Higher levels of pollution were associated with more heart attacks.
Regular physical activity may reduce the risk of heart attack, even in areas with moderate-to-high levels of traffic pollution, a study claims.
"While exercise is known to reduce cardiovascular disease risk; pollution can increase the risk of cardiovascular disease, including heart attacks, asthma and chronic obstructive lung disease," said Nadine Kubesch from the University of Copenhagen in Denmark.
"Currently there is little data on whether poor air quality cancels out the protective benefits of physical activity in preventing heart attacks," said Kubesch, lead author of the study published in the Journal of the American Heart Association.
Researchers in Denmark, Germany and Spain evaluated outdoor physical activity levels (sports, cycling, walking and gardening) and nitrogen dioxide (NO2 pollutant generated by traffic) exposure in 51,868 adults, aged 50-65.
Over a 17.7-year period, there were 2,936 first heart attacks and 324 recurrent heart attacks.
Higher levels of pollution were associated with more heart attacks, however, the risk was lower among those who were physically active, the researchers found.
Moderate cycling for four or more hours per week cut risk for recurrent heart attack by 31 per cent; and there was a 58 per cent reduction when all four types of physical activity (together totalling four hours per week or more) were combined, regardless of air quality.
Those who participated in sports had a 15 per cent lower rate of initial heart attacks and there was a 9 per cent risk reduction associated with cycling, regardless of air quality, the researchers said.
Compared to participants with low residential NO2 exposure, those in higher risk areas had a 17 per cent increase risk in first heart attack and 39 per cent for recurrent heart attack, they said.