The microphysics of surrogates of exhaled aerosols from the upper respiratory tract

Mucin Colocalizes with Influenza Virus and Preserves Infectivity in Deposited Model Respiratory Droplets

The stability of influenza virus in respiratory particles varies with relative humidity (RH) and protein content. This study investigated the decay, or loss of infectivity, of influenza A virus (IAV) in 1-μL respiratory droplets deposited on a surface with varying concentrations of mucin, one of the most abundant proteins in respiratory mucus, and examined the localization of virions within droplets. IAV remained stable at 0.1% and 0.5% mucin in phosphate-buffered saline (PBS) over 4 h at 20%, 50%, and 80% RH, with a maximum decay of 1.2 log10/mL. In contrast, in pure PBS droplets, the virus decayed by at least 2.6 log10/mL after 4 h at 50% and 80% RH. Mucin's protective effect was independent of its concentration, except at 80% RH after 4 h. Confocal microscopy of the particles revealed that at 20% and 50% RH, mucin led to thicker coffee rings and dendritic patterns where virions colocalized with mucin. At 80% RH, no morphological difference was observed between PBS-only and mucin-containing droplets, but virions still colocalized with mucin in the center of droplets with 0.5% mucin. Analysis by digital droplet PCR showed that mucin helped maintain virus integrity. To our knowledge, this is the first study to localize influenza virus in model respiratory droplets. The results suggest that mucin's colocalization with virions in droplets may protect the virus from environmental stressors, enhancing its stability.

Evaporation Kinetics and Final Particle Morphology of Multicomponent Salt Solution Droplets

In both nature and industry, aerosol droplets contain complex mixtures of solutes, which in many cases include multiple inorganic components. Understanding the drying kinetics of these droplets and the impact on resultant particle morphology is essential for a variety of applications including improving inhalable drugs, mitigating disease transmission, and developing more accurate climate models. However, the previous literature has only focused on the relationship between drying kinetics and particle morphology for aerosol droplets containing a single nonvolatile component. Here we investigate the drying kinetics of NaCl-(NH4)2SO4, NaCl-NH4NO3, and NaCl-CaCl2 mixed salt aqueous aerosol droplets (25-35 μm radius) and the resulting morphology and composition of the dried microparticles. A comparative kinetics electrodynamic balance was used to measure evaporation profiles for each mixed salt aerosol at a range of relative humidities (RH) (0-50% RH); measurements of the evaporation kinetics are shown to be consistent with predictions from the "Single Aerosol Drying Kinetics and Trajectories" model. Populations of the mixed salt droplets were dried in a falling droplet column under different RH conditions and imaged using scanning electron microscopy to observe the impact of the drying kinetics on the morphology. Energy dispersive spectroscopy was used in tandem to obtain atomic maps and view the impact of drying kinetics on the composition of the resultant particles. It has been shown that the relationship between drying kinetics and dry particle morphology in mixed salt solution droplets is compositionally dependent and determined by the predominant salts that crystallize (i.e., (NH4)2SO4, Na2SO4, or NaCl). The degree of homogeneity in composition throughout the particle microstructure is dependent on the drying rate.

Ambient carbon dioxide concentration correlates with SARS-CoV-2 aerostability and infection risk

An improved understanding of the underlying physicochemical properties of respiratory aerosol that influence viral infectivity may open new avenues to mitigate the transmission of respiratory diseases such as COVID-19. Previous studies have shown that an increase in the pH of respiratory aerosols following generation due to changes in the gas-particle partitioning of pH buffering bicarbonate ions and carbon dioxide is a significant factor in reducing SARS-CoV-2 infectivity. We show here that a significant increase in SARS-CoV-2 aerostability results from a moderate increase in the atmospheric carbon dioxide concentration (e.g. 800 ppm), an effect that is more marked than that observed for changes in relative humidity. We model the likelihood of COVID-19 transmission on the ambient concentration of CO2, concluding that even this moderate increase in CO2 concentration results in a significant increase in overall risk. These observations confirm the critical importance of ventilation and maintaining low CO2 concentrations in indoor environments for mitigating disease transmission. Moreover, the correlation of increased CO2 concentration with viral aerostability need to be better understood when considering the consequences of increases in ambient CO2 levels in our atmosphere.