Modification by influenza on air pollution effects In this study, we studied the impact of influenza on health effects of air pollution at lag 0–1 days. These lag days were recom‑ mended and chosen a priori for presentation of the main results in the PAPA studies. However, when considering the impact of influenza on health effects of air pollutant at longer lag days, we observed more statistical significant estimates for modifying effects of influenza on hospitalization outcomes (data not shown). We did not adjust for potential changes in mortality counts in year 2001 due to a switch of mortality codes from ICD‑ 9 to ICD‑10. Unfortunately , because the data were collected through a computerized system during the routine medical practice, we were unable to do the sensitivity analysis to assess the impact of switching codes on our results. However, because the changes in mortality counts were estimated to be small (Tsang and Cheung 2005), and because they affected only a short time period in the middle of the study period, we believe the impacts of inconsistent ICD codes on the results would be minimal. Conclusions In this epidemiologic study of interactions between air pollution and influenza activity, we have shown that influenza modifies the effects of air pollution on mortality and hospi‑ talization of RDs. These results support a link between air pollution mainly for O3 and influ‑ enza activity in their effects on human health, which have been demonstrated in animal stud‑ ies. Further research is needed to clarify what conditions produce observable interactions between effects of air pollution and influenza and to provide the evidence for causality. c orectin In Table 1, the values for ARD were incor‑ rect in the manuscript originally published online. They have been corrected here. 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