Using a Novel Spatiotemporal Exposure Assessment to Estimate Respiratory Health Effects during the 2008 Northern California Wildfires

Background: We investigated the health effects associated with fine particulate matter during a particularly long-lived, large wildfire complex in northern California in the summer of 2008. Methods: We estimated exposure to PM_2.5 for each day using an optimal exposure prediction model created through data-adaptive machine learning methods from a large set of spatiotemporal data including output from a chemical transport model, two measures of satellite aerosol optical depth, distance to the nearest fires from satellite-derived fire detection points, meteorological data, and land use, traffic, spatial location, and temporal characteristics. We then used Poisson generalized estimating equations (GEE) to calculate the population-averaged effect of exposure to PM_2.5 on cardiovascular and respiratory hospitalizations and ED visits during these fires. We further assessed effect modification by sex, age, and area-level socio-economic status (SES). Results: Our exposure model performed well with a cross-validated R² of 0.72. The variables that contributed most to the model were GASP AOD, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) model PM_2.5 estimates, and surface pressure. Asthma hospitalizations (RR=1.08 (95% CI 1.04, 1.11) per 10 µg/m³ increase) and ED visits (RR = 1.20 (95% CI: 1.09, 1.34) per 10 µg/m³ increase) were significantly associated with PM_2.5 during the fires, but not in the periods before and after the fires. These effects were more pronounced in women compared to men and adults compared to children and the elderly. We did not find consistently significant effects of wildfire smoke on cadiovascular outcomes. Conclusion: Using a novel spatiotemporal exposure model that combined information from satellite data, chemical transport models, and ground observations, we found that hospitalization and ED visits for asthma were significantly associated with PM_2.5 from the 2008 northern California wildfires. Given that wildfire smoke exposure is increasing in many parts of the world due to climate change and other drivers, more research is needed to better understand the impacts of these exposures on health and which subpopulations are more affected.