As Baby Boomers hit retirement, about 1 in 6 Americans is now over the age of 65. The number of Americans living with dementia is projected to skyrocket—but the proportion of older Americans who develop dementia has actually decreased. The exact reason why is uncertain, but various lifestyle and environmental factors can influence a person's risk of cognitive decline.

One recently discovered risk is air pollution. Studies have linked exposure to a type of air pollution called fine particulate matter, or PM2.5, with an increased risk of developing dementia, and researchers suspect that some sources of PM2.5 may pose a greater risk than others.

New research led by the University of Washington has found that wildfire smoke is especially hazardous. An analysis of the healthcare records of 1.2 million Southern California residents found that higher long-term smoke exposure was associated with a significant increase in the odds that a person would be diagnosed with dementia.

The researchers presented their findings at the Alzheimer's Association International Conference in July and have published the full study in JAMA Neurology.

"There have been studies that have found total PM2.5 is related to people developing dementia, but no one had looked specifically at wildfire PM2.5," said lead author Joan Casey, a UW associate professor of environmental & occupational health sciences. "Wildfire smoke is a different thing, in that it's much spikier. There are many days where there's no wildfire smoke, and there are some days where exposure is really, really extreme."

Researchers analysed the health records of 1.2 million members aged 60 and older of Kaiser Permanente Southern California between 2008 and 2019, all of whom were free from dementia at the start of the study period. They estimated each person's long-term exposure to both wildfire and non-wildfire PM2.5 as a three-year rolling average, and then identified people who received a dementia diagnosis.

Researchers found that for every 1 microgram per cubic meter (µg/m³) increase in three-year average wildfire PM2.5 concentration, the odds of a dementia diagnosis increased by 18%. Exposure to non-wildfire PM2.5 also increased a person's risk of dementia, but to a much lesser degree.

"One microgram per meter cubed might sound fairly small, but we have to think about how people are exposed to wildfire smoke," Casey said. "Most days they aren't exposed at all, so this might represent a few days of exposure at a concentration of something like 300 µg/m³, where the AQI is over 200 in someone's community. When you think about it, it's actually a few really severe wildfire smoke days that might translate into increased risk."

That risk further increased among racialised people and those living in high-poverty census tracts, following long-term trends in which vulnerable populations often experience disproportionate effects of environmental hazards. The authors suggested that disparities might be related to lower-quality housing, which can increase the amount of smoke that enters people's homes, or lower-income families' inability to afford air filtration systems.

The study period does not include the summers of 2020 and 2021, which produced the most extreme wildfire seasons recorded in California. The climate crisis has drastically increased the frequency and severity of wildfires across the American West, introducing "smoke season" in many West Coast regions. The influx of smoke has chipped away at air quality improvements made over the last century.

"The main culprit here is climate change," Casey said. "It's a global problem. While individuals can protect themselves with air filters and masks, we need a global solution to climate change. It's going to have to be many-pronged—many people have to be involved to solve this highly complex problem."

Co-authors on this study are Holly Elser of the University of Pennsylvania; Timothy Frankland of the Kaiser Permanente Hawaii Center for Integrated Health Research; Chen Chen and Tarik Benmarhnia of the Scripps Institution of Oceanography at UC San Diego; Sara Tartof and Gina Lee of Kaiser Permanente Southern California; Elizabeth Rose Mayeda of UCLA; Dr Alexander Northrop of Columbia University; and Jacqueline Torres of UC San Francisco.