The Association between Wildfire Smoke Exposure and Respiratory Morbidity and Mortality in North America

Epidemiological studies consistently show an association between wildfire-related smoke exposure and adverse respiratory health outcomes, indicated by hospital and emergency department (ED) admissions, and mortality. Non-wildfire specific studies of air pollution and human health show that impacts can be heterogeneous, however there has been no prior review of evidence pertaining to heterogeneity of effects from wildfire smoke exposure. Through systematic review of published literature, we abstracted relative risk estimates for the association between wildfire smoke exposure and risk of ED and/or hospital admissions for respiratory conditions in North America. We calculated the within-study ratio of relative risks (RRR) and 95% confidence intervals (CI) to examine heterogeneity of effect across socio-demographic groups or by other factors, and then summarized the RRRs across studies using meta-analysis. We found evidence of slightly greater effect of wildfire smoke on respiratory health outcomes for females than for males, for all respiratory conditions (RRR: 1.006, 95% CI: 1.001, 1.011) and also specifically, for asthma (RRR: 1.016, 95% CI: 1.006, 1.026). There was evidence of slightly higher relative risks for all respiratory-related hospital or ED admissions for youth than for adults (RRR: 1.011, 95% CI: 1.006, 1.017). Wildfire smoke effects reported in the literature were also stratified by categories of income, race, educational attainment, health behaviors, access to care, percent occupied housing, geographic region, urban/rural status and other aspects of the physical environment. However, data were insufficient calculation of RRRs by these characteristics. We find evidence of small differences in risk of respiratory health outcomes by sex and gender, and this information might be used to protect vulnerable groups in wildfire planning and response.