Association between Classroom Relative Humidity Levels and Incidence of Influenza-Like Illness among Teachers

Abstract

Under laboratory conditions, seasonal influenza, a group of viruses typically having the most activity during the winter, is most stable at low relative humidity (RH) levels. During an epidemic, transmission of influenza within a community is increased when school is in session. Thus, decreasing the spread of influenza in schools is of utmost importance. We examined the association between classroom RH levels and self-reported influenza-like illness (ILI) among a cohort of teachers (n=122) followed for up to 12 weeks during the school year. Data were analyzed using modified Poisson regression models for correlated binary data, clustered by participant. On days when the buildings were occupied, classroom RH observations taken during the heating season (outdoor temperature: <15oC) had higher risk of low RH (<30%) compared to observations taken during transitional periods (outdoor temperature: 15-23oC) [Risk Ratio (RR): 4.32 (95% Confidence Interval (CI): 3.73, 5.00)]. Teachers in classrooms with low RH had 1.20 (95% CI: 0.29, 4.95) times the risk of ILI compared to teachers in classrooms with recommended RH. Low classroom relative humidity levels during the heating season may allow influenza to be easily transmissible.