Disruption of outdoor activities caused by wildfire smoke shapes circulation of respiratory pathogens

Abstract

The consequences of wildfires on public health extend beyond injury. Smoke can traverse vast distances, compromising air quality in unaffected areas and exacerbating chronic respiratory conditions. But smoke may affect the circulation and burden of communicable diseases, too. The disruption in air quality and adherence to safety guidelines can impact the time people spend indoors, and this in turns may increase exposure to airborne pathogens like influenza, SARS-CoV-2, RSV. However, the quantification of such disruptions and their implications for the transmission of respiratory diseases remain unclear. Here we study the effects of smoke generated by severe wildfires in the U.S. states of California, Oregon, Washington in September 2020. We assess the impact on human behavior and the potential consequences for the emergence of respiratory diseases. Our findings reveal a significant shift towards indoor activities in counties within Oregon and Washington during wildfires. However, a discernible change in mobility patterns is not evident in California. This discrepancy may arise from the familiarity of Californian residents with wildfires and air quality index alerts, which have become integrated into their daily routines. Consequently, their mobility patterns may be less affected during such incidents compared to individuals in other regions. We then use a deterministic compartmental model of epidemic spread to quantify the impact of the describe behavioral changes on epidemic circulation. We found that counties with disrupted air exhibited higher cumulated and peak incidence of cases compared to unaffected counties, with the exception of California. Additionally, we found that flu-like epidemics – low reproduction ratio and short generation time – are most affected by the behavioral changes under study. Our findings may help improve public health response in a context of larger, more frequent wildfires triggered by climate change.