at new cases of cardiovascular disease in previously healthy subjects and local air pollution levels within metropolitan areas. Researchers used data from the multisite Women's Health Initiative Observational Study, which is funded by the National Heart Lung and Blood Institute of the National Institutes of Health (NIH) and coordinated through a center based at the Fred Hutchinson Cancer Research Center in Seattle. The U.S. Environmental Protection Agency (U.S. EPA) and the National Institute of Environmental Health Sciences provided funding for the study of the effects of air pollution.

Scientists don't understand exactly how fine particulate matter may be leading to cardiovascular disease, but some believe that the soot particles are accelerating atherosclerosis, or hardening of the arteries, which is the major precursor of heart disease.

"This could be a cellular and biochemical process that starts in the lung and then proceeds from there into the cardiovascular system," Kaufman explained. "Or it could be that these very small particles actually enter the blood stream through vessels in the lung, and then begin affecting blood vessels throughout the body."

Kaufman is leading a major new U.S. EPA-funded study to uncover these mechanisms--an air-pollution study based on NIH's Multi-Ethnic Study of Atherosclerosis (MESA). The MESA Air Pollution Study is tackling two key areas, Kaufman said: investigating the mechanisms through which particulate matter leads to cardiovascular disease and identifying the sources of pollution that cause the problem. "Preventing these effects requires reducing the pollution at the source," he said.

The implications of this connection could be very significant.

"More than one out of three deaths in the United States are due to cardiovascular disease--it's the leading cause of death," said Kristin Miller, first author of the study and a doctoral student in epidemiology at UW. "If the annual average concentration of fine particulate air pollution can be reduced, it would potentially translate on a national scale to the prevention or delay of thousands and thousands of heart attacks, strokes, and bypass surgeries, not to mention fewer early deaths."

Note: To determine the average annual concentration of fine particulate matter for a particular city or county, visit U.S. EPA's Air Trends Web site at www.epa.gov/airtrends/factbook.html and look for "PM 2.5 Wtd AM" in the tables provided. The most recent data available from U.S. EPA are from 2005.