Of 3320 men and women who lived in a specific south London region and had a first stroke between 1995 and 2005, Dr. Ravi Maheswaran, at the University of Sheffield, and colleagues found more deaths among those exposed to higher estimated traffic-related pollution over more than a decade.

Maheswaran's team used 2002 estimates of two common traffic pollutants -- nitrogen dioxide and small, inhalable particles called particulate matter -- linked to breathing difficulties and other health problems.

Their report, in the journal Stroke, shows risk of dying increased 28 percent when nitrogen dioxide levels rose by just 10 micrograms per 3 square meters of air. A likewise increase in particulate matter increased death risk by 52 percent, they report.

All of the areas were typical of London. The low-pollution areas typically did not have major roads running through them. The higher pollution areas had levels of nitrogen dioxide that the U.S. Environmental Protection Agency consider the U.S. average.

The researchers report more deaths in the higher polluted neighborhoods - 975 of 1659 patients, or about 59 percent, in high nitrogen dioxide neighborhoods and 967 of 1658 patients, or about 58 percent, in high particulate matter areas.

That compared to 881 of 1661 patients, or about 53 percent, in an area less polluted by high nitrogen dioxide, and 889 of 1662 patients, or about 53 percent, in an area less polluted by high particulate matter.

Risk for death remained higher with greater air pollution exposure after the investigators took into account a number of other factors associated with stroke death including age, gender, ethnicity, smoking and alcohol use, high blood pressure, heart disease, and diabetes.

If future investigations show air pollution causes death among stroke patients, a 10 microgram reduction in nitrogen dioxide exposure - about a 10 percent decrease from the average in the highest pollution areas -- would be associated with a 22 percent decrease in mortality after stroke, Maheswaran and colleagues write.

It's unclear why stroke patients may be more vulnerable to the long-term effects of air pollution, notes Dr. Jiu-Chiuan Chen, at the University of Southern California Keck School of Medicine, in Los Angeles, in a related editorial. However, some studies have linked living near high levels of traffic pollution to clogged arteries.