Abstract
The airborne dynamics of respiratory droplets, and the transmission routes of pathogens embedded within them, are governed primarily by the diameter of the particles. These particles are composed of the fluid which lines the respiratory tract, and is primarily mucins and salts, which will interact with the atmosphere and evaporate to reach an equilibrium diameter. Measuring organic volume fraction (OVF) of cough aerosol has proved challenging due to large variability and low material volume produced after coughing. Here, the diametric hygroscopic growth factors (GF) of the cough aerosol produced by healthy participants were measured in situ using a rotating aerosol suspension chamber and a humidification tandem differential mobility analyser. Using hygroscopicity models, it was estimated that the average OVF in the evaporated cough aerosol was 0.88 ± 0.07 and the average GF at 90% relative humidity (RH) was 1.31 ± 0.03. To reach equilibrium in dry air the droplets will reduce in diameter by a factor of approximately 2.8 with an evaporation factor of 0.36 ± 0.05. Hysteresis was observed in cough aerosol at RH = ∼35% and RH = ∼65% for efflorescence and deliquescence, respectively, and may depend on the OVF. The same behaviour and GF were observed in nebulized bovine bronchoalveolar lavage fluid.
Highlights
Understanding the mechanisms governing the transmission and survival of respiratory pathogens has been of considerable importance in attempting to control the spread of airborne pathogens such as SARS-CoV-2 [1,2]
Physical models with simple assumptions of the overall composition were compared to the measured values to estimate the organic volume fraction (OVF) of the aerosol, and it was found that organics account for an estimated 0.88 ± 0.07 of the dry solute volume
Hysteresis behaviour was observed in one sample at approximately 65% and 35% relative humidity (RH) for deliquescence and efflorescence, respectively
Summary
Understanding the mechanisms governing the transmission and survival of respiratory pathogens has been of considerable importance in attempting to control the spread of airborne pathogens such as SARS-CoV-2 [1,2]. The composition of ASL is understood to consist primarily of water and solids, including mucins (mucin 5AC and 5B), inorganic salts (NaCl, KCl), and pulmonary surfactants [3,4,5]. These inorganic salts, and to a lesser extend the mucins, are hygroscopic, so that when droplets containing these species are exposed to the ambient environment, the droplets will grow or evaporate to reach a thermodynamic equilibrium with the relative humidity (RH) of the environment. Understanding the hygroscopicity and state hysteresis of human respiratory aerosol is an important step in determining both the composition of the micro-environment to which airborne pathogens are exposed, and the equilibrium particle sizes required to predict airborne dynamics
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