Research links exposure to wildfire smoke with an increased risk of dementia

As baby boomers retire, about 1 in 6 Americans is over 65. The number of Americans with dementia is expected to skyrocket, but the share of older Americans who develop dementia has actually declined. The exact reason why is uncertain, but several lifestyle and environmental factors can influence a person’s risk of cognitive decline.

A recently discovered risk is air pollution. Studies have made a connection Exposure to a type of air pollution called fine particulate matter, or PM2.5, carries 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 particularly dangerous. An analysis of the health care records of 1.2 million Southern California residents found that higher long-term smoke exposure was associated with a significant increase in the likelihood that someone 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 shown that total PM2.5 is linked to people developing dementia, but no one has looked specifically at PM2.5,” said lead author Joan Casey, an associate professor of environmental and occupational health sciences at the UW. “Wildfire smoke is a different animal because it’s much more spiky. There are a lot of days where there’s no wildfire smoke, and there are days where the exposure is really, really extreme.”

Researchers analyzed the health records of 1.2 million members aged 60 and older at Kaiser Permanente Southern California between 2008 and 2019, all of whom were free of 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 diagnosed with dementia.

Researchers found that for every 1 microgram per cubic meter (μg/m³) increase in the three-year average PM2.5 concentration during wildfires, the chance of a diagnosis of dementia increased by 18%. Exposure to PM2.5, other than wildfires, also increased the risk of dementia, but to a much lesser extent.

“One microgram per square meter may sound pretty small, but we need to think about how people are exposed to wildfire smoke,” Casey said. “Most days they are not exposed at all, so this could amount to a few days of exposure to a concentration of around 300 µg/m³where the AQI is more than 200 in one’s community. If you think about it, it’s actually some really bad wildfire smoke days that could translate into increased risk.”

That risk further increased among racialized people and people living in high-poverty census tracts, reflecting long-term trends in which vulnerable populations often experience disproportionate impacts from environmental hazards. The authors suggested that the differences may be related to lower-quality housing, which can increase the amount of smoke entering people’s homes, or to the inability of lower-income families to afford air filtration systems.

The study period does not include the summers of 2020 and 2021, which produced the most extreme wildfire seasons in California. The climate crisis has dramatically increased the frequency and severity of wildfires in the American West, introducing a “smoke season” in many West Coast regions. The influx of smoke has wiped out improvements in air quality over the past century.

“The main culprit here is climate change,” Casey said. “It is a global problem. While individuals can protect themselves with air filters and masks, we need a global solution to climate change. It will have to be on multiple fronts: many people need to be involved to solve this extremely complex problem.”

Co-authors of 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.