Moderate Air Pollution Linked To Stroke, Cognitive Decline

Chronic exposure to air pollution, even at levels typically considered safe by federal regulations, increases the risk of stroke by 34 percent and may accelerate cognitive decline in older adults, according to two separate studies published this week in the Archives of Internal Medicine.

In one study, researchers studied more than 1,700 stroke patients in the Boston area over a 10-year period, and found exposure to ambient fine particulate matter (PM), generally from vehicle traffic, was associated with a significantly higher risk of ischemic strokes on days when the EPA’s air quality index for PM was yellow instead of green.

The researchers focused on tiny particles with a diameter of 2.5 millionths of a meter, less than 1/30th the width of a human hair, referred to as PM2.5.

These particles come from a variety of sources, including power plants, factories, trucks and automobiles, and can travel deep into the lungs.  They have been associated in previous studies with increased numbers of hospital visits for cardiovascular diseases, including heart attacks.

“The link between increased stroke risk and these particulates can be observed within hours of exposure and are most strongly associated with pollution from local or transported traffic emissions,” said the study´s senior author, Dr. Murray Mittleman, a physician at Beth Israel Deaconess Medical Center and an Associate Professor of Medicine at Harvard Medical School.

“Any proposed changes in regulated pollution levels must consider the impact of lower levels on public health.”

“Considering that almost everyone is exposed to air pollution and is at risk for stroke, that’s actually a pretty large effect,” said lead author Gregory Wellenius, ScD, an Assistant Professor of Community Health at Brown University.

The 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 researchers then abstracted data on the time of symptom onset and clinical characteristics to estimate the hour the stroke systems first occurred.   The team included only strokes confirmed by attending neurologists, and did not rely upon insurance billing codes, which can sometimes be vague.

They compared this data with Harvard’s hourly measurements of pollution within 13 miles of 90 percent of the stroke patients’ homes to allow 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 said.

The team was able to calculate that the peak risk to patients from air pollution exposure occurs 12-14 hours before a stroke.

Such 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.

The researchers 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.

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 researchers estimate that reducing PM2.5 pollution by about 20 percent could have prevented 6,100 of the 184,000 stroke hospitalizations in the northeastern U.S. in 2007.

Although the researchers acknowledge their results need to be replicated in other cities, they note that Boston is considered to have relatively clean air.

“The levels of PM2.5 in Boston are lower than those seen in many in other parts of the country, yet we still find that within these moderate levels the risk of stroke is higher on days with more particles in the air,” Mittleman said.

Meanwhile, a separate study by researchers at Rush University Medical Center suggests that chronic exposure to particulate air pollution may accelerate cognitive decline in older adults.

The large, prospective study found that women who were exposed to higher levels of ambient particulate matter over the long term experienced more decline in their cognitive functioning over a four-year period.

These associations were present at levels of PM exposure typical in many areas of the United States.

The researchers concluded that higher levels of long-term exposure to both coarse PM (PM2.5-10) and fine PM (PM2.5) were associated with a significant acceleration of cognitive decline.

The study is the first to examine changes in cognitive function over a period of time, and whether exposure to the size of particulate matter is important.

Jennifer Weuve, MPH., ScD, assistant professor of the Rush Institute of Healthy Aging and the principal investigator of the study, and colleagues evaluated air pollution, both coarse and fine, in relation to cognitive decline in older women.  They used a study population from the Nurses’ Health Study Cognitive Cohort, which included 19,409 U.S. women ages 70 to 81 over a 14-year period dating back to 1988.

“Very is little known about the role of particulate matter exposure and its association with cognitive decline,” said Weuve.

Exposure to particulate air pollution is known to be associated with cardiovascular risk, which may itself play a role in causing or accelerating cognitive decline, the researchers said.

“Unlike other factors that may be involved in dementia such as diet and physical activity, air pollution is something we can intervene on as a society at large through policy, regulation and technology,” said Weuve.

“Therefore, if our findings are confirmed in other research, air pollution reduction is a potential means for reducing the future population burden of age-related cognitive decline, and eventually, dementia,” she said.

Both studies are published this week in the Archives of Internal Medicine.

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