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For the first time, scientists analyzed previous studies from five continents to verify and quantify the links between air pollution and heart health. They found that short-term exposure – less than seven days – to all major air pollutants except ozone was associated with an increase in heart attacks.
The team reported in the Journal of the American Medical Assn. that the magnitude of the risk “is relatively small” compared to other factors, such as smoking, high blood pressure and diabetes. But they stressed that so many people worldwide are breathing fine particulates, carbon monoxide, sulfur dioxide and other contaminants that the numbers of people at risk are substantial.
“Thus an improvement in air quality could have a significant effect on public health,” wrote the authors, led by Dr. Hazrije Mustafic of the Paris Cardiovascular Research Center at University Paris Descartes.
Published on Valentine’s Day, the new study shows that the human heart is perhaps the most vulnerable part of the body when it comes to air pollution.
Dr. Jesus Araujo, an assistant professor of medicine and director of environmental cardiology at UCLA, said there is now “more than enough evidence” from human, animal and cellular studies that air pollution kills.
One of the most important findings of the new research is that it confirms that heart attacks increase even when exposures to worsening air quality are short in duration.
“We don’t have to be exposed for weeks or months or years,” Araujo said.
Air in most urban areas is made up of an array of contaminants, some gases, some microscopic particles, all containing a variety of chemical ingredients. In recent years, most of the attention has focused on fine particulates – microscopic pieces of soot from diesel engines and other sources. Studies conducted in numerous cities have shown that whenever fine particles increase, deaths and hospitalizations from cardiovascular and respiratory diseases increase, too.
In the new analysis, the researchers examined more than 100 studies from around the world and included 34 that met certain standards, then combined them to calculate the risk of heart attacks associated with fine particles, coarse particles, ozone, nitrogen dioxide, carbon monoxide and sulfur dioxide. Most are pollutants related to the combustion of fossil fuels, emitted by vehicles or industry.
“One strength of our study is the comprehensive nature of our search that spanned multiple databases and was not restricted to particular publication language or a single pollutant,” wrote the authors, who are from several institutions in France as well as the Mayo Clinic in Minnesota.
Araujo said it is “quite significant” that the authors combined the results of several dozen studies and found that heart attacks increased with all of the air pollutants except ozone.
“What this study is showing us is that the gaseous pollutants are important, too. It shows that not only particulate pollution is associated with deaths but also the other major gaseous pollutants,” said Araujo, who did not participate in the new study.
Jean Ospital, health officer of the agency responsible for cleaning up the Los Angeles basin's air, said Tuesday that although the link between air pollution and heart attacks has been documented by individual studies for years, the new analysis is global and gives it “more statistical power and a larger sample size.”
“The studies are overall consistent,” said Ospital, who has been health officer at the South Coast Air Quality Management District since 2000. “There are a number of investigators looking at alternative explanations, but it always seems to come out that air pollutants are associated with premature deaths. And as the testing becomes more sophisticated, we find more effects at lower levels.”
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.
For about 50 years, scientists have tried to unravel the human health impacts of air pollution. For most of those years, the focus has been on the lungs.
“We know now that the mortality is mostly from cardiovascular causes,” Araujo said. “That’s something that has become more clear over the last seven years or so.”
Still, there are many more questions to address.
What happens when people are exposed to multiple pollutants at the same time? How do particles and gases interact? What exactly does each pollutant do to the heart? Which sources pose the most risk? Is it mostly size of particles that matters or the ingredients?
And perhaps the most critical question of all: How much cleaner does the air need to be?
“The more scientists look, the more they find effects at lower exposures,” Ospital said. “This is a question that always comes up, how low do we need to go to protect public health? It seems to be a moving target in terms of where the health effects are, where we really need to go to have health protection.”
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