Abstract
Ambient air pollution and seasonal influenza both contribute to respiratory morbidity, yet their potential synergistic effects remain unclear. Understanding how these two exposures interact is crucial for developing integrated health protection strategies. We conducted a time-series analysis to assess whether community-level influenza activity modifies the short-term associations between air pollution and respiratory emergency department (ED) visits. We acquired daily ED visits for all respiratory illnesses, pneumonia, asthma, and chronic obstructive pulmonary disease (COPD) from six U.S. metropolitan areas (2005-2017). Weekly influenza activity was measured by laboratory-confirmed influenza hospitalization rates from hospital surveillance system. We used quasi-Poisson models to estimate associations between PM(2.5) and NO(2) levels and ED visits during the influenza season. Effect modification by influenza activity was examined through quartile level and spline-based interaction terms. PM(2.5) was consistently associated with all respiratory outcomes, while NO₂ associations were more variable across locations. Adjustment for influenza activity slightly attenuated air pollution and ED visit associations. We observed evidence of positive effect modification by influenza activity, which varied by pollutants, outcomes and locations. For example, in Atlanta the relative risk for respiratory ED visits per interquartile range increase in PM(2.5) was 1.035 (95% CI: 1.020, 1.051) during peak influenza periods (4th quartile) versus 0.998 (0.975, 1.021) during low influenza periods (first quartile). In San Francisco, the corresponding relative risks were 1.013 (95% CI: 1.006, 1.024) versus 0.999 (0.993, 1.006). Highest NO2 associations were also during peak influenza periods in pooled analyses of pneumonia and COPD ED visits. Influenza activity modifies short-term associations between air pollution and respiratory ED visits in complex, non-linear ways, with amplification during moderate or high influenza circulation. These findings highlight the need for integrated air quality and infectious disease surveillance to mitigate seasonal respiratory health burdens.