Living in a crowded city or near a busy highway may be tied to a higher chance of having a stroke or losing your memory, new research suggests.

A study published Monday in the Archives of Internal Medicine found a higher risk of stroke after "moderate" compared to "good" air quality days in Boston-area residents, especially when traffic-related pollution was high.

And another report in the same journal documented a faster long-term decline in thinking and memory skills in women living in higher-pollution areas of the United States.

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

"Individuals that may be susceptible to air pollution health effects may consider staying inside during high air pollution levels," Wellenius told Reuters Health.

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, he added, which might trigger a stroke in people who are already at risk.

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," Wellenius said. "There may be effects... on blood flow to the brain that we're not yet aware of that could be affecting cognitive function."

A faster decline

In the other study, researchers led by Jennifer Weuve of Rush University Medical Center in Chicago analyzed 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, Weuve's team 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 U.S. are diagnosed with dementia.

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

He said the new cognition findings, because they're preliminary, shouldn't cause women to be too concerned -- but they should encourage future research into the effects of pollution on thinking and memory.

In a separate research letter also published with the studies, exposure to pollutants in secondhand smoke was found to be linked to a 39 percent higher risk of dementia in Chinese women.

Time to adjust standards?

None of the findings can prove that pollutants, themselves, are to blame for memory problems or strokes. But previous studies support negative effects on the heart and blood vessels.

While there are ways that people can limit their exposure to air pollution, researchers said that for widespread health benefits, the EPA might have to reconsider acceptable upper limits for pollutants.

"I think this is really a public health problem" with regard to stroke risk in particular, Chen told Reuters Health. "The policymakers need to realize this real threat to human health."

"Our air is of the quality where people are generally not keeling over dead because it's so bad," Weuve said.

"We've made a lot of progress in reducing ambient air pollution, however the reductions haven't been the same everywhere... and we're still (seeing) health effects, even at the current levels of exposure."

Nearly 800,000 Americans suffer a stroke every year, according to the American Heart Association.