The dirtier the air, the more likely people are to suffer sudden cardiac arrest, new research from Australia shows.
Particulate matter -- tiny specks of soot, dust, and other pollutants in the air that can be breathed deep into the lungs -- has been "consistently" linked to increases in deaths from heart disease and clogged arteries, Dr. Martine Dennekamp of Monash University in Melbourne and her colleagues note.
But studies looking at whether air pollution specifically ups the risk of heart attack or cardiac arrest have had mixed results.
Airborne particles are harmful to people with existing health problems, the researchers add, but they could also trigger heart attack or even sudden death in people with no apparent symptoms of cardiovascular disease.
To investigate, Dennekamp and her team looked at 8,434 cases of sudden cardiac arrest among people 35 and older that occurred in metropolitan Melbourne between 2003 and 2006.
After a rise in concentration of the tiniest airborne particles (particles less than 2.5 microns across, known as PM2.5), the likelihood of cardiac arrest rose, and stayed higher than average for two days. For every 4.26 micrograms per cubic meter increase in PM2.5 concentrations, the risk of cardiac arrest was 4 percent higher than average for the next 48 hours.
An individual's risk of suffering sudden cardiac arrest is quite low; the American Heart Association estimates that there's about one cardiac arrest per 2,000 people every year in North America. And the study does not prove that pollution causes more cardiac arrests, as the researchers did not find out whether participants in the study also smoked or had other risk factors for heart disease.
Carbon monoxide levels also were associated with increases in cardiac arrest risk, although the effect wasn't as strong as it was for PM2.5. None of several other pollutants the researchers measured, including larger airborne particles, affected risk. The effect was strongest for people 65 to 74 years old, and weakest for those 75 and older.
Australia currently has an "advisory standard" limiting PM2.5 concentrations to 25 micrograms per cubic meter or less, the researchers note.
Given that an increase of less than 5 micrograms per cubic meter was tied to significant health effects, they add, "the present study suggests an increase in the risk of cardiac effects at concentrations below the current air quality standards in Australia."