Research 3

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

Background

Scientists examined the role of relative humidity in the aerosol transmission of influenza 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 mM, 1–4 mM, and .4 mM 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 a relative humidity of 23% but only 14.6–22.2% at a relative humidity of 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.

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 centimetres.

Figure2: 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 (20uC), 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 mm, 1–4 mm, and ,1 mm) 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 means 6 standard errors (n = 5).

Conclusion

At low relative humidity, influenza retains maximal infectivity and inactivation of the virus at higher relative humidity occurs rapidly after coughing. Although viruses carried on aerosol particles ,4 mM 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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