The increase in heart-attack risk was small on the individual level, but it can have a substantial impact at the population level, says Hazrije Mustafic, M.D., the lead author of the analysis and a researcher at the University Paris Descartes.
“We must keep in mind that the entire population is exposed to air pollution in industrial countries, so the effect on public health is not negligible,” Mustafic says.
For instance, she says, although an increase in carbon monoxide levels of 10 micrograms per cubic meter raises an individual’s short-term heart-attack risk by just 5% or so, a change in air quality of that magnitude could be expected to account for 4.5% of all heart attacks in the exposed population.
The top sources of air pollution are the burning of fossil fuels (such as gas, oil, and coal) and industrial emissions. The pollutant Wallenius and his team focused on, fine particulate matter, consists of microscopic particles of metal, carbon, sulfates, and other materials. These specks of dust are about 30 times smaller than the thickness of a human hair, so they can be inhaled deep into the lungs.
Breathing fine particulate matter may harm the cardiovascular system in two ways, Wallenius says. If particles find their way into the bloodstream they can make blood vessels less elastic, and they may also boost activity of the sympathetic nervous system, which tends to increase heart rate and blood pressure and trigger the release of stress hormones.
“That would change how the blood flows through the body,” Wellenius says. “That could then take somebody [who's] susceptible to stroke and push them over the edge to actually having a stroke.”
In addition to affecting blood flow, air pollution appears to increase inflammation, an immune-system response that is believed to contribute to both heart disease and strokes.
If inhaled pollutants reach the alveoli—the tiny sacs in the lungs where oxygen and carbon dioxide pass into and out of the bloodstream, respectively—they trigger an inflammatory reaction, Mustafic says. “These pollutants can also spread through the blood stream and reach the heart,” she adds.
Wellenius and Mustafic agree that people who are already at high risk of stroke or heart attack should consider taking steps to cut their exposure to very high levels of pollution.
But healthy people shouldn’t view the findings as a call to go out and buy face masks, says Robert D. Brook, M.D., a cardiologist at the University of Michigan, in Ann Arbor, who coauthored an editorial accompanying Wallenius’ study.
“The wrong thing to do is to get alarmed…that every time you’re in traffic or every time you’re exposed to a little bit of smog, you are going to get a heart attack or die,” Brook says. The best way to prevent a heart attack or stroke, he says, is to control personal factors such as blood pressure, cholesterol, smoking, and exercise.