Department of Research & Evaluation News

Wildfire smoke has increasingly been recognized as a serious public health threat. Now, 2 new Kaiser Permanente studies in Southern California have examined how health risks for both young and old people might be associated with exposure.

One study showed that long-term exposure increased the risk of death among older adults, while another found that prenatal exposure late in pregnancy is associated with a higher risk of autism for offspring.

Together, these findings emphasize the need for stronger awareness and protection for people in wildfire zones as wildfires continue to intensify across the nation.

Study examining mortality followed more than 1 million people

Published in the journal Proceedings of the National Academy of Sciences in December 2025, the first study followed more than 1.25 million Kaiser Permanente Southern California members aged 60 and older from 2008 to 2019 to understand how long-term wildfire smoke exposure influenced mortality.

Researchers examined 5 different wildfire smoke metrics to capture how often and how intensely people had been exposed.

The study found that the most highly exposed people faced 7% higher odds of death compared to those with minimal exposure. Adults between ages 60 and 75, people who were Black or lived in higher-poverty census tracts experienced stronger effects. Cumulative wildfire smoke exposure, meaning the combined impact of smoke duration and intensity over time, was the strongest predictor of mortality.

Wildfire exposure had long term impact on health

“These findings underscore that wildfire smoke exposure was not just a short-term nuisance. It was a long-term health threat,” said study author Sara Tartof, PhD, MPH, an infectious disease epidemiologist at the Kaiser Permanente Southern California Department of Research & Evaluation. “Even lower levels of smoke exposure, when repeated over years, could increase the risk of death.”

The study also highlighted differences in vulnerability across populations.

Adults younger than 75 showed higher sensitivity to long-term smoke exposure, which may reflect differences in health status, behavior, or access to indoor air protection. When looking at differences by poverty level, those who lived in higher poverty census tracts had higher risk of mortality from wildfire smoke than those who lived in lower poverty census tracts, although the differences were not statistically significant for any exposure metric.

“Our results suggest that high-poverty census tracts may be more vulnerable to the effects of wildfire smoke, but further studies are needed,” Dr. Tartof said. “These disparities highlight the need for targeted public health strategies to protect people who face the greatest risk.”

Second study examined wildfire smoke and pregnancy  

A second study examined whether wildfire smoke exposure during pregnancy increased the risk of autism. The study evaluated more than 200,000 mother and child pairs within Kaiser Permanente Southern California between 2006 and 2014. Researchers assessed daily address-specific wildfire fine inhalable particulate matter known as PM2.5 to identify trimester-specific exposure levels.

This study used a different population and time frame than the mortality research. While the mortality study looked at census tract exposure over many years, the autism study focused on daily exposure of pregnant members’ home addresses to identify sensitive windows during fetal development.

Third trimester appears most vulnerable

This study did not find that exposure to average wildfire-related PM2.5 particulates across pregnancy was associated with autism risk. However, exposure to more than 10 wildfire smoke days during the third trimester was associated with a 23% higher risk of autism by age 5. Short periods of exposure, known as smoke waves, lasting 2 or 3 consecutive days during the third trimester were also associated with increased autism risk.

“Although this study showed an association, and not causation, and requires additional studies, it does provide us with important early evidence that wildfire smoke may affect fetal brain development, particularly during late pregnancy,” said the study’s co-senior author Anny Xiang, PhD, MS, the director of the Department of Research & Evaluation’s Division of Biostatistics Research. “As wildfires become more frequent, understanding these risks is crucial to protecting pregnant people and their babies.”

The study was published in Environmental Science & Technology on Jan. 20, 2026.

Wildfire smoke and mortality study authors from the Department of Research & Evaluation include Sara Y. Tartof, PhD, MPH; Gina S. Lee, PhD; and Yuqian M. Gu, PhD.

Wildfire smoke and autism study authors from the Department of Research & Evaluation include Anny H. Xiang, PhD; Jane C. Lin, PhD; Ting Chow, PhD, and Mayra P. Martinez, PhD.