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, 8 (2), e57485

High Humidity Leads to Loss of Infectious Influenza Virus From Simulated Coughs


High Humidity Leads to Loss of Infectious Influenza Virus From Simulated Coughs

John D Noti et al. PLoS One.


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 µM, 1-4 µM, and >4 µM 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 µM 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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.


Figure 1
Figure 1. Three-dimensional view of the simulated examination room.
National Institute of Occupational Safety and Health (NIOSH) samplers collected aerosols through the mouth, 10 cm on either side of the manikin’s mouth, and at 3 other positions (P1, P2, P3) as shown. The mouths of the coughing and breathing simulators and sampler inlets at P1, P2, and P3 were located 152 cm above the floor (approximate mouth height of a patient sitting on an examination table and a standing healthcare worker). All dimensions adjacent to white arrows within the room are in centimeters.
Figure 2
Figure 2. Cough aerosol particle optical size distribution.
A particle counter was positioned just below the coughing manikin’s mouth. Each bar represents the total volume of the aerosol particles in that size range expelled during a single cough. The amount of virus in the particles is proportional to the aerosol volume. The plot shows the mean and standard deviation of 30 coughs (six sets of five coughs each performed as described in the Methods).
Figure 3
Figure 3. High humidity reduces the infectivity of influenza.
Influenza virus was coughed into the examination room and NIOSH samplers collected aerosol samples for 60 minutes from the manikin’s mouth, 10 cm to the right and left of the mouth, and at positions P1 and P2 within the room. At constant temperature (20°C), the RH was varied over 7–73%.The percentage of virus that retained infectivity relative to that prior to coughing is shown. A, The percentage of infectious virus from all fractions (>4 µm, 1–4 µm, and <1 µm) was determined by the viral plaque assay (VPA) and is shown. B–D, The percentage of infectious virus within each aerosol fraction is shown. Data are means ± standard error (n = 5).
Figure 4
Figure 4. Loss of infectivity at moderate humidity occurs rapidly after coughing.
Influenza virus was coughed into the examination room and NIOSH samplers collected aerosol samplers positioned on the outside wall of the examination room (P3) to enable immediate processing of the collected samples. Aerosol samples were collected at 16–30 min, 31–45 min, 46–60 min, and 4–5 h after coughing at 20% RH and 45% RH. The temperature of the examination room was maintained at 20°C throughout the collection periods. A,C,E,G, Amounts of total virus (infectious and noninfectious) from all aerosol fractions (>4 µm, 1–4 µm, and <1 µm) collected at each time interval was determined by quantitative polymerase chain reaction (qPCR). B,D,F,H, The number of infectious virus collected at each timepoint from all aerosol fractions was determined by viral plaque assay. The amount of virus collected at each 15 minute interval during the initial 60 minutes was totaled and shown as the “Total” on the X-axis of each graph. Data are means ± standard errors (n = 3 for each aerosol fraction assayed).

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Grant support

This work was supported by the National Institute for Occupational Safety and Health and the Centers for Disease Control and Prevention. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.