Humidity-Dependent Decay of Viruses, but Not Bacteria, in Aerosols and Droplets Follows Disinfection Kinetics

Abstract

The transmission of some infectious diseases requires that pathogens can survive (i.e., remain infectious) in the environment, outside the host. Relative humidity (RH) is known to affect the survival of some microorganisms in the environment; however, the mechanism underlying the relationship has not been explained, particularly for viruses. We investigated the effects of RH on the viability of bacteria and viruses in both suspended aerosols and stationary droplets using traditional culture-based approaches. Results showed that viability of bacteria generally decreased with decreasing RH. Viruses survived well at RHs lower than 33% and at 100%, whereas their viability was reduced at intermediate RHs. We then explored the evaporation rate of droplets consisting of culture media and the resulting changes in solute concentrations over time; as water evaporates from the droplets, solutes such as sodium chloride in the media become more concentrated. Based on the results, we suggest that inactivation of bacteria is influenced by osmotic pressure resulting from elevated concentrations of salts as droplets evaporate. We propose that the inactivation of viruses is governed by the cumulative dose of solutes or the product of concentration and time, as in disinfection kinetics. These findings emphasize that evaporation kinetics play a role in modulating the survival of microorganisms in droplets.

COVID-19 vulnerability: the potential impact of genetic susceptibility and airborne transmission

“…In recent laboratory testing, aerosolized SARS-CoV-2 remained viable for up tõ 3 h [21] when tested at a relative humidity of 65%. Inactivation studies suggested that survival on surfaces, and in the air, may be further enhanced at relative humidities of less than 50% [40]. These results are consistent with other airborne viruses-including SARS-CoV-1 [21,41], influenza H1N1 [42,43], and MERS-CoV [44][45][46], which also show evidence of airborne transmission.…”

Virus survival in evaporated saliva microdroplets deposited on inanimate surfaces

“…The dramatic increase in salt concentrations, and/or the time of exposure to high salt concentrations and reduced pH likely cause damage to the virus. This in turn suggests that the evaporation kinetics of suspended droplets on surfaces may affect the survival of viruses, in a similar manner as that suggested in aerosol droplets 24 . Thus, complete dryness could in fact protect the virions from those high concentrations of dissolved salts and low pH 5,19 .…”

“…While survival varies between virus species, increased survival at both low (<40%) and very high (>90%) RH is often observed, with decreased survival at intermediate RH levels [20][21][22]25,27 . The underlying mechanism of this U-shaped survival as a function of RH is not clear 24 . Only a few studies have attempted to gain mechanistic understanding of how RH affects virus stability in microdroplets.…”

“…Still, it is not clear how well viruses survive in saliva microdroplets following their deposition on surfaces, and what factors are in play therein. The impact of RH on the stability and viability of SARS-CoV-2 and other enveloped viruses has been studied mostly in airborne droplets or aerosols [20][21][22][23][24][25][26] and less on surfaces [27][28][29] .…”

“…Prominent among these are the pioneering studies of the Marr group 5,24,26,30 , which point to the role of the solvent composition, evaporation dynamics, and RH on virus survival in aerosols and sessile droplets.…”