A small increase in exposure to fine particle air pollution may increase a person's risk for dementia, a study published Wednesday by the journal Environmental Health Perspectives found.

Based on data from two long-running studies of air quality in the Seattle area, it appears that small rises in levels of a specific type of air pollution called PM2.5, or microscopic particulate matter, affected the dementia risk for those exposed, the researchers said.

An increase in PM2.5, or particulate matter measuring 2.5 micrometers or smaller which is about one-eighth the diameter of a human hair, of 1 microgram per cubic meter of exposure -- essentially a droplet invisible to the human eye -- elevated dementia risk by 16%, the data showed.

"Over an entire population, a large number of people are exposed. So, even a small change in relative risk ends up being important on a population scale," study co-author Rachel Shaffer said in a press release.

"These data can support further policy action on the local and national level to control sources of particulate air pollution," said Schaffer, a researcher at the University of Washington in Seattle.

The findings are based on an analysis of data from two large, long-running studies of air quality in the Puget Sound region, which includes Seattle, according to Schaffer and her colleagues.

The first study started measuring air pollution in the region in the late 1970s.

The second study, called Adult Changes in Thought, or ACT, focused on risk factors for dementia, beginning in 1994.

Both projects were led by University of Washington researchers.

For the new analysis, Schaffer and her colleagues looked at more than 4,000 Seattle-area residents enrolled in the ACT study, which is also being led by health system Kaiser Permanente Washington.

Of the residents included in the analysis, more than 1,000 had been diagnosed with dementia since the ACT Study began in 1994, according to the researchers.

Once a patient with dementia was identified, researchers compared the average pollution exposure of each participant leading up to the age at which they were diagnosed.

For instance, if a person was diagnosed with dementia at age 72, the researchers compared the pollution exposure of other participants over the previous decade to when each reached the same age.

The researchers accounted for the difference years in which these individuals were enrolled in the study, since air pollution has dropped dramatically in the decades since the ACT study began, they said.

In 2019, there was approximately 1 microgram per cubic meter difference in PM2.5 pollution -- enough to raise dementia risk by 16% -- between Pike Place Market in downtown Seattle and the residential areas around Discovery Park in the city, 6 miles away, the researchers said.

While there are many factors such as diet, exercise and genetics are associated with the increased risk of developing dementia, air pollution exposure is now recognized to be key as well.

"We know dementia develops over a long period of time -- it takes years, even decades, for these pathologies to develop in the brain," Shaffer said.

"There are some things that individuals can do, such as mask-wearing, which is becoming more normalized now because of COVID-19, but it is not fair to put the burden on individuals alone," according to the researchers.

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