Writing in the Feb. 14, 2012 issue of the Archives of Internal Medicine, researchers who studied more than 1,700 stroke patients in the Boston area over a 10-year period found exposure to ambient fine particulate matter, generally from vehicle traffic, was associated with a significantly higher risk of ischemic strokes on days when the EPA’s air quality index for particulate matter was yellow instead of green.
Researchers focused on particles with a diameter of 2.5 millionths of a meter, referred to as PM2.5. These particles come from a variety of sources, including power plants, factories, trucks and automobiles and the burning of wood. They can travel deeply into the lungs and have been associated in other studies with increased numbers of hospital visits for cardiovascular diseases such as heart attacks.
The team was able to calculate that the peak risk to patients from pollution exposure occurs 12-14 hours before a stroke. That information may be useful to researchers who want to trace how PM2.5 might be working in the body to increase the likelihood of stroke.
They also found that black carbon and nitrogen dioxide, two pollutants associated with vehicle traffic, were closely linked with stroke risk, suggesting that pollution from cars and trucks may be particularly important.
Researchers estimate reducing PM2.5 pollution by about 20 percent could have prevented 6,100 of the 184,000 stroke hospitalizations in the northeastern United States in 2007.
Researchers analyzed the medical records of more than 1,700 patients who went to the hospital for treatment of confirmed strokes between 1999 and 2008. They matched the onset of stroke symptoms in each patient to hourly measurements of particulate air pollution taken at the nearby Harvard School of Public Health’s environmental monitoring station.
The team was able to estimate the hour the stroke symptoms first occurred, rather than relying on the more coarse measure of when patients were admitted to the hospital. They also included only strokes confirmed by attending neurologists, rather than relying on more vague insurance billing codes.
Meanwhile, Harvard’s hourly measurements of pollution within 13 miles of 90 percent of the stroke patients’ homes allowed for close matching in time of exposure and stroke onset.
“We think that this study is novel in that it has high-quality data on both air pollution exposure and stroke diagnosis,” Wellenius says.
The estimated odds ratio (OR) of ischemic stroke onset was 1.34 following a 24-hour period classified as moderate (PM2.5 15-40 µg/m3) by the US Environmental Protection Agency’s (EPA) Air Quality Index compared with a 24-hour period classified as good (less than 15 µg/m3). Considering PM2.5 levels as a continuous variable, we found the estimated odds ratio of ischemic stroke onset to be 1.11 per interquartile range increase in PM2.5 levels (6.4 µg/m3). The increase in risk was greatest within 12 to 14 hours of exposure to PM2.5 and was most strongly associated with markers of traffic-related pollution.
Stroke is a leading cause of long-term disability and the third leading cause of death in the United States. An estimated 795,000 Americans suffer a new or recurrent stroke every year, resulting in more than 135,000 deaths and 829,000 hospital admissions.
The finding that days of moderate air quality substantially elevate stroke risk compared to days of good air quality suggest that the Environmental Protection Agency may need to strengthen the language it uses to describe the health consequences of moderate air quality, researchers say.