Living in city smog may be bad for the lungs, but whether it also plays a role in jacking up stroke risk, as studies have suggested, warrants another look, new Canadian research concludes.

Health Canada’s Dr. Paul Villeneuve and colleagues did find a higher incidence of stroke among residents of the smoggiest areas of Edmonton, Canada, but once income levels were taken into account, “the air pollution effect was lowered and disappeared,” the study co-author told Reuters Health.

Previous research has “consistently” shown that short-term exposure to air pollution can increase the risk of stroke-related death, but long-term exposure had not been studied, the Canadian researchers note in the journal Stroke.

To address that question, they looked at environmental and health data gathered between 2003 and 2007, at the neighborhood level, in Edmonton, Alberta, a city surrounded by oil, gas and coal processing plants.

The authors found more than 7,300 hospitalizations for stroke and cross-referenced them to neighborhood pollution levels. Although a first glance at the numbers suggested an association between long-term exposure to smog and strokes, “once we adjusted for neighborhood characteristics such as income, the increased risk disappeared,” Villeneuve said.

Only those Edmontonians living in traffic congested areas of the city appeared to face a higher risk of stroke, but smog was only one of any number of explanations. “It could also be explained by the tendency for populations more prone to stroke and with greater need for access to large health care centers to live in higher traffic areas than in remote areas,” they write.

The traffic congestion-stroke association could help influence public policy, Dr. Jiu-Chiuan Chen, of the University of Southern California Keck School of Medicine, told Reuters Health.

“From the public health perspective, results of chronic health effect studies (like this one) will have larger impact on environmental regulation, as compared with the results of short-term effect studies,” said Chen, who has published research on the subject but was not involved in the current study.

US regulators have been keenly interested in “epidemiologic evidence on chronic health effects, especially those emerging health data on the toxicity of traffic-related air pollutants,” Chen noted.

The Canadian authors acknowledge their findings contradict not only previous stroke research but also a similar Canadian study into a possible connection between pollution and heart disease. They urge caution in interpreting the results because the reasons for the discrepancies “are not entirely clear.”

“Things like affluence are associated with a number of other risk factors for stroke, for example people of lower socio-demographic status may be more likely to smoke or may be more inactive or may have exposure to other things that could increase the risk of stroke,” he said.

Villeneuve characterized his research as an interim study. More research is underway, he said, to “disentangle” the risk posed by long-term exposure to air pollution and other more well-established stroke risks such as smoking, physical inactivity, and being overweight.

SOURCE: Stroke, online June 10, 2010.