September 5, 2023

Air pollution linked to dementia cases

At a Glance

  • Higher levels of a type of air pollution called PM2.5 were linked to a higher number of dementia cases developing over time.
  • The strongest links between pollution and dementia were seen for PM2.5 from agriculture and wildfires.
  • Reducing such exposures might help lower the incidence of dementia.
Silhouette of elderly man walking alone outside a city on a hazy, polluted day. The study adds to a growing body of evidence that air pollution can have effects in the body well beyond the lungs. Alex Linch / Shutterstock

Dementia is a loss of thinking, remembering, and reasoning skills. It’s not a normal part of aging. But the risk of developing dementia grows as people get older. Millions of Americans, along with their families and caregivers, are living with the effects of dementia. This number is expected to continue to grow as the population ages.

Exposure to a type of air pollution called fine particulate matter, or PM2.5, has recently been identified as a potential risk factor for dementia. The reason for the connection is not yet well understood. Researchers do know that fine particulates can affect the lungs, circulate in the blood, and move into the brain where they might be able to cause direct damage.

But there are many sources of fine particulate matter, and it hasn’t been clear whether PM2.5 pollution from some sources pose greater risks than others. Particulates from various sources, such as traffic, agriculture, and smoke, can be physically and chemically different.

An NIH-funded study led by Drs. Boya Zhang and Sara Adar from the University of Michigan examined the links between different types of PM2.5 air pollution and dementia. They looked at data from more than 27,000 adults aged 50 and older in the Health and Retirement Study, an ongoing national study of aging, collected between 1992 and 2016. As part of the study, participants underwent cognitive testing every two years or had caretakers report on their memory and cognitive function. The mean period of follow-up was 10.2 years. The average age of participants was 60.

The researchers estimated pollution exposures for the participants using models that included real-time pollution measurements and aspects of their homes like geography, land use, and local emissions sources. They also considered other factors that can influence dementia risk. These included age, sex, race and ethnicity, education, and household wealth. In addition, they estimated how other airborne pollutants, such as ozone, might affect the results. The study was published on August 14, 2023, in JAMA Internal Medicine.

The team found that 4,105 of the people studied—15%—developed dementia during the follow-up period. Those who did were more likely to be non-White, to have less formal education, less wealth, and to live in places with higher levels of PM2.5.

Overall, higher PM2.5 exposure was linked to an increased risk of dementia. The team also examined nine specific sources of PM2.5: agriculture, road traffic, nonroad traffic, burning coal for energy, burning coal for industry, other energy production, other industry, wildfires, and windblown dust. After consideration of all sources, PM2.5 from agriculture and wildfires were specifically associated with an increased risk of dementia.

The researchers estimated that, if PM2.5 exposure truly is a cause of cognitive decline and dementia, as many as 188,000 cases of dementia per year might be due to PM2.5.

“As we experience the effects of air pollution from wildfires and other emissions locally and internationally, these findings contribute to the strong evidence needed to best inform health and policy decisions,” says Dr Richard J. Hodes, director of NIH’s National Institute on Aging.

Further study is needed to confirm these results and better understand if reducing specific types of PM2.5 pollution would help lower the burden of dementia in the population.

Related Links

References: Comparison of Particulate Air Pollution From Different Emission Sources and Incident Dementia in the US. Zhang B, Weuve J, Langa KM, D'Souza J, Szpiro A, Faul J, Mendes de Leon C, Gao J, Kaufman JD, Sheppard L, Lee J, Kobayashi LC, Hirth R, Adar SD. JAMA Intern Med. 2023 Aug 14:e233300. doi: 10.1001/jamainternmed.2023.3300. Online ahead of print. PMID: 37578757.

Funding: NIH’s National Institute on Aging (NIA) and National Institute of Environmental Health Sciences (NIEHS); Social Security Administration.