Levels of air pollution that fall within the amounts deemed safe in current U.S. standards for air quality were associated with a significant increase the risk for acute ischemic stroke after short-term exposure and accelerated cognitive decline after long-term exposure in two separate studies.

Previous studies of the effects of ambient fine particulate matter air pollution, defined as particulate matter less than 2.5 mcm in diameter (PM_2.5), on ischemic stroke risk have not provided unequivocal results and have not adequately examined the etiology of such strokes. Studies of the effects of air pollution on cognitive decline are even rarer, and none have assessed the longitudinal effects of PM_2.5 on cognition, according to the authors of the reports, which were published online Feb. 13 in Archives of Internal Medicine.

The odds of having a stroke can climb dramatically after continued exposure to polluted, low-quality air.

Gregory Wellenius, Sc.D., of Brown University, Providence, R.I., and his colleagues reviewed data from patients admitted to Beth Israel Deaconess Hospital, Boston, with ischemic stroke between 1999 and 2008. They used a time-stratified case-crossover study design to examine the association between ischemic stroke risk and PM_2.5 levels in the hours and days before each stroke (Arch. Intern. Med. 2012;172:229-34).

Overall, a 24-hour period of exposure to "moderate" air quality (as defined by the Environmental Protection Agency Air Quality Index) raised the odds of having a stroke by 34%, compared with a 24-hour period of "good" air quality. The estimated odds ratio of ischemic stroke was 1.11 for each interquartile range increase in pollution levels (defined as 6.4 mcg/m³).

The increased stroke risk was highest within 12-14 hours of exposure to PM_2.5 and was most strongly associated with traffic-related pollution, the researchers noted.

The mean age of the patients was 73 years; 55% were white, and 68% were women. The most common determined causes of the strokes were small-vessel strokes (26%), cardioembolism (25%), and large-artery atherosclerosis (20%).

When the patients were examined by clinical subgroups, increased pollution levels were associated with stroke in patients with large-artery atherosclerosis (odds ratio 1.24) and small-vessel strokes (1.19), but not in patients with strokes due to cardioembolism. There was no evidence that comorbid diabetes, hypertension, atrial fibrillation, or a history of stroke increased susceptibility to pollution-related strokes, the researchers noted.

Although the observed relative risk of stroke was modest, the findings suggest that "if the association between stroke and pollution is causal and a linear dose-response occurs, a 2-microgram/m³ reduction in mean PM_2.5 levels (approximately 20%) during this time period might have averted approximately 6,100 of the 184,000 stroke hospitalizations observed in the U.S. Northeast region in 2007 alone," the researchers said.

If pollution levels decline, further data on stroke timing and patient demographics can be used to show whether pollution control impacts stroke risk, they added.

In a related finding, Jennifer Weuve, Sc.D., of Rush University Medical Center, Chicago, and colleagues found that long-term exposure to both coarse and fine PM was significantly associated with faster cognitive decline in older adults. They reviewed data from 19,409 women aged 70-81 years in the Nurses’ Health Study Cognitive Cohort and used geographic information to estimate short-term exposure (1 month) and long-term exposure (7-14 years) before the women underwent baseline cognitive testing.

Overall, the 2-year cognitive decline as measured by a global score was 0.020 standard units worse per 10 mcg/m³ increment of exposure to coarse PM, defined as particles from 2.5-10 mcm in diameter (PM_2.5-10), and 0.018 standard units worse per 10 microgram/m³ increment of exposure to PM_2.5, the researchers said. The average age at the time of baseline cognitive assessment was 74 years (Arch. Intern. Med. 2012;172:219-27).

The trend of cognitive decline across quintiles of pollution exposure bordered on statistical significance. But when air pollution exposure was treated as a continuous variable, both the long-term PM_2.5 exposure and PM_2.5 exposure in the 5 years before the initial cognitive assessment were associated with significantly worse decline in global cognition. "Decline in the individual cognitive domains generally was more strongly predicted by long-term than recent exposure to PM_2.5," the investigators wrote.

The results were limited by indirect estimates of pollution exposure. But the findings support data from previous studies showing that ambient particles in the air may have a negative effect on cognition, they said.

The associations with cognitive decline were observed in women with levels of PM exposure typical in many areas of the United States, the researchers said. "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."

Dr. Wellenius’s study was funded by the National Institutes of Health and the Environmental Protection Agency. Dr. Weuve’s study was funded by the National Institute of Environmental Health Sciences and the EPA. The Nurses Health Study is separately funded by the National Cancer Institute. None of the authors of either study had relevant financial disclosures.

Air Pollution May Hurt Cognition, Increase Strokes