Multi-Day Wildfire Smoke Exposure Thresholds & Distributions for an Oregon Health Impacts Study

Wildfire smoke has drastically impacted air quality in Oregon, increasing fine particulate matter (PM_2.5) levels during wildfire events by up to 100% in the past two decades. Recent years have seen the most severe wildfire seasons on record with fires growing larger, intensifying, and spreading more rapidly due to climate change. Fires like the 2020 Labor Day fires, which burned over 1 million acres in Oregon, may become more frequent under future climate scenarios. Smoke PM_2.5 poses significant respiratory and cardiovascular health risks, increasing the burden on healthcare systems. Few studies have examined the duration of exposure to smoke PM_2.5, especially among children and various demographic groups. Utilizing daily census tract-level smoke PM_2.5 data from 2006 to 2020 provided by the Environmental Change and Human Outcomes (ECHO) Lab at Stanford University, we defined high smoke days (smoke PM_2.5 > 15 µg/m³) and medium smoke days (smoke PM_2.5 between 9-15 µg/m³). The 15 µg/m³ threshold aligns with the World Health Organization (WHO) guideline that recommends 24-hour average exposures should not exceed this concentration more than 3-4 days per year, and both thresholds are consistent with Doubleday et al. (2020). Across 834 Oregon census tracts from 2006 to 2020, we identified 25,317 total smoke events (one or more consecutive days with smoke PM_2.5 ≥ 9 µg/m³ in a single census tract). Several census tracts experienced over 60 events and 300 total event days. Over 60% of these days exceeded the high threshold, predominantly from 2016 to 2020. Long-term events (7+ days) had a greater fraction of high threshold days than shorter events, with the longest event spanning 40 days. Over 900 events exceeded a cumulative smoke PM_2.5 concentration of 1000 µg/m³. These findings will inform a case-crossover study using Oregon hospitalization data from 2012 to 2020 to investigate the associations between smoke PM_2.5 exposure duration and hospitalizations among different age groups and demographics.