Air pollution linked to higher risk 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," says Lianne Sheppard. (Credit: Getty Images)

Researchers have identified a link between air pollution and a greater hazard of all-cause dementia. They found a similar association for Alzheimer’s-type dementia.

The team from the University of Washington used 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 that began in 1994 on risk factors for dementia.

The findings show 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 1 microgram per cubic meter of exposure corresponded to a 16% greater hazard of all-cause dementia. There was a similar association for Alzheimer’s-type dementia,” says Rachel Shaffer, who conducted the research as a doctoral student in the environmental & occupational health sciences department and is lead author of the paper in Environmental Health Perspectives.

Extended periods of exposure

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

Once researchers identified a patient with dementia, they 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 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 1 microgram per cubic meter difference between residences was associated with 16% higher incidence of dementia. To put that difference into perspective, Shaffer says, in 2019 there was approximately 1 microgram per cubic meter difference in PM2.5 pollution between Pike Street Market in downtown Seattle and the residential areas around Discovery Park.

Air pollution’s effect on the brain

“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 says.

Because of long-running efforts 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 let us obtain more precise air pollution estimates for study participants,” says senior author Lianne Sheppard, a 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.”

What can individuals do to lower their risk?

While there are many factors such as diet, exercise, and genetics associated with the increased risk of developing dementia, air pollution is now recognized to be among the key potentially modifiable risk factors. The new 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,” Sheppard says.

“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,” Shaffer says. “There are some things that individuals can do, such as mask-wearing, which is becoming more normalized now because of COVID.

“But it is not fair to put the burden on individuals alone. These data can support further policy action on the local and national level to control sources of particulate air pollution.”

Additional coauthors are from the University of Michigan and the University of Washington. The National Institute for Environmental Health Sciences, the National Institute on Aging, the University of Washington Retirement Association Aging Fellowship, and the Seattle chapter of the Achievement Rewards for College Scientists Foundation funded the work.

Source: University of Washington