Even a small increase in the levels of fine particle pollution (PM2.5) is associated with a greater risk of dementia for people living in those areas, according to a study conducted in the US.

The researchers at the University of Washington used data from two large, long-running study projects -- one that began in the late 1970s measuring air pollution and another on risk factors for dementia that began in 1994.

They identified a link between PM2.5 or particulate matter 2.5 micrometres or smaller and dementia.

"We found that an increase of 1 microgramme per cubic metre of exposure corresponded to a 16 per cent greater hazard of all-cause dementia. There was a similar association for Alzheimer''s-type dementia," said study lead author Rachel Shaffer, who conducted the research as a doctoral student at the University of Washington.

The study, published in the journal Environmental Health Perspectives on August 4, looked at over 4,000 Seattle-area residents enrolled in the Adult Changes in Thought (ACT) Study.

Of those residents, the researchers identified more than 1,000 people who had been diagnosed with dementia at some point since the ACT Study began in 1994.

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

For instance, if a person was diagnosed with dementia at 72 years old, the researchers compared the pollution exposure of other participants over the decade prior to when each one reached 72.

The researchers found that just a 1 microgramme per cubic meter difference between residences was associated with 16 per cent higher incidence of dementia.

While there are many factors such as diet, exercise and genetics associated with the increased risk of developing dementia, air pollution is now recognised to be among the key potentially modifiable risk factors, the researchers said.

The latest results add to this body of evidence suggesting air pollution has neurodegenerative effects and that reducing people''s exposure to air pollution could help reduce the burden of dementia.

"How we've understood the role of air pollution exposure on health has evolved from first thinking it was pretty much limited to respiratory problems, then that it also has cardiovascular effects, and now there's evidence of its effects on the brain," said Ms Shaffer.

"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," she added.