Two studies published in the Archives of Internal Medicine on Monday suggest that air pollution at levels experienced by most Americans or considered safe by the Environmental Protection Agency is linked to higher risk of cognitive decline and stroke.

Cognitive decline is a normal process of aging and is characterized by difficulties with memory, information processing, language, and other thinking skills.

An ischemic stroke is a stroke that occurs when an artery to the brain is blocked as a result of build-up of fatty deposits lining the vessel walls (atherosclerosis).

For the first study, first author Dr Jennifer Weuve from the Institute for Healthy Aging at Rush University Medical Center, Chicago, Illinois, and colleagues, examined results of cognitive tests undergone by nearly 19,500 women aged 70 to 81 during 1995 to 2005 who took part in the Nurses' Health Study Cognitive Cohort.

The cognitive assessments were done by telephone, three times approximately every two years. They included tests of general cognition or thinking skills, verbal memory, working memory and attention.

The researchers then compared the test results to estimates of exposure to particulate matter air pollution, both in the short term (one month preceding the cognitive tests) and in the longer term (7 to 14 years leading up to the tests) for all the geographic areas that the participants lived in. The exposure data covered both coarse (2.5 to 10 micrometers or µm in diameter) and fine (under 2.5 µm) particulate matter.

Previous studies have suggested there is a link between chronic exposure to particulate air pollution and accelerated cognitive decline in older adults, but the evidence is either limited or patchy.

The results showed that higher levels of long-term exposure to both coarse and fine particulate matter were significantly linked to faster cognitive decline.

The results also showed a dose-response relationship, the more polluted the air the women were exposed to, the faster the cognitive decline. Every 10 µg per cubic metre increase in concentration of particulate matter the women were exposed to in the long term was equivalent to aging by about 2 years. (The researchers worked this out by comparing the cognitive decline graphs over time of the women with the highest exposures against those with the lowest).

The researchers concluded that long-term exposure to both coarse (2.5 to 10 µm) and fine (under 2.5 µm) particulate matter "at levels typically experienced by many individuals in the United States is associated with significantly worse cognitive decline in older women".

In the second study, first author Dr Gregory A. Wellenius, from the Center for Environmental Health and Technology, at Brown University, Providence, Rhode Island, and colleagues, used data from the medical records of 1,705 patients from the Boston area who had been hospitalized with neurologist-confirmed ischemic stroke.

Then they compared the time when the symptoms first appeared, step by step for the relevant time periods, to the fine particulate matter (under 2.5 µm diameter) concentrations recorded at a central monitoring station in Boston to see if there were any significant links between these and risk of onset of a stroke in the hours and days preceding each event.

The results showed there was a significant 34% raised risk of ischemic stroke after a 24-hour period of what the US Environmental Protection Agency's (EPA) Air Quality Index classes as moderate fine particle pollution (15 to 40 µg/m3), compared to a 24-hour period the EPA index classes as good (15 µg/m3).

The researchers also found a dose-response relationship, in that they saw an 11% increased risk of ischemic stroke onset for each 6.4 µg/m3 increase in concentration of fine particulate matter.

The increase in risk was greatest within 12 to 24 hours of exposure to fine particles, and "was most strongly associated with markers of traffic-related pollution," they write, before concluding that exposures to fine particulate matter ( under 2.5 µm diameter) that is "considered generally safe by the US EPA increase the risk of ischemic stroke onset within hours of exposure".

Written by Catharine Paddock PhD