Associations between respiratory health and ozone and fine particulate matter during a wildfire event

Wildfires have been increasing in frequency in the western United States (US) with the 2017 and 2018 fire seasons experiencing some of the worst wildfires in terms of suppression costs and air pollution that the western US has seen. Although growing evidence suggests respiratory exacerbations from elevated fine particulate matter (PM_2.5) during wildfires, significantly less is known about the impacts on human health of ozone (O₃) that may also be increased due to wildfires. Using machine learning, we created daily surface concentration maps for PM_2.5 and O₃ during an intense wildfire in California in 2008. We then linked these daily exposures to counts of respiratory hospitalizations and emergency department visits at the ZIP code level. We calculated relative risks of respiratory health outcomes using Poisson generalized estimating equations models for each exposure in separate and mutually-adjusted models, additionally adjusted for pertinent covariates. During the active fire periods, PM_2.5 was significantly associated with exacerbations of asthma and chronic obstructive pulmonary disease (COPD) and these effects remained after controlling for O₃. Effect estimates of O₃ during the fire period were non-significant for respiratory hospitalizations but were significant for ED visits for asthma (RR = 1.05 and 95% CI = (1.022, 1.078) for a 10 ppb increase in O₃). In mutually-adjusted models, the significant findings for PM_2.5 remained whereas the associations with O₃ were confounded. Adjusted for O₃, the RR for asthma ED visits associated with a 10 μg/m³ increase in PM_2.5 was 1.112 and 95% CI = (1.087, 1.138). The significant findings for PM_2.5 but not for O₃ in mutually-adjusted models is likely due to the fact that PM_2.5 levels during these fires exceeded the 24-hour National Ambient Air Quality Standard (NAAQS) of 35 μg/m³ for 4976 ZIP-code days and reached levels up to 6.073 times the NAAQS, whereas our estimated O₃ levels during the fire period only occasionally exceeded the NAAQS of 70 ppb with low exceedance levels. Future studies should continue to investigate the combined role of O₃ and PM_2.5 during wildfires to get a more comprehensive assessment of the cumulative burden on health from wildfire smoke.