High pollution levels increase dementia risk: Study

Researchers found that just a one microgram per cubic meter difference between residences was associated with 16 per cent higher incidence of dementia

High pollution levels increase dementia risk: Study

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There is a link between improving air quality and a reduction in the risk of dementia, a new research in the US has shown.

Using data from two large, long-running study projects in the Puget Sound region -- one that began in the late 1970s measuring air pollution and another on risk factors for dementia that began in 1994 -- University of Washington (UW) researchers identified a link between air pollution and dementia.

In the UW-led study, a small increase in the levels of fine particle pollution (PM2.5 or particulate matter 2.5 micrometers or smaller) averaged over a decade at specific addresses in the Seattle area was associated with a greater risk of dementia for people living at those addresses.

"We found that an increase of one microgram per cubic meter 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 lead author Rachel Shaffer, who conducted the research as a doctoral student in the UW Department of Environmental and Occupational Health Sciences.

The study, published on August 4 in the journal Environmental Health Perspectives, looked at more than 4,000 Seattle-area residents enrolled in the Adult Changes in Thought (ACT) Study run by Kaiser Permanente Washington Health Research Institute in collaboration with UW.

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.

"The ACT Study is committed to advancing dementia research by sharing its data and resources, and we're grateful to the ACT volunteers who have devoted years of their lives to supporting our efforts, including their enthusiastic participation in this important research on air pollution," said Eric Larson, ACT's founding principal investigator and a senior investigator at KPWHRI.

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.

In these analyses, the researchers also had to account for the different years in which these individuals were enrolled in the study, since air pollution has dropped dramatically in the decades since the ACT study began.

In their final analysis, the researchers found that just a one microgram per cubic meter difference between residences was associated with 16 per cent higher incidence of dementia.

To put that difference into perspective, Shaffer said, in 2019 there was approximately one microgram per cubic meter difference in PM2.5 pollution between Pike Street Market in downtown Seattle and the residential areas around Discovery Park.

"We know dementia develops over a long period of time. It takes years -- even decades -- for these pathologies to develop in the brain, and so we needed to look at exposures that covered that extended period," Shaffer said.

And, because of long-running efforts by many UW faculty and others to build detailed databases of air pollution in our region, "we had the ability to estimate exposures for 40 years in this region. That is unprecedented in this research area and a unique aspect of our study."

In addition to extensive air pollution and dementia data for the region, other study strengths included lengthy address histories and high-quality procedures for dementia diagnoses for the ACT Study participants.

"Having reliable address histories lets us obtain more precise air pollution estimates for study participants," said senior author Lianne Sheppard, a UW professor of environmental and occupational health sciences and of biostatistics.

"These high-quality exposures combined with ACT's regular participant follow-up and standardized diagnostic procedures contribute to this study's potential policy impact."