Pollution tied to stroke risk

By Anthony J. Brown, MD

NEW YORK (Reuters Health) – Increases in particles
polluting the air are associated with an increase in the number
of strokes caused by a blood clot in the brain — but not the
type caused by an artery rupture in the brain — new research
shows.

Previous reports have shown a link between air pollution
and overall risk of heart attacks and other cardiovascular
events, but the specific effect on stroke risk has not been
well studied, lead author Dr. Gregory A. Wellenius, from Beth
Israel Deaconess Medical Center in Boston, told Reuters Health.

“Our study is the first large study in the US to address
this topic,” he said

The researchers evaluated the link between air pollution
and stroke among Medicare recipients in nine US cities.
Specifically, they analyzed data on 155,503 artery-blockage
(ischemic) strokes and 19,314 bleeding (hemorrhagic) strokes
recorded as hospital admissions between 1986 and 1999.

As reported in the American Heart Association’s journal
Stroke, the team found that an increase in particulate air
pollution from the lowest to the highest levels raised ischemic
stroke admissions by 1.03 percent on the same day. Further
analysis yielded similar results for levels of carbon monoxide,
nitrogen dioxide, and sulfur dioxide.

By contrast, the investigators found no association between
air pollution and hospital admission for hemorrhagic stroke.

Wellenius cited three possible mechanisms, which alone or
in combination might explain how air pollution promotes stroke.
“One possibility is through inflammatory effects. The other is
through pulmonary reflexes that trigger changes in blood
pressure or heart rate.” A third possibility is changes in
clotting factors that tend to promote more blood clots.

Although the increase in ischemic stroke risk is small, the
number of excess strokes can be high because pollution affects
the whole population.

“Taken together with previous reports, the results suggest
that reducing exposure to air pollution is likely to reduce the
risk of a number of health problems, including heart disease
and stroke,” Wellenius concluded.

SOURCE: Stroke, December 2005.