Air pollution raises risk of stroke and cognitive decline

By Dr. Ananya Mandal, MDFeb 15 2012

According to data from two studies published in the Archives of Internal Medicine exposure to air pollution may have serious cognitive and cardiovascular health consequences, even at levels currently deemed acceptable by the U.S. Environmental Protection Agency (EPA).

Gregory Wellenius, of Brown University in Providence, R.I., and colleagues found that short-term exposure to fine particulate matter at levels the EPA considers safe can increase the risk for ischemic stroke. In the other study, Jennifer Weuve, of Rush University Medical Center in Chicago, and colleagues found that long-term exposure to fine particulate matter was associated with significantly expedited cognitive decline among older women.

The Wellenius et al study “adds to the already strong evidence” linking air pollution to cardiovascular effects, Rajiv Bhatia, from the Department of Environmental Health at the San Francisco Department of Public Health, wrote in an accompanying editorial. Bhatia concluded that more effort is needed to curb human exposure to air pollution at the community level, as well as through “more stringent and creative regulatory initiatives, and political support.”  He called on healthcare providers to advocate for these protections.

Air pollution and risk of stroke

Wellenius and colleagues analyzed data from 1,705 patients admitted to a single institution in Boston with neurologist-confirmed ischemic stroke between April 1, 1999 and Oct. 31, 2008. They looked at association between fine particulate matter (defined as particles less than 2.5 micrometers in diameter) and stroke.

The study period included only days in which the air quality was good or moderate as measured at a Central Monitoring Station. EPA regulations define good air quality as concentrations of fine particulate matter at or below 15 micrograms per cubic meter of air, and moderate as between 15 and 40 micrograms. The researchers excluded 11 days on which air pollution surpassed 40 micrograms per cubic meter.

They found that the estimated odds ratio of stroke onset was 1.34 following a 24-hour period during which air quality was classified as moderate compared with a similar, good-air quality timeframe.  This association was linear, with the association between stroke onset and fine particulate matter strongest within 12 hours of exposure among those with large-artery atherosclerosis or small-vessel occlusion.

When they looked at the type of particulate matter, the researchers determined that the risk was more strongly associated with traffic pollution, such as black carbon and NO₂, versus non-traffic associated particulate matter.

Air pollution and cognitive decline

Weuve and colleagues examined data from 19,049 women aged 70 years or older without a history of stroke, who participated in the Nurses Health Study between 1995 and 2001 and agreed to undergo cognitive testing.

The researcher performed cognitive testing via telephone three times during the study period, at two-year intervals looking for associations between cognitive changes in both fine (smaller than 2.5 micrometers in diameter) and course particulate matter exposure (2.5-10 micrometers).

Significantly faster cognitive decline occurred among participants with higher levels of long-term exposure to both grades of pollutants, the researchers determined. For every 10 micrograms per cubic meter increase in long-term exposure to coarse particles, participants experienced an average 0.020 unit decline in global score during a two-year period. Similarly, the two-year global score decline for every 10 microgram per cubic meter increase in fine particle exposure was 0.018. These changes were on par with differences in the cognitive trajectories of women approximately two years apart in age, the researchers noted.

“These associations were present at levels of PM exposure typical in many areas of the United States. 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,” Weuve et al wrote.

“We, as a society, are on the verge of dealing with an unprecedented number of people having dementia,” said Jennifer Weuve, lead author of the study and a researcher at Chicago’s Rush University Medical Center. “We know relatively little about how to prevent dementia, but we do know cognitive decline is related to dementia.”

“It turns out that cardiovascular disease may play a role in cognitive decline," said Weuve, who is a researcher at Rush’s Institute for Healthy Aging. "So if we understand how to prevent or delay these cognitive increments, maybe we can prevent or delay dementia.” And not just at an individual level, she said. “What’s interesting about air pollution," Weuve said, is that “other factors that may cause dementia are generally found at the more individual level – diet, weight, smoking. And we can help to try to prevent them at that level. But in this case, we’re looking at something that we can do to intervene at a broad scale, with society at large…It's a whole new way to think about prevention for dementia and cognitive decline,” she said.

She said more research needs to be done. For instance, is the cognitive decline they observed due to cardiovascular issues, or are pollutants having a direct effect on the brain?

“We keep learning about more adverse effects (from pollution) than we thought possible,” said Jean Ospital, health effects officer with the South Coast Air Quality Management District, who was not involved with the current research. “I’m not sure I find these results surprising,” he said, “but I’m also not sure I would have expected them if you’d asked me 10 years ago.”

“The bottom line,” said Sam Atwood, a spokesman for the South Coast Air Quality Management District, “is that in Southern California, we have some of the highest levels of particulate matter in the country, and we are working as quickly as possible at reducing those levels.”

“While overall levels of fine particles and ozone have declined significantly in the past two decades, these two pollutants still pose a burden to public health,” the EPA statement said. “One of the hardest things to explain to the public is that while the air is cleaner, we continue to find that we have underestimated the health effects of breathing in air pollution,” said Joe Lyou, president and CEO of the Coalition for Clean Air and a governing board member of the South Coast Air Quality Management District. “Yes, we have made significant accomplishments, but we still have a long way to go. The public needs to understand that this is a life-and-death situation.”