Wildfire smoke impacts respiratory health more than fine particles from other sources: observational evidence from Southern California

Abstract

The Western U.S. and regions worldwide are experiencing more frequent and destructive wildfires in a changing climate. Fine particulate matter, PM2.5, is the main component of wildfire smoke that adversely impacts human health. Wildfires account for 29% of total PM2.5 emissions in the US, 50% in California, and this proportion is expected to increase in the coming years. Recent animal toxicological studies suggest that wildfire particulate matter may be more toxic than equal doses of PM2.5 from other sources. Current air quality regulations in the US however assume that the toxicity of PM2.5 does not vary across different sources of emission. Assessing whether PM2.5 from wildfires is more or less harmful than PM2.5 from other sources is a pressing public health concern. To answer this question, we developed and compared results from a series of statistical approaches that isolate the wildfire-specific PM2.5 from other sources using different exposure definitions including smoke plumes and the presence of wildfire upwind. Our study covered a large population in Southern California where Santa Ana (SAW) wind-driven wildfires are most damaging. Strong SAWs in the region can increase PM2.5 concentrations by spreading smoke downwind. Overall, we conclude that wildfire-specific PM2.5 is up to 10 times more harmful than PM2.5 from other sources. Specifically, we found increases in respiratory hospital admissions of 9.8% (95% CI: 3.3, 16.3) and 0.90% (95% CI: 0.59, 1.2) associated with a 10 μg m-3 increase in wildfire-specific PM2.5 and non-wildfire PM2.5 respectively. Our conclusions are robust across several approaches used to isolate wildfire-specific PM2.5, and point to the need for air quality policies to consider the variability in PM2.5 impacts on human health according to the sources of emission. This is especially relevant in California and other regions where most PM2.5 is expected to come from wildfires.