Tiny Air Pollutants
Linked to Heart Attacks

By Cat Lazaroff

DALLAS, Texas, June 12, 2001 (ENS) - As few as two hours after being inhaled, tiny, invisible air pollutants can penetrate the lungs' natural defenses and may trigger a heart attack, says a new report. The study, which appears in today's "Circulation: Journal of the American Heart Association," warns of particular problems for people who are already at risk for heart disease.

Smoke from diesel buses is a major source of fine particle air pollution in urban areas
(Photo courtesy EPA)

"Studies of hospital admissions and emergency department visits have linked exposure to particulate air pollution with increased risk of cardiovascular diseases," said study author Dr. Murray Mittleman, director of cardiovascular epidemiology at Boston's Beth Israel Deaconess Medical Center. "But the current study is the first to examine short term transient effects of air pollution on the risk of heart attack."

Between January 1995 and May 1996, researchers interviewed 772 Boston area heart attack patients about four days after their heart attack to establish when their symptoms began. Participants were enrolled in the Determinants of Myocardial Infarction Onset Study, which is aimed at gathering information about factors associated with myocardial infarction, or heart attack.

Researchers compared the times heart attack symptoms began with daily air pollution measurements collected in Boston during the study period. They paid special attention to levels of the smaller pollutants.

"These tiny particles are known as PM2.5 because they measure less than 2.5 micrometers in diameter," explained coauthor Dr. Douglas Dockery, professor of environmental epidemiology at the Harvard School of Public Health. "They are so small that they can get past the normal defense mechanisms in the lungs and penetrate deeply into the air exchange regions, or alveoli."

Incinerators, like this one in Michigan, emit fine particle air pollution
(Photo courtesy Lake Michigan Federation)

The risk of heart attack was higher in patients who had been exposed to elevated PM2.5 in the two hours before the onset of their symptoms. The researchers also found that a higher heart attack risk was related to a higher average exposure over the full day before the onset of symptoms, indicating a delayed response to the particles.

The study concluded that there was a 48 percent higher risk of heart attack when PM2.5 concentrations increased by 25 micrograms per cubic meter of air in the two hours before symptoms began.

Fine particulate air pollution is produced primarily by automobile engines, power plants, refineries, smelters and other industries, Dockery said. Larger, more readily noticed particles of airborne dust and debris from farming, construction work and mining are far less likely to trigger heart attack, he added.

Stationary sources like this Michigan steel mill are also sources of particulate pollution
(Photo courtesy EPA)

Some recent data suggest that exposure to high levels of PM2.5 may cause increased systemic inflammation, increased plasma viscosity (thicker blood) and an increase in certain proteins in the blood that can cause clots to form.

"It's too early to predict what types of medical intervention might be effective in preventing the serious cardiovascular consequences of fine particle exposure," Mittleman adds. "More research is needed to determine the exact mechanisms by which inhaling fine particles can set off heart attacks."

Many other major metropolitan areas have higher average levels of PM2.5 pollution than Boston, the researchers note, meaning that residents of those cities may face even greater risk of pollution related heart attacks than Bostonians.

"If the Boston exposure data can be generalized to other communities, we would expect proportionately higher effects in more heavily polluted cities," Dockery said. "But despite the widespread assumption that particulate air pollutants are primarily an urban problem, they can also affect large regions located downwind from the cities. Some of the highest PM2.5 concentrations are often found far from major urban areas, in places where we would expect the air to be cleaner."

Mittleman said one bit of encouraging news is that levels of the tiny pollutants have decreased somewhat in most urban areas over the past few years.

The Clean Air Act has brought about substantial reductions in air pollution since this photo of Los Angeles was taken in 1972 (Photo by Gene Daniels, courtesy EPA)

Fine particle pollution is largely a summer phenomenon, Dockery points out.
"Pollution monitors show seasonal variations where hot, hazy days have higher levels of fine particles on average," he said. Also, Dockery said it is much more difficult for individuals to take protective measures against PM2.5 than against gaseous pollutants like carbon monoxide, which can be removed from indoor air.

"Because of their size, these particles readily penetrate indoor spaces," noted Dockery, "but air conditioning helps somewhat, reducing indoor concentrations by 30 percent to 50 percent. The best advice is to avoid outdoor activity on hot, hazy days. If a person exercises outside, the increased respiratory activity also increases the dose of PM2.5."

Many urban areas have trouble meeting clean airstandards, in part due to exhaust from trucks and buses (Photo courtesy EPA)

The U.S. Environmental Protection Agency's (EPA) current acceptable standard is 65 micrograms of PM2.5 per cubic meter of air.

"Even at PM2.5 concentrations below that standard, our study shows that the risk of a heart attack is increased," said Mittleman. "Our findings suggest that people who have heart disease or an elevated risk of heart attack would be well served to avoid being outdoors for extensive periods of time when the air quality is poor. This is especially the case on the hot, hazy days of summer when the problem is much more prominent."

Mittleman said that more research is needed to determine how a person's exposure to high levels of fine particulate air pollution can lead to a heart attack. Understanding that triggering mechanism, he believes, could spur the development of new drugs that could protect individuals during peak exposure to air pollution, such as they would experience during rush hour traffic or outside on a hot summer day.