For people with existing heart problems, exposure to high levels of air pollution can trigger the irregular heartbeats that may lead to a stroke or heart attack, according to a new study.
Past research has linked air pollution to ventricular fibrillation, electrical confusion in the lower chambers of the heart which can cause sudden death.
The new study also finds an association with atrial fibrillation (AF), erratic quivering in the heart's upper chambers and the most common type of irregular heartbeat.
"As in all epidemiological studies we do not prove causation, but rather an association," said lead author Dr. Mark Link, a cardiologist at Tufts Medical Center in Boston.
His study included people with so-called implantable cardioverter defibrillators (ICDs), which record when the heart's electrical activity is abnormal and deliver shocks to try to right the rhythm.
Link and his colleagues analyzed data from the ICDs of 176 heart patients and compared it to air quality data collected in the region.
Over about two years, 49 of those people had a total of 328 AF episodes.
The researchers found that the level of air pollution, including soot-like particles, on a given day was directly tied to heart rhythm problems.
With every 6 microgram per cubic meter increase in fine particulate pollution, for example, people were 26 percent more likely to have an AF episode in the next two hours, the study team reported in the Journal of the American College of Cardiology.
That extra risk is greater than the 1 percent increased risk of death from heart attack and the 18 percent increased risk of stroke seen with each 10 microgram per cubic meter rise in pollution in other studies, Link noted.
The daily average particulate pollution level in Massachusetts, where the study took place, was 8.4 micrograms per cubic meter, well below the upper limit of 35 set by the Environmental Protection Agency (EPA).
More than two million Americans have AF, according to the Centers for Disease Control and Prevention. Although it can cause rapid heartbeat, lightheadedness and fatigue, AF doesn't always come with symptoms.
"This study does show that there is an increased association, especially within two hours of air pollutant levels being high, with a person having an irregular heart rhythm," said Dr. Comilla Sasson, who studies community-wide risk for cardiac arrest at the University of Colorado in Denver.
But it does not look at how often those irregular rhythms lead to more deadly problems such as heart attack or sudden death, she added.
"Although this is interesting, it still leaves a lot more questions than providing answers," Sasson told Reuters Health in an email.
She questioned whether or not the EPA should reevaluate its air quality standards and if doctors should be talking to patients about increased risks on pollution-heavy days.
"There is much more research that will need to be done, especially in other cities, to see if these results hold true," she said.
Although the study focused on people at unusually high risk already, an increase in the chance of AF could have implications for anyone, Link said.
"Unfortunately, all of us are at risk for AF, especially as we age. It is by far the most common arrhythmia in the U.S. and for that matter, the entire world," he said.
Boston has relatively clean air, which makes the results all the more troubling, Link told Reuters Health.
"Imagine what the effect of air pollution is in cities without the clean air of Boston," he said.