This is an archived article.

February 8, 2007

Soot in air increases heart attack risk

The yellowish haze you may have seen floating at the edge of the horizon on a sunny Seattle day is doing more than clouding the view of Mount Rainier. It might also be hurting your heart and circulatory system.

That haze is mostly caused by airborne fine particulate matter, tiny particles of soot or dust that come from a variety of sources, mainly vehicle exhaust and coal-fired power plants. The particles are less than 2.5 microns in diameter — about 30 to 40 of them would equal the diameter of a human hair. They’re typically invisible to the human eye once they’re in the atmosphere, though they may be visible in dense clouds as they come out of a tailpipe, smokestack, or chimney.

Airborne soot has long been associated with respiratory problems, but in recent years, scientists have begun to connect it with cardiovascular problems such as heart disease. The tiny, toxic particles are breathed into the lungs and either cause a negative reaction originating in blood vessels there, researchers theorize, or they might enter the bloodstream and cause problems throughout the circulatory system. They don’t understand the exact mechanism just yet, but some scientists believe that the soot particles may be 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,” explained Dr. Joel Kaufman, UW professor of environmental and occupational health sciences, epidemiology and medicine. “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 and Kristin Miller, a UW doctoral student in epidemiology, have recently completed a study that further connects this type of air pollution with cardiovascular disease and related health problems. Their study appeared in the Feb. 1 issue of the New England Journal of Medicine.

Miller and Kaufman found that women living in areas with higher levels of fine particulate matter pollution had a greater risk of developing cardiovascular disease and dying from cardiovascular or related causes. They also found a much stronger connection between pollution and heart disease than previous studies, suggesting the effects of particulate matter may be worse than previously thought.

The study looked at the cardiovascular health of more than 65,000 post-menopausal women, age 50 to 79, in 36 cities around the country. Researchers studied the health of the women, who did not initially have cardiovascular disease, and followed them for several 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 long-term 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.

Fine particulate matter is measured in micrograms (or millionths of a gram) per cubic meter; 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 taking into account known risk factors such as blood pressure, cholesterol, and smoking. 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.

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

“Our findings show that both what city a woman lived in, and where she lived in that city, affected her exposure level and her disease risk,” said Miller, who was first author of the study.

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 multi-site 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. The EPA and the National Institute of Environmental Health Sciences provided funding for the study of the effects of air pollution.

Kaufman is leading a major new EPA-funded study to uncover these mechanisms — an air-pollution study based on the NIH’s Multi-Ethnic Study of Atherosclerosis, or MESA. Where the Women’s Health Initiative study looked at clinical cardiovascular disease and its related events — heart attack and stroke, for instance — the MESA Air Pollution Study will look at how air pollution may be related to early pre-clinical stages of heart disease and diseases of the blood vessels and lungs.

“You wouldn’t know if you had any atherosclerosis or hardening of the arteries without an imaging test, such as a CT scan of the coronary artery or an ultrasound of the carotid arteries,” explained Miller. “If you do a series of these tests on the same individuals across several years, it’s possible to see progression or changes in how much blockage is present in the arteries. So in MESA Air, the main study objective is to investigate how long-term air pollution exposure influences the progression of atherosclerosis.”

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, making it the leading cause of death in this country. Reducing fine particulate air pollution might translate to preventing or delaying thousands of heart attacks, strokes and bypass surgeries, and fewer early deaths from cardiovascular causes.

That means cutting back on the use of fossil fuels, in vehicles and power plants — an issue that has come to the forefront in recent years during the debate over climate change.

“Recently, there is increased awareness of global warming and its potential implications for future human well-being,” Miller said. “It is important to realize air pollution is also affecting us presently and locally in terms of human disease. Steps to control air pollution by reducing emissions may have multiple benefits for human health, and for the global ecosystem in general.”