A Danish study hints that air pollution from car exhaust might trigger strokes, although much more study is needed to confirm this, the study team notes.

In the study, short-term exposure to certain pollutants affected admission rates for mild, “ischemic” strokes at hospitals around Copenhagen, Denmark. Ischemic stroke, the most common type of stroke, occurs when a clot disrupts blood flow to the brain.

The Danish findings, reported in the European Heart Journal, follow another recent study that showed a link between long-term exposure to traffic-related pollution and stroke death rates in the United Kingdom.

“The key observation is that the traffic-generated air pollution seems to be the primary source of the exposure,” Dr. Jiu-Chiuan Chen, of the Keck School of Medicine of the University of Southern California, Los Angeles, told Reuters Health.

Stroke is the second leading cause of the death worldwide, accounting for about 4.4 million deaths every year. In the United States, about 700,000 people suffer a stroke every year, costing the country an estimated $43 billion dollars.

Dr. Zorana Andersen, of the Danish Cancer Society in Copenhagen, and colleagues used a background air pollution monitor to measure the amount of small polluting particles produced by cars over four years. Then they compared the levels of these particles on certain days with the number of hospital admissions for different kinds of strokes at nine hospitals within 15 kilometers (about 9 miles) of the monitor.

Over the course of the study, about 7,500 people were hospitalized for a stroke.

Short-term exposure to these particles was linked to hospital admissions for minor, ischemic strokes that happened a few days after exposure.

Among patients with mild, ischemic strokes who did not have heart abnormalities that would make them stroke-prone, the researchers estimated a 21 percent increase in hospital admissions following exposure. They calculated that 147 new cases of these strokes per year can be attributed to small, traffic-generated particles.

Short- and long-term exposure to air pollution probably have different impacts that both contribute to stroke risk, Chen said. Long-term exposure has been shown to increase the buildup of fatty deposits in the arteries.

Exactly how short-term exposure might increase the risk of stroke is still unknown, and the data do not show that air pollution definitely causes strokes, Andersen said.

“Our data suggest that exposure to increased levels of air pollution over several days can trigger stroke hospitalization,” Andersen told Reuters Health in an email. “But we don’t know yet how air pollution triggers stroke.”

“This finding does add to our current knowledge of the effects of exposure to ambient air pollution” on cardiovascular health, said Chen, who was not involved in the study. “It’s a profound public health problem.”

Andersen said the problem is not limited to the elderly or others who might normally be at risk for stroke, but that air pollution could potentially trigger strokes in otherwise healthy people. However, there are ways that people can take precautions to protect their health.

“Reduced exposure to air pollution can be accomplished, to a limited extent, by personal choices,” such as taking roads with less traffic or avoiding living in a traffic-heavy area, she said.

SOURCE: European Heart Journal, online June 10, 2010.