Modification by influenza on health effects of air pollution in Hong Kong.

Background: Both influenza viruses and air pollutants have been well documented as major hazards to human health, but few epidemiologic studies have assessed effect modification of influenza on health effects of ambient air pollutants.
Objectives: We aimed to assess modifying effects of influenza on health effects of ambient air pollutants.
Methods: We applied Poisson regression to daily numbers of hospitalizations and mortality to develop core models after adjustment for potential time-varying confounding variables. We assessed modification of influenza by adding variables for concentrations of single ambient air pollutants and proportions of influenza-positive specimens (influenza intensity) and their cross-product terms.
Results: We found significant effect modification of influenza (p < 0.05) for effects of ozone. When influenza intensity is assumed to increase from 0% to 10%, the excess risks per 10-microg/m(3) increase in concentration of O(3) increased 0.24% and 0.40% for hospitalization of respiratory disease in the all-ages group and >or= 65 year age group, respectively; 0.46% for hospitalization of acute respiratory disease in the all-ages group; and 0.40% for hospitalization of chronic obstructive pulmonary disease in the >or= 65 group. The estimated increases in the excess risks for mortality of respiratory disease and chronic obstructive pulmonary disease in the all-ages group were 0.59% and 1.05%, respectively. We found no significant modification of influenza on effects of other pollutants in most disease outcomes under study.
Conclusions: Influenza activity could be an effect modifier for the health effects of air pollutants particularly for O(3) and should be considered in the studies for short-term effects of air pollutants on health.