Dementia Incidence Tied to Long-Term Exposure to Wildfire Smoke

PHILADELPHIA — Long-term exposure to wildfire smoke was linked to later diagnoses of dementia in Southern California, a retrospective cohort study of older adults found.

An increase of 1 μg/m³ of fine particulate matter 2.5 micrometers (PM_2.5) or less in diameter from wildfires in the 3-year average exposure was associated with a 21% increase in odds of dementia diagnosis (adjusted OR 1.21, 95% CI 1.06-1.38), reported Holly Elser MD, PhD, of the Hospital of the University of Pennsylvania in Philadelphia, and colleagues in a poster presented at the 2024 Alzheimer’s Association International Conference.

They also found that for non-wildfire PM_2.5, a 3 μg/m³ increase was associated with a 3% increase in odds of dementia diagnosis (OR 1.03, 95% CI 1.02-1.04). Subgroup analyses suggested slightly stronger, though statistically insignificant, links between wildfire PM_2.5 exposure and dementia diagnoses among Asian, Black, and Hispanic populations compared with non-Hispanic white populations.

“In the U.S., environmental exposures often disproportionately impact racialized and economically marginalized groups,” Elser noted in an email to MedPage Today. The effects of environmental exposures may be further compounded by economic constraints, like an inability to pay for an air filtration system, “and members of marginalized groups may have amplified physiologic responses to environmental exposures due to the compounding effects of discrimination and chronic exposure to psychosocial stressors,” she suggested.

Fine particulate matter exposure in the U.S. is linked to the greatest proportion of adverse health effects related to air pollution, and wildfires are a major source. Several studies have investigated links between neurologic health and the environment, especially between air quality and incident dementia. Air pollution is considered a key modifiable risk factor for dementia, according to a 2020 Lancet Commission report.

Boya Zhang, PhD, of the University of Michigan in Ann Arbor, found in a 2023 study that higher concentrations of PM_2.5 from all sources, including wildfire particulates, were associated with greater rates of incident dementia.

In an email to MedPage Today, Zhang noted that the authors of the current study found a stronger association with wildfire smoke in Southern California, where wildfires seem to be a more severe issue compared to other parts of the U.S. “This finding suggests the need for future studies to evaluate how the association between wildfire smoke and incident dementia varies by region,” she said.

Researchers used electronic health record data from Kaiser Permanente Southern California from 2008-2019 for 1.23 million members ages 60 and older. Those with a dementia diagnosis before entry or missing address information were excluded. Of this cohort, 53% were women, and 6.6% were diagnosed with dementia during the study period. Most identified as non-Hispanic white (49%) or Hispanic (26%) individuals.

Using PM_2.5 estimates from participant census tracts, researchers calculated 3-year rolling mean wildfire and non-wildfire particulate matter concentrations between 2006-2019, updated quarterly. PM_2.5 estimates were measured via monitoring and remote sensing data. Dementia diagnoses were identified using ICD-9 and ICD-10 diagnostic codes. Statistical models were adjusted for characteristics including age, sex, race, ethnicity, marital status, and smoking status derived from electronic health records. Further adjustments were made for calendar year, census tract-level poverty, and population density.

Though the study did not explore mechanisms for this connection, Elser explained, “Wildfire PM_2.5 contains higher concentrations of oxidative and pro-inflammatory compounds, has a smaller average particle size, and is generated at substantially higher temperatures. These factors may collectively contribute to its risk for human health.”

Because fine particulate matter from wildfires and other sources may pose a risk for dementia, Elser said, patients should stay indoors on days where the air quality index exceeds 100, or wear N95 or KN95 masks. Box fan-based air filtration too, can offer an inexpensive air quality fix, she added, and patients with respiratory conditions should plan for wildfire conditions with a healthcare provider.

Researchers acknowledged that subgroup analyses of minoritized groups were “imprecise.” Co-author of the study, Joan Casey, PhD, of the University of Washington, in Seattle, added that because of the data used, long-term exposure to particulate matter was only measured for a 3-year period on average, though dementia typically develops over a much longer period. Dementia subtype could not be evaluated because of the diagnostic codes used.