Air Pollution Tied to Acute Stroke, Cognitive Decline

Megan Brooks

February 13, 2012

February 13, 2012 — Breathing in particulate matter, even at levels deemed safe by Environmental Protection Agency (EPA) standards, may raise the risk for acute ischemic stroke and for cognitive decline, 2 new studies suggest.

In an accompanying commentary, Rajiv Bhatia, MD, MPH, from the San Francisco Department of Public Health in California, notes that the reported association between ambient fine particulate matter, defined as less than 2.5 μm (PM 2.5), and ischemic stroke "adds to the already strong evidence" linking PM 2.5 to cardiovascular effects, and adds that the analysis on cognitive function shows that "we may not fully understand the breadth of PM health burdens."

"The strong and growing evidence on the harms of PM 2.5 demands scrutiny of societal efforts to reduce exposure," Dr. Bhatia concludes.

The studies and commentaries were published online February 13 in the Archives of Internal Medicine.

34% Higher Stroke Risk

In the first study, Gregory A. Wellenius, ScD, from the Center for Environmental Health and Technology, Brown University, Providence, Rhode Island, and colleagues reviewed the medical records of 1705 patients admitted to Beth Israel Deaconess Medical Center in Boston, Massachusetts, between 1999 and 2008 with acute ischemic stroke, and estimated the hour when the stroke symptoms first occurred.

Dr. Gregory A. Wellenius

"Using hourly measurements of ambient air pollution from the nearby Harvard School of Public Health environmental monitoring station, we found that the risk of stroke was linked to levels of ambient air particles," Dr. Wellenius told Medscape Medical News.

The estimated odds ratio (OR) of ischemic stroke onset was 1.34 (95% confidence interval [CI], 1.13 -1.58; P < .001) after a 24-hour period with PM 2.5 levels in the "moderate" range by EPA standards (15 - 40 μg/m3) compared with a similar period with levels in the "good" range (≤15 μg/m3). During the study, PM 2.5 levels in the Boston area did not exceed current EPA standards.

"We also found that the increased risk of stroke peaked 12 to 14 hours after levels of ambient air particles went up — a shorter time interval than previously found," Dr. Wellenius said.

In a second invited commentary, Robert D. Brook, MD, from the University of Michigan, Ann Arbor, and Sanjay Rajagopalan, MD, from the Ohio State University Medical Center in Columbus, say these findings are important because current US and World Health Organization air quality standards focus only on daily and annual PM 2.5 mean concentrations.

"The findings by Wellenius et al remind us that clinical events could be initiated by even briefer subdaily periods of PM2.5 inhalation," they write.

The risk was most strongly associated with components of traffic pollution, mainly black carbon and nitrogen dioxide. Ozone or sulfates were not associated with ischemic stroke risk.

"Based on our findings, we estimate that lowering the levels of ambient air particles by about 20% could have prevented about 6000 of the 184,000 stroke hospitalizations that occurred in the Northeast US in 2007," Dr. Wellenius told Medscape Medical News.

Given that air pollution differs around the country, he said his team is starting a study to look at the link between air pollution and stroke in multiple cities across the United States.

Air Pollution Ups Risk for Cognitive Decline

In the second study, Jennifer Weuve, MPH, ScD, from Rush University Medical Center in Chicago, Illinois, and colleagues evaluated both fine and coarse PM (2.5 - 10 μm) in relation to cognitive decline in 19,409 older women, using the Nurses' Health Study Cognitive Cohort.

Dr. Jennifer Weuve

The researchers estimated each woman's recent (1 month) and long-term (7 - 14 years) exposure to fine and coarse PM, based on data from EPA air monitors located throughout the contiguous United States, accounting for multiple factors that affect exposure, including roadways, wind patterns, and altitude. Cognitive function tests were obtained when the women were 70 to 81 years old, and were repeated 3 times at approximately 2-year intervals.

The researchers found that higher levels of long-term exposure to both fine and coarse PM were associated with significantly faster cognitive decline. The 2-year decline on a global cognitive score was 0.020 (95% CI, −0.032 to −0.008) and 0.018 (95% CI, −0.035 to −0002) standard units worse for every 10 μg/m3 increment in coarse and fine PM exposure, respectively.

To put these findings into perspective, 10 μg/m3 higher exposure to PM was cognitively equal to aging by up to 2 years, the authors note in their report.

"On an individual basis," Dr. Weuve told Medscape Medical News, "the magnitude of the association (about 2 years of additional cognitive aging for each additional 10 μg/m3 of average annual exposure) does not seem like much. But if this association turns out to be real, then at the level of the population, it is potentially meaningful."

It has been projected, she explained, that a broadly applied intervention that delays the onset of Alzheimer's disease by 2 years could reduce the number of prevalent cases in the United States by approximately 2 million during a 40-year interval.

"This notion of a 'broadly applied intervention' may sound like wishful thinking," Dr. Weuve admitted. Yet, unlike other factors that may be involved in dementia, such as lack of physical activity and diet, "air pollution is something that we can intervene on as a society at large through policy, regulation and technology, offering a population-wide means of preventing or delaying dementia if, indeed, its development is related to PM exposure."

The studies were supported by grants from the National Institute of Environmental Health Sciences and the US Environmental Protection Agency. The study authors and commentary authors have disclosed no relevant financial relationships.

Arch Intern Med. 2012;172:219-236. Wellenius abstract, Weuve abstract, Bhatia extract, Brook and Rajagopalan extract

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