Abstract
The detailed physico-chemical characteristics of respiratory droplets in ambient air, where they are subject to evaporation, are poorly understood. Changes in the concentration and phase of major components in a droplet—salt (NaCl), protein (mucin) and surfactant (dipalmitoylphosphatidylcholine)—may affect the viability of any pathogens contained within it and thus may affect the efficiency of transmission of infectious disease by droplets and aerosols. The objective of this study is to investigate the effect of relative humidity (RH) on the physico-chemical characteristics of evaporating droplets of model respiratory fluids. We labelled these components in model respiratory fluids and observed evaporating droplets suspended on a superhydrophobic surface using optical and fluorescence microscopy. When exposed to continuously decreasing RH, droplets of different model respiratory fluids assumed different morphologies. Loss of water induced phase separation as well as indication of a decrease in pH. The presence of surfactant inhibited the rapid rehydration of the non-volatile components. An enveloped virus, ϕ6, that has been proposed as a surrogate for influenza virus appeared to be homogeneously distributed throughout the dried droplet. We hypothesize that the increasing acidity and salinity in evaporating respiratory droplets may affect the structure of the virus, although at low enough RH, crystallization of the droplet components may eliminate their harmful effects.
Highlights
There is growing evidence that transmission of some infectious diseases via the airborne route is important [1,2,3,4,5]
We investigated the effect of relative humidity (RH), mediated through evaporation of water, on the physico-chemical characteristics of droplets of model respiratory solutions, whose composition is listed in table 1
We assessed the detailed physico-chemical characteristics of respiratory droplets in ambient air exposed to different RH
Summary
There is growing evidence that transmission of some infectious diseases via the airborne route is important [1,2,3,4,5]. There appears to be a relationship between transmission and environmental conditions for a subset of diseases. Multiple lines of evidence, including epidemiological studies [6,7,8] and laboratory studies with animal models [9,10] suggest that there is a relationship between incidence or transmission and humidity. What remains unclear is how humidity could affect virus viability. A common misperception is that airborne viruses are naked particles floating around in air. If the virion itself is not in direct contact with air, how could its viability be affected by humidity?
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