A variety of theories exist as to how weather and climate might exert some influence on influenza seasonality. Low temperatures enhance viral stability [4] , reduce mucosal blood flow [11] , and/or diminish mucociliary clearance [12] . Correlations exist between the number of upper respiratory infections and cold-air outbreaks [13] . It is commonly assumed that winter indoor crowding enhances influenza virus transmission, though direct evidence is lacking [8] . Low humidity conditions, which are often but not always accompanied by low temperatures, enhance survival times of viral aerosols [14] , [15] , [16] . If correct, these relationships suggest that indoor winter heating without humidification could enhance influenza transmission [8] , as indoor absolute humidity tends to be correlated with outdoor values and thus is typically lower in winter [6] .