A recent study sponsored and authored by the Center for Disease Control (CDC) and the National​ Institute for Occupational Safety and Health (NIOSH) discovered that maintaining humidity levels above 40%RH reduces the activity of the flu virus significantly.

Scientists used a mannequin and a pump to simulate a cough containing the flu virus.  Samples were then collected for 60 minutes to determine the infectivity of the virus.  At low levels of relative humidity (<23%) the virus retained over 70% infectivity.  However, when the humidity was raised to 43% the virus retained only 15 – 22% of its infectivity.