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Review
. 2021 Mar;47(1):1-29.
doi: 10.1007/s10867-020-09562-5. Epub 2021 Feb 10.

Relative humidity in droplet and airborne transmission of disease

Anže Božič  1 , Matej Kanduč  2
Affiliations
Review

Relative humidity in droplet and airborne transmission of disease

Anže Božič et al. J Biol Phys. 2021 Mar.

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.

Keywords: Aerosol; Airborne transmission; Droplets; Efflorescence; Relative humidity; Viruses.

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Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

Fig. 1
Fig. 1
Seasonality of four different respiratory viruses (influenza virus, adenovirus, respiratory syncytial virus (RSV), and coronavirus) in Paris during years 2012 to 2015. Shaded regions show the winter period (November to April). Image adapted from Ref. [19] under a Creative Commons Attribution (CC BY 4.0) License
Fig. 2
Fig. 2
Indoor RH (blue bars; left scale) and outdoor temperature (red line; right scale) over the period of one year in a residential building. Shaded region shows the winter period (November to April). Data for Gothenburg, Sweden [30, 35]
Fig. 3
Fig. 3
Droplet and airborne spread of respiratory droplets. The size of the droplets influences whether they sediment quickly (droplet/fomite spread) or persist in the air for a longer period of time (airborne spread)
Fig. 4
Fig. 4
Droplet size distribution while talking. Blue circles show measurements by Xie et al. [46], performed at RH = 70%, and the solid red line shows the fit of (1)
Fig. 5
Fig. 5
a Droplet falling through air with a constant, sedimentation (terminal) velocity vsed. The gravitational force is balanced by the Stokes drag. b Evaporating droplet stagnant in air. The vapor density profile (inset) is given by (4). c Wells falling-evaporation diagram, showing evaporation (blue lines; (3)) and sedimentation (orange line; (6)) times of droplets as a function of the initial radius of the droplet
Fig. 6
Fig. 6
Drying out of a respiratory droplet. The droplet progressively evaporates and shrinks in size. Its final size depends on the amount of non-water solutes in the droplet and on the ambient RH—the larger the RH, the larger the final size of the droplet residue
Fig. 7
Fig. 7
a Illustration of hygroscopic shrinking and growth of a NaCl-water droplet. Upon dehydration, a liquid droplet undergoes efflorescence at RH ≈ 40% and crystallizes, whereas upon hydration, a NaCl particle undergoes deliquescence at RH = 75% and turns into a liquid droplet. b Radius of NaCl-water droplets containing bovine serum albumin protein with an initial dry mass fraction of 10%, measured upon dehydration (Data points replotted from Mikhailov et al. [85]). The red dashed line is a fit of (8) to the data points for RH > 50%. The green-shaded region denotes the most typical ambient conditions of RH = 30 to 50%. c Size distribution of droplet residues at different RH, recalculated from Fig. 4 using (8)
Fig. 8
Fig. 8
a Fraction of the initial pathogen load from a cough according to (11). The distribution of droplet residues is taken from the distributions in Fig. 7c, assuming ϕ0 = 10%. Shown for RH = 50% and RH = 30%; in the latter case both in the presence (dashed line) and in the absence (solid line) of efflorescence. b Ratio of the pathogen loads at RH = 50% and RH = 30% from panel (a)
Fig. 9
Fig. 9
a Airflow that influences small aerosol droplets in a typical indoor setting (from left to right): Air exchange, walking, door vortices, thermal plume, heating convection. b Different deposition mechanisms of aerosol particles: Interception, inertial impaction, and diffusion
Fig. 10
Fig. 10
a Particle deposition loss rate coefficient βR as a function of particle radius R, measured in a 14 m3 furnished room, either without ventilation or ventilated by fans of two different intensities (data from Ref. [109]). b Decay of aerosol concentration of different sizes, mimicking the effect of RH on a respiratory droplet residue with R = 5 μ m at RH = 50% (blue solid line) and R = 3 to 4.5 μ m at RH = 30% (orange-shaded band). Calculated using (12) with the air-exchange rate of λ = 5 h− 1
Fig. 11
Fig. 11
a Viability of Langat virus with respect to RH at different times after aerosolization (data replotted from [137]). b Viability of influenza A virus after aerosolization in different media: Mainly salts (phosphate-buffered saline (PBS), Dulbecco’s modified Eagle’s medium (DMEM)), salts with addition of proteins (fetal calf serum (FCS)), and mucus (data replotted from [145]). Lines are guides to the eye
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References

    1. Fernstrom, A., Goldblatt, M.: Aerobiology and its role in the transmission of infectious diseases. J. Pathog. 2013 (2013) - PMC - PubMed
    1. La Rosa G, Fratini M, Libera SD, Iaconelli M, Muscillo M. Viral infections acquired indoors through airborne, droplet or contact transmission. Ann. Ist. Super. Sanita. 2013;49:124–132. - PubMed
    1. Verreault D, Moineau S, Duchaine C. Methods for sampling of airborne viruses. Microbiol. Mol. Biol. Rev. 2008;72:413–444. - PMC - PubMed
    1. Belser, J.A., Maines, T.R., Tumpey, T.M., Katz, J.M.: Influenza a virus transmission: contributing factors and clinical implications. Expert Rev. Mol. Med. 12 (2010) - PubMed
    1. Gralton J, Tovey E, McLaws M-L, Rawlinson WD. The role of particle size in aerosolised pathogen transmission: a review. J. Infection. 2011;62:1–13. - PMC - PubMed

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