We critically investigate the claim put forward by Sagripanti and Lytle ( Photochem. Photobiol. 83, 1278–1282) that inactivation of influenza A virus by solar radiation can explain the seasonality of influenza epidemics. We correct an error in the Sagripanti and Lytle paper and show that changes in relative humidity and temperature affect influenza virus inactivation as strongly as variation in solar radiation. Furthermore, it appears unlikely that transmission in outdoor settings plays an important role during influenza outbreaks, because influenza A virus is sensitive to a wide range of environmental factors.