Characterizing Impacts of Wildfire Smoke Exposure on Medication Refills and Outpatient Visits for Asthma during the 2015 Wildfire Season in the Western United States

Smoke from wildfires impacted several states in the western United States (U.S.) during the summer months of 2015. Our primary objective was to evaluate associations between wildfire smoke concentrations and adverse health outcomes for asthma, specifically medication refills and outpatient visits, using state-of-the-art spatio-temporal exposure models and epidemiological regression techniques

Our assessment examined employer-based health insurance claims for asthma for a period between June 1^st and September 30^th, 2015, covering 11 states in the western U.S. We linked approximately 450,000 asthma medication refills and 85,000 outpatient visit records with population-weighted daily average smoke concentrations estimated from a geographically weighted ridge regression (GWR) model. The GWR model blends information from surface monitors, a chemical-weather model, and satellite data products to estimate smoke particulate matter ≤2.5 microns in diameter (PM_2.5). A conditional quasi-Poisson regression with month/day/place strata in combination with a distributed lag non-linear model (DLNM), was used to measure the overall cumulative effect of smoke concentrations. This regression framework explored how the associations were distributed over a 7-day lag period and included additional predictors, such as, cross-basis terms for daily measures of ozone, relative humidity, and temperature.

The estimated (95% Confidence Interval [CI]) risk ratios [RRs]) associated with a 10 µg/m³ increase in smoke PM_2.5 exposure for asthma medication refills and outpatient visits were 1.066 (1.047 – 1.085) and 1.108 (1.065 – 1.152), respectively. RRs estimated on the day of smoke exposure were the highest, but delayed effects of smoke on these health outcomes were statistically significant and persisted until 5 days following the smoke exposure.

Asthma medication refills and outpatient visits are important morbidity measures to consider during wildfires. A comprehensive strategy to mitigate adverse health impacts associated with wildfire disasters necessitates a thorough understanding of population-level exposures and health impacts associated with surface smoke PM_2.5 concentrations.