Living in a crowded city or near a busy highway where the air quality is poor might lead to a higher chance of stroke or memory loss, according to two US studies.

Both reports appeared in the Archives of Internal Medicine, with one finding a higher risk of stroke among Boston-area residents after days where the air quality was "moderate" as opposed to "good," especially when traffic-related pollution was high.

The other study, which looked at thousands of women, documented a faster long-term decline in thinking and memory skills in women living in higher-pollution areas of the United States.

None of the findings could prove that pollutants were to blame for strokes or memory problems, but previous studies have supported findings of negative effects on the heart and on blood vessels.

"One of the important points is that at levels that are considered to be generally safe by the US EPA (Environmental Protection Agency), we're seeing important health effects," said Gregory Wellenius of Brown University in Providence, Rhode Island, who is the lead author of the stroke study.

He and his colleagues reviewed the medical records of about 1,700 patients who were admitted to Beth Israel Deaconess Medical Center in Boston with a stroke between 1999 and 2008.

Using data from a local air pollution monitoring station, the team found that the risk of having a stroke was 34 percent higher in the 24 hours after "moderate" EPA pollution readings compared to "good" pollution days.

That increased risk was greatest within 12 to 14 hours of pollution exposure, and was linked to nitrogen dioxide, a traffic-related pollutant.

Wellenius said that blood vessels dilate and constrict in response to the outside environment in an attempt to keep blood pressure constant. But air pollution might affect the body's ability to regulate blood pressure, which in turn could trigger a stroke in people who are already at risk, he added.

That same effect could explain why, over a longer period of time, being exposed to air pollution might be associated with declining thinking and memory skills.

"The blood flow to the brain is incredibly important for cognitive function. There may be effects ... on blood flow to the brain that we're not yet aware of that could be affecting cognitive function," he said.

In the other study, researchers led by Jennifer Weuve of Rush University Medical Center in Chicago analysed a series of cognitive tests given to close to 20,000 women, mostly in their 70s, and also estimated the air pollution around their homes through the EPA's monitoring system.

They found that more air pollution was tied to faster rates of cognitive decline.

For two different sizes of pollution particles, the difference in thinking and memory skills between women with some of the highest and lowest exposures was similar to a year or two of age-related decline, they reported.

That's probably not a mental change an individual person would notice, Weuve told Reuters Health. But on a population-wide scale, cleaner air might mean fewer people in the US are diagnosed with dementia.

Researchers have speculated "whether the damage of exposure to ambient air pollution may also reach the human brain," said Jiu-Chiuan Chen, an environmental health researcher at the University of Southern California's Keck School of Medicine, Los Angeles, who was not involved in either study.

He added that the findings about cognition, since they are preliminary, shouldn't cause women to be too concerned - but they should encourage future research into the effects of pollution on thinking and memory. – Reuters