Comparison of wildfire smoke estimation methods and associations with cardiopulmonary-related hospital admissions

Abstract

Climate forecasts predict an increase in frequency and intensity of wildfires. Associations between health outcomes and population exposure to smoke from Washington 2012 wildfires were compared using surface monitors, chemical-weather models, and a novel method blending three exposure information sources. The association between smoke particulate matter ≤2.5 μm in diameter (PM_2.5) and cardiopulmonary hospital admissions occurring in Washington from 1 July to 31 October 2012 was evaluated using a time-stratified case-crossover design. Hospital admissions aggregated by ZIP code were linked with population-weighted daily average concentrations of smoke PM_2.5 estimated using three distinct methods: a simulation with the Weather Research and Forecasting with Chemistry (WRF-Chem) model, a kriged interpolation of PM_2.5 measurements from surface monitors, and a geographically weighted ridge regression (GWR) that blended inputs from WRF-Chem, satellite observations of aerosol optical depth, and kriged PM_2.5. A 10 μg/m³ increase in GWR smoke PM_2.5 was associated with an 8% increased risk in asthma-related hospital admissions (odds ratio (OR): 1.076, 95% confidence interval (CI): 1.019-1.136); other smoke estimation methods yielded similar results. However, point estimates for chronic obstructive pulmonary disease (COPD) differed by smoke PM_2.5 exposure method: a 10 μg/m³ increase using GWR was significantly associated with increased risk of COPD (OR: 1.084, 95%CI: 1.026-1.145) and not significant using WRF-Chem (OR: 0.986, 95%CI: 0.931-1.045). The magnitude (OR) and uncertainty (95%CI) of associations between smoke PM_2.5 and hospital admissions were dependent on estimation method used and outcome evaluated. Choice of smoke exposure estimation method used can impact the overall conclusion of the study.

Figure 1

Time series of the range (minimum to maximum) of ZIP code population‐weighted PM_2.5 concentrations of wildfire smoke from 1 July to 31 October 2012 for WRF‐Chem smoke, kriging smoke, and geographically weighted ridge regression (GWR) smoke by ecology region.

Figure 2

Counts of days in each ZIP code impacted by smoke in Washington from 1 July to 31 October 2012. A smoke day is any day where GWR smoke population‐weighted PM_2.5 was >10 μg/m³. Smoke counts were not estimated for some areas on the map (indicated in white) as there were no events reported at these ZIP codes.

Figure 3

Association between a 10 μg/m³ increase in smoke PM_2.5 (using three estimation methods) and risk for a cardiopulmonary emergency department or urgent care hospital admission, adjusting for temperature, relative humidity, wind speed, and precipitation.

Figure 4

Lag analysis (0–5 days) for associations between a 10 μg/m³ increase in smoke PM_2.5 (using either WRF‐Chem smoke or GWR smoke) and risk for a cardiopulmonary emergency department or urgent care hospital admissions.

Figure 5

Age‐stratified association between a 10 μg/m³ increase in WRF‐Chem smoke and GWR smoke PM_2.5 and risk for a cardiopulmonary emergency department or urgent care hospital admission, adjusting for temperature, relative humidity, wind speed, and precipitation.

Figure 6

Sex‐stratified association between a 10 μg/m³ increase in WRF‐Chem smoke and GWR smoke PM_2.5 and risk for a cardiopulmonary emergency department or urgent care hospital admission, adjusting for temperature, relative humidity, wind speed, and precipitation.