Background: The role of relative humidity in the aerosol transmission of influenza was examined in a simulated examination room containing coughing and breathing manikins. Methods: Nebulized influenza was coughed into the examination room and Bioaerosol samplers collected size-fractionated aerosols (<1 micorM, 1-4 micorM, and >4 microM aerodynamic diameters) adjacent to the breathing manikin's mouth and also at other locations within the room. At constant temperature, the RH was varied from 7-73% and infectivity was assessed by the viral plaque assay. Results: Total virus collected for 60 minutes retained 70.6-77.3% infectivity at relative humidity = / <23% but only 14.6-22.2% at relative humidity = / >43%. Analysis of the individual aerosol fractions showed a similar loss in infectivity among the fractions. Time interval analysis showed that most of the loss in infectivity within each aerosol fraction occurred 0-15 minutes after coughing. Thereafter, losses in infectivity continued up to 5 hours after coughing, however, the rate of decline at 45% relative humidity was not statistically different than that at 20% regardless of the aerosol fraction analyzed. Conclusion: At low relative humidity, influenza retains maximal infectivity and inactivation of the virus at higher relative humidity occurs rapidly after coughing. Although virus carried on aerosol particles <4 microM have the potential for remaining suspended in air currents longer and traveling further distances than those on larger particles, their rapid inactivation at high humidity tempers this concern. Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus.
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