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This is the VOA Special English Health Report.

A woman wears a face mask to protect her from polluted air in Lanzhou, China, in December

A new study shows that air pollution may be more of a risk for heart disease than scientists have thought. The research involved more than sixty-five thousand women in the United States.

Kristin Miller, a doctoral student at the University of Washington in Seattle, was the lead author of the study. She says the study showed that disease risk was related not just to which city a woman lived in, but also where in the city.

The study found that estimates of the effects of air pollution were often larger within cities than between cities. Yet averages between cities have served as the main measure of the long-term effects of pollutants.

The new findings lead some experts to suggest that current pollution limits may not be strong enough.

The New England Journal of Medicine published the study. The scientists examined rates of heart attack, stroke and other cardiovascular events in women with long-term exposure to air pollution. The cardiovascular system is the heart and all of the passages that carry blood throughout the body.

The study involved women over the age of fifty who had no sign of cardiovascular disease at the start of the research. The study followed the women for as long as nine years to see how many developed cardiovascular problems.

The researchers used information from a government project, the Women's Health Initiative.

The researchers also examined levels of fine particles in the air in thirty-six areas across the country. That information came from the Environmental Protection Agency. The extremely small particles come from industrial smoke and traffic along with things like wood-burning fireplaces in houses.

In the study, every ten-microgram increase in pollution was linked to a twenty-four percent increase in the risk of a cardiovascular event. But it was related to a seventy-six percent increase in the risk of death from cardiovascular disease.

But just how do particles in the air damage the cardiovascular system? Douglas Dockery and Peter Stone at Harvard University in Massachusetts offer some theories in a related report. They say the particles may cause the lungs to swell and release chemicals from the pollutants into the blood. The chemicals then could damage the heart.

And that's the VOA Special English Health Report, written by Caty Weaver. I'm Barbara Klein.