Repeated, long-term exposure to wildfire smoke raises risk of death years after the air clears
- 2 min. read ▪ Published
Wildfire smoke has become an increasingly common hazard around the world, due to climate-driven environmental changes, such as droughts and rising temperatures.
Although in recent years, much data has been gathered about the acute health effects of exposure to wildfire particulate matter (PM2.5), less has been known about the potential long-term effects from repeated exposures over time.
A UC Berkeley School of Public Health study published online December 15, 2025 in the prestigious Proceedings of the National Academy of Sciences (PNAS) helps fill that research gap.
Led by Lara Schwarz, a postdoctoral scholar in environmental health sciences at the School of Public Health, the research team followed a cohort of 1.25 million members of Kaiser Permanente Southern California over 60 years of age from 2009 to 2019. They found a 7% higher chance of mortality for those highly exposed to wildfire smoke than for those minimally exposed over a three year period.
Within the group that had been more exposed, researchers found that the mortality risk was greatest in individuals between age 60 and 75 and who were of Black race/ethnicity. By looking at five different metrics of wildfire smoke they found that cumulative smoke exposure better explained the differences in mortality burden than the number of exposures to smoke events. Cumulative smoke exposure captures both the intensity of short-term exposure and long-duration of low-level wildfire smoke exposure.
“Having lived in Southern California for six years, in addition to growing up in the state,” Schwarz said, “I have experienced many major wildfire smoke events and always wondered about the long-term health consequences of breathing in smoke—in addition to the short-term health effects.
“As wildfires become more frequent and severe in the context of climate change, with California being at high risk, understanding how these harmful wildfires will affect us in the long term is critical to informing better public health action and response.”
Schwarz and her co-authors noted that clean air centers are often used to provide better air quality to the public during a wildfire smoke event. Most of these clean air centers in California are activated only when the air quality exceeds a certain threshold, such as a daily air quality index of 100 to 150.
“Our results suggest that activation of public health measures such as clean air centers should not only depend on intensity of smoke, as the long-term effects can be harmful, even at low levels,” the investigators noted.
In addition, they wrote that since wildfire-smoke-related deaths are preventable it is critical to protect those who are most vulnerable.
The project is part of a broader research agenda on wildfire smoke, in collaboration with Kaiser Permanente Southern California and the University of Washington.
