Key Takeaways

• Wildfire smoke may be particularly hazardous to brain health, according to a 10-year study of more than 1.2 million southern Californians.
• Exposure to wildfire smoke increases the risk of being diagnosed with dementia more than other forms of air pollution.
• The risk of exposure to fine particulate matter (PM_2.5) air pollution is much more pronounced when the source is wildfire smoke than when it comes from other sources, such as motor vehicles and factories.
• To lower their risk, people should update their home air filtration systems when possible, stay inside when the air quality is unhealthy, and wear an N95 mask outside when the Air Quality Index reaches 100.

Exposure to wildfire smoke increases the risk of being diagnosed with dementia more than other types of air pollution, according to a decade-long study of more than 1.2 million people in southern California. The findings, reported today at the Alzheimer’s Association International Conference^® (AAIC^®) 2024, in Philadelphia and online, suggest the brain health threat posed by wildfire smoke is higher than other forms of air pollution.

Wildfire smoke, motor vehicles and factories all emit a type of air pollution called fine particulate matter (PM_2.5). This is a microscopic mixture of solid and liquid droplets in the air that are 30 times smaller than the width of an average human hair. Researchers found that the risk of dementia diagnosis due to exposure to PM_2.5 in wildfire smoke was notably stronger — even with less exposure — than the risk due to the other sources of PM_2.5 air pollution. Exposure to non-wildfire PM_2.5 raised the risk of dementia diagnosis, but not as much as wildfire smoke.

High levels of PM_2.5 also have been shown to raise the risk of heart disease, asthma and low birth weight.

“With the rising global incidence of wildfires, including in California and the western U.S., exposure to this type of air pollution is an increasing threat to brain health,” said Claire Sexton, DPhil, Alzheimer’s Association senior director of scientific programs and outreach. “These findings underscore the importance of enacting policies to prevent wildfires and investigating better methods to address them.”

Researchers analyzed the health records of 1,227,241 socioeconomically diverse Kaiser Permanente southern California members who were 60 years or older between 2009-2019, none of whom had been diagnosed with dementia at the beginning of the study. Total PM_2.5 was estimated from various sources, including satellite-derived aerosol properties and Environmental Protection Agency monitoring. Researchers used air quality monitoring data, satellite imagery and machine learning techniques to separate wildfire and non-wildfire PM_2.5. They determined each study participant’s exposure to both sources of PM_2.5 according to where they lived. They compared that information to subsequent diagnoses of dementia in participants’ health records.

Reported for the first time at AAIC 2024, the researchers observed a 21% increase in the odds of dementia diagnosis for every increase of 1 microgram per meter — or µg/m³, which is the amount of particulate matter in a cubic meter of air — in the three-year average wildfire PM_2.5 exposure. Comparatively, they determined study participants had a 3% increased risk of dementia diagnosis for every increase of 3 µg/m³ in the three-year average of non-wildfire PM_2.5 exposure.

“Previous research has found that exposure to PM_2.5 is associated with dementia, but in light of our large, long-term study, it’s apparent the risk from exposure due to wildfire smoke is an even bigger concern,” said Holly Elser, M.D., Ph.D., the study’s first author and a neurology resident at the Hospital of the University of Pennsylvania, Philadelphia. “Air pollution produced by wildfires now accounts for more than 70% of total PM_2.5 exposure on poor air quality days in California. This is a real problem.”

Dr. Elser noted several reasons why PM_2.5 produced by wildfires might be more hazardous to health: they are produced at higher temperatures, contain a greater concentration of toxic chemicals and, on average, are smaller in diameter than PM_2.5 from other sources. She said more research needs to be done to determine the exact mechanisms.

“The findings appeared most pronounced among individuals from racially and ethnically minoritized groups and in high poverty areas,” said Joan A. Casey, Ph.D., senior author of the study and assistant professor in the Department of Environmental and Occupational Health Sciences at the University of Washington, Seattle. “These findings underscore that clinical and health policies seeking to prevent dementia-associated disparities should include efforts to reduce exposure to long-term wildfire and non-wildfire PM_2.5.”

Drs. Elser and Casey recommend that people update their air filtration systems and check the air quality on their weather app if they use one. An Air Quality Index (AQI) number of 100 or higher means the air is unhealthy to breathe. To reduce their risk when the AQI is 100 or higher, people should stay inside when possible and close the windows, and wear an N95 mask when they go outside.

Exposure to Wildfire Smoke Raises Risk of Dementia | alz.org