Competing Health Impacts of Smoke from Wildfires, Agricultural Burning, and Prescribed Fires in California

Makoto Kelp, Minghao Qiu, Sam Heft-Neal, Rebecca Sandoval, Claire L Schollaert, Miriam Marlier, Marshall Burke, Noah S Diffenbaugh

Wildfire smoke is a dominant threat to air quality and public health in North America, but smoke from other fire sources can also affect health during wildfire seasons. In California, smoke from agricultural burning and prescribed fire co-occurs with wildfire smoke, yet the competing health impacts of these sources remain poorly quantified. To investigate these differential impacts, we estimate daily source-specific fine particulate matter (PM2.5) from wildfires, agricultural burning, and prescribed fires across California from 2014 to 2020 by combining empirical smoke estimates with emissions-based source attribution; and link these concentrations to statewide emergency department (ED) records of asthma-related visits. A 10 µg/m3 increase in smoke PM2.5 is associated with similar increases in asthma-related visits for wildfire (3.0% [95% CI: 2.6%, 3.5%]) and agricultural burning (2.7% [1.2%, 4.3%]), while the estimate for prescribed fire smoke is smaller and more uncertain (1.5% [-0.1%, 3.1%]). Although wildfire dominates the statewide burden, agricultural burning and prescribed fire account for 24% and 11% of smoke-attributable asthma ED visits during 2014-2020. Using the historical agricultural burning record to evaluate how planned burns relate to health impacts, we find that with 5-7% of burn days accounting for 20-25% of ED visits across the Central Valley, indicating that impacts are concentrated on a few meteorologically unfavorable days rather than scaling directly with emissions or burned area. We also find that 24% of nonburn days have atmospheric conditions associated with lower ED impacts, suggesting that additional lower impact burn windows may have existed.