Background: Influenza remains a significant public health threat with pandemic potential. Understanding environmental factors influencing virus spread and severity is critical, particularly as wildfires become more frequent and intense. While temperature and humidity´s roles in virus seasonality and persistence are well understood, the impacts of air pollution ─ especially wildfire-specific particulate matter (PM2.5) ─ on respiratory infections are less explored.

Objectives: This study aimed to investigate the association between wildfire PM2.5 exposure and influenza or influenza-like illness (ILI) incidence. Specifically, we assessed (1) the long-term impact of PM2.5 exposure during the preceding wildfire season on influenza/ILI risk in the following flu season, and (2) the effects of short-term PM2.5 exposure during the active flu season.

Methods: We utilized ILI and influenza data from state health departments in six Western U.S. states (Arizona, Colorado, Montana, Nevada, Oregon, and Washington) from 2010 to 2019. We applied generalized linear distributed lag models to assess the impact of PM2.5 exposure during the preceding wildfire season on influenza or ILI risk in the subsequent flu season, as well as the effect of short-term PM2.5 exposure during the current flu season.

Results: Long-term exposure to wildfire PM2.5 was associated with increased influenza risk in states with influenza data: Arizona ([Rate Ratio (RR) = 1.061 (1.026-1.100)]), Colorado [RR = 1.067 (1.056-1.078)], Montana [RR = 1.038 (1.013-1.063)], and Oregon [RR = 1.049 (1.041-1.057)], per 10 μg/m3 PM2.5 increase. However, the states with only ILI data did not follow this pattern, revealing no observed effect in Nevada [RR = 1.005 (0.920-1.097)] and a negative effect in Washington [RR = 0.884 (0.842-0.919)]. Similarly, but to a lesser degree, short-term PM2.5 exposure effects were noted in states with only influenza data but not ILI data.

Discussion: Our findings underscore a positive association between wildfire-specific PM2.5 and influenza risk in states with influenza data, suggesting a differential effect of PM2.5 on respiratory infections. This study supports further investigation into the causative mechanisms behind these correlations, particularly considering the increasing frequency of wildfires and the resulting air quality impacts.