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

Abstract

BackgroundThe role of relative humidity in the aerosol transmission of influenza was examined in a simulated examination room containing coughing and breathing manikins.MethodsNebulized 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.ResultsTotal 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.ConclusionAt 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.

Update of the scientific evidence for specifying lower limit relative humidity levels for comfort, health, and indoor environmental quality in occupied spaces (RP-1630)

“…Influenza virus survival exhibited a canonical dip between 40% and 80% RH in many studies (Harper, 1961;Hemmes et al, 1962;Lowen et al, 2007;Noti et al, 2013). For nearly all cases, virus survival declined relative to increased length of exposure.…”

“…The virus literature featured a combination of laboratory testing (Akers et al, 1966;Lowen et al, 2007;McDevitt et al, 2010;Noti et al, 2013;de la Noue et al, 2014), epidemiology (Shaman and Kohn, 2009;Shaman et al, 2010;Barreca and Shimshack, 2012;van Noort et al, 2012;Jaakkola et al, 2014), modeling (Shaman and Kohn, 2009;Yang and Marr, 2011), and review papers (Morawska, 2006;Brankston et al, 2007;Weber and Stilianakis, 2008;Tang, 2009;Memarzadeh, 2012;; test conditions are shown in Figure 4. The review paper by Weber and Stilianakis (2008) identified three methods of transmission: droplet (sneezing or coughing), airborne, and contact; the review paper by Brankston et al (2007) also discussed influenza survival and transmission.…”

“…Several studies considered influenza transmission (Lowen et al, 2007;Noti et al, 2013;Lowen and Steel, 2014). Lowen et al (2007) demonstrated reduced influenza transmission at 50% RH compared to 20% and 35% RH in a guinea pig model.…”

“…In a subsequent paper, Lowen and Steel, 2014 analyzed the data as a function of absolute humidity instead of RH; they did not observe a trend with absolute humidity. Noti et al (2013) simulated influenza transmission via coughing using manikins, and observing five times more infectious virus in the low humidity range (7%-23% RH) than above 43% RH. The evidence suggested that influenza viability and transmission is higher at lower humidities, with minimal transmission and survival occurring at midrange relative humidities (around 50% RH).…”

“…Bacteria are constructed with different envelopes, usually termed Gram-positive or Gram-negative, but even in these subcategories, the behavior at low humidities varies (Theunissen et al, 1993). The review by Tang Comparisons of laboratory studies of pathogens (Dunklin and Puck, 1948;Akers et al, 1966;Theunissen et al, 1993;Ko et al, 2000;Lowen et al, 2007;Lopez et al, 2013;Noti et al, 2013) posed a challenge in this analysis due to different lethality levels amongst organism types. For example, many studies reported a starting virus titer and percent survival as a function of time.…”