“Wildfire smoke isn’t just a respiratory issue — it affects mental health, too,” said corresponding author Kari Nadeau, John Rock Professor of Climate and Population Studies and chair of the Department of Environmental Health. “Our study suggests that — in addition to the trauma a wildfire can induce — smoke itself may play a direct role in worsening mental health conditions like depression, anxiety, and mood disorders.”

The study will be published April 4 in JAMA Network Open.

The study is the first to isolate the short-term impact of wildfire-specific PM2.5, offering more precise insights into its impacts on mental health. A growing body of research suggests that PM2.5 may influence mental health outcomes, but few studies have investigated the effects of wildfire-specific PM2.5. Most studies of wildfire-specific PM2.5 have focused on its relationship to respiratory and cardiovascular outcomes.

The researchers analyzed data on wildfire-specific PM2.5 levels and emergency department visits for mental health conditions throughout California between July and December 2020, a period covering the state’s most severe wildfire season on record. Daily wildfire-specific PM2.5 levels and visits to the ED for mental health conditions — including psychoactive substance use disorders, psychotic disorders, mood-affective disorders, depression, and anxiety — were ascertained for each zip code in the state.

Throughout the study period, there were 86,588 mental health ED visits. The average daily concentration of wildfire-specific PM2.5 was 6.95 micrograms per cubic meter of air (μg/m3), a level that rose to 11.9 μg/m3 during peak wildfire months and to 24.9 μg/m3 during the highest peak in September.

The study found that exposure to wildfire smoke substantially increased mental health ED visits. A 10 μg/m3 increase in wildfire-specific PM2.5 was linked to a higher number of visits, including for depression, anxiety, and other mood-affective disorders, for up to seven days post-exposure. Women, children and young adults, Black and Hispanic individuals, and Medicaid enrollees showed the highest risk of mental health ED visits from exposure to wildfire-specific PM2.5.

“The disparities in impact by race, sex, age, and insurance status suggest that existing health inequities may be worsened by wildfire smoke exposure,” added lead author YounSoo Jung, research associate in the Department of Environmental Health. “We need to make sure everyone has access to mental health care during wildfire seasons, particularly the most vulnerable groups and particularly as wildfires become more frequent and severe as a result of climate change.”

Other Harvard Chan authors included Mary Johnson and Caleb Dresser.

The study was supported by the National Heart, Lung, and Blood Institute (grant P01HL152953).