Relative humidity in droplet and airborne transmission of disease

Abstract

A large number of infectious diseases are transmitted by respiratory droplets. How long these droplets persist in the air, how far they can travel, and how long the pathogens they might carry survive are all decisive factors for the spread of droplet-borne diseases. The subject is extremely multifaceted and its aspects range across different disciplines, yet most of them have only seldom been considered in the physics community. In this review, we discuss the physical principles that govern the fate of respiratory droplets and any viruses trapped inside them, with a focus on the role of relative humidity. Importantly, low relative humidity—as encountered, for instance, indoors during winter and inside aircraft—facilitates evaporation and keeps even initially large droplets suspended in air as aerosol for extended periods of time. What is more, relative humidity affects the stability of viruses in aerosol through several physical mechanisms such as efflorescence and inactivation at the air-water interface, whose role in virus inactivation nonetheless remains poorly understood. Elucidating the role of relative humidity in the droplet spread of disease would permit us to design preventive measures that could aid in reducing the chance of transmission, particularly in indoor environment.

Effect of relative humidity on hydrogen peroxide production in water droplets

“…Thus, we hypothesize that the water droplets at the high end of the percentage relative humidity that we studied would also be a more potent bactericide and virucide. This conjecture is consistent with the observations presented elsewhere that viruses are effectively killed in small water droplets when the relative humidity is in the range of 40% to 60% but survive much better when the relative humidity is less than 40% or greater than 60% (Lin and Marr, 2020;Birks and Rowlen, 2020;Božič and Kanduč, 2021). Although not without some controversy (Heneghan et al, 2021;Greenhalgh et al, 2021;Niazi et al,2021), it does seem well established that viral respiratory diseases are primarily transmitted by airborne droplets.…”

“…All droplets formed from the mist maker are believed to start with the same H 2 O 2 concentration but then undergo evaporation at different rates depending on the relative humidity, The rate of evaporation depends on many factors and has been reviewed elsewhere (Božič and Kanduč, 2021). H 2 O 2 becomes increasingly concentrated in the remaining water droplet (Hultman et al, 2007).…”

“…Because respiratory droplets are saturated (Walker and Wells, 1961) while ambient air typically is not, a vapor pressure gradient is produced between the droplet surface and the ambient air that is the driving force for evaporation, which has been shown to be influenced by relative humidity (Marr et al, 2019). The equilibrium size of a droplet, which is attained within a few seconds (Shaman and Kohn, 2009;Halloran et al, 2012;Nicas and Jones, 2006) is dependent on RH (Marr et al, 2019), and the size of the droplet determines its gravitational settling speed (Xie et al, 2007;Božič and Kanduč, 2021). The changes in chemistry within these smaller droplet sizes have been proposed as a cause of influenza virus degradation.…”

“…The stability of viruses may depend on chemical events such as osmotic bursting or changes in pH that may alter a protein's geometry (Marr et al, 2019). Virus inactivation at the air-water interface of an aerosolized droplet has also been proposed although the mechanism is poorly understood (Božič and Kanduč, 2021).…”

Recent advancements in the measurement of pathogenic airborne viruses

“…RH and temperature are also main environmental factors that can affect the stability and viability of a virus in air. Previous studies suggested that non-enveloped viruses are able to survive longer at high RHs (70 to 90%) while enveloped viruses survive longer at low RHs (<30%) ( Božič and Kanduč, 2021 ); however, these observations are not always consistent. Moreover, enveloped viruses are observed to be more stable at low temperature and susceptible to damage at high temperature due to dysfunction of their phospholipid bilayers ( Price et al, 2019 ).…”