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

Abstract

Wildfires are becoming more frequent and destructive in a changing climate. Fine particulate matter, PM_2.5, in wildfire smoke adversely impacts human health. Recent toxicological studies suggest that wildfire particulate matter may be more toxic than equal doses of ambient PM_2.5. Air quality regulations however assume that the toxicity of PM_2.5 does not vary across different sources of emission. Assessing whether PM_2.5 from wildfires is more or less harmful than PM_2.5 from other sources is a pressing public health concern. Here, we isolate the wildfire-specific PM_2.5 using a series of statistical approaches and exposure definitions. We found increases in respiratory hospitalizations ranging from 1.3 to up to 10% with a 10 μg m^-3 increase in wildfire-specific PM_2.5, compared to 0.67 to 1.3% associated with non-wildfire PM_2.5. Our conclusions point to the need for air quality policies to consider the variability in PM_2.5 impacts on human health according to the sources of emission.

Fig. 1. Wildfire perimeters in Southern California (1999–2012).

Wildfire perimeters shown here represent the total area burned for a given fire during our study period comprising 1999–2012, excluding summer months (June, July, and August). The inset figure shows the location of our study region, which includes zip codes within the Santa Ana Wind domain in Southern California, USA.

Fig. 2. Mean values of PM_2.5 and rate of respiratory admissions by ZIP code.

a Mean PM_2.5 concentrations at available zip codes (county boundaries shown in black) and b mean rate of respiratory admissions (per 100,000 individuals) per zip code during 1999–2012 (summer months not considered).

Fig. 3. Wildfire-specific concentrations of PM_2.5.

Mean wildfire-specific PM_2.5 estimated by a imputation and b seasonal interpolation methods and using the fire upwind and strong SAW exposure definition.

Fig. 4. Wildfire upwind exposure estimation.

The relationship between a given zip code centroid and a fire centroid is assessed by means of geospatial tools. Fires burning upwind are considered within the context of the north-easterly direction of Santa Ana Winds. We use the distance between centroids, as well as the angle of the spatial relationship as shown here, to classify a zipcode day as exposed (or not).