The risk of each pollutant was calculated. For fine particle pollution, heart attacks increased 2.5 percent for every incremental increase of pollution in the air.
That means if fine particles in one city reached a concentration of 10 micrograms per cubic meter while in a nearby city, it reached 20 micrograms, heart attacks in that second city would be 2.5 percent higher.
Such variations in pollution levels are commonplace, even in the same city from day to day. Miraloma, a city in Southern California's Riverside County that has some of the nation's worst particulate pollution, measured at about 55 micrograms per cubic meter on a day in early January 2010, and then dropped three days later to about 7 micrograms, according to data from the California Air Resources Board. That would raise the heart attack risk sharply on that earlier day.
“The good news is that air pollutants have been going down here in Southern California. Particles, for example, are down 20 to 25 percent over the past decade,” said Ospital.
“I think [the new study] tells you that probably the current levels may not be where we want them to be, but we are making progress toward attaining our air quality standards.”
In most U.S. cities, levels of all six of the pollutants studied have declined over the past few decades as cars, trucks, industries and consumer products have been forced to get cleaner. But many cities still have a long way to go. More than 30 metropolitan areas exceed the federal government’s health standard for fine particles. Nine areas violate the sulfur dioxide standard, 43 violate the carbon monoxide standard and 45 exceed the coarse particles standard.
Areas with excessive levels of one or more of the five pollutants include the Los Angeles basin, California's San Joaquin Valley, the Salt Lake City area, Phoenix, New York City and Philadelphia.
The analysis included several studies from each populated continent, except Africa, where pollution is largely unstudied.
Araujo said the risk is not just among people who are sick with pre-existing heart conditions. Some people are more at risk than others, including those who are obese or have hypertension, “but that is not to say that somebody who doesn’t have these conditions is at no risk of having a cardiovascular event” brought on by air pollution, he said.
He suggested that people avoid exercising in highly congested areas near busy roads and freeways, particularly during rush hours.
“A very small percentage of patients are aware of this problem,” he said. “It’s a relatively low increase for heart attacks but the population at stake is larger than it is for the other risk factors.” For example, all 17 million residents of the Los Angeles basin are exposed to air pollution while only a small fraction of them are smokers, obese or have diabetes.
Scientists are uncertain how air pollution triggers heart attacks. One major theory is it causes inflammation. Another is that it disrupts heart rate variability, which is how the heart responds to stress. Still another is that it increases the viscosity of blood, leading to more clots or hardening of the arteries.
The authors do not know why no association was found between heart attacks and ozone, which was somewhat surprising. The main ingredient of smog, it is formed when the sun reacts with hydrocarbons and nitrogen oxides from vehicles, industries and consumer products.
One reason may be that heart attacks decline on hot summer days, when ozone is the worst, so it may have been difficult to find a link.
“Either there is no true association or the association is more difficult to reveal,” Araujo said.
Some previous studies have found a link between ozone and heart attacks, while others have not. It is more clear that when ozone levels rise, deaths from asthma and other respiratory problems seem to increase.