In situ measurements of human cough aerosol hygroscopicity

Weather-related changes in the dehydration of respiratory droplets on surfaces bolster bacterial endurance

HYPOTHESIS

The study shows for the first time a fivefold difference in the survivability of the bacterium Pseudomonas Aeruginosa (PA) in a realistic respiratory fluid droplet on fomites undergoing drying at different environmental conditions. For instance, in 2023, the annual average outdoor relative humidity (RH) and temperature in London (UK) is 71 % and 11 °C, whereas in New Delhi (India), it is 45 % and 26 °C, showing that disease spread from fomites could have a demographic dependence. Respiratory fluid droplet ejections containing pathogens on inanimate surfaces are crucial in disease spread, especially in nosocomial settings. However, the interplay between evaporation dynamics, internal fluid flow and precipitation and their collective influence on the distribution and survivability of pathogens at different environmental conditions are less known.

EXPERIMENTS

Shadowgraphy imaging is employed to study evaporation, and optical microscopy imaging is used for precipitation dynamics. Micro-particle image velocimetry (MicroPIV) measurements reveal the internal flow dynamics. Confocal imaging of fluorescently labelled PA elucidates the bacterial distribution within the deposits.

FINDINGS

The study finds that the evaporation rate is drastically impeded during drying at elevated solutal concentrations, particularly at high RH and low temperature conditions. MicroPIV shows reduced internal flow under high RH and low temperature (low evaporation rate) conditions. Evaporation rate influences crystal growth, with delayed efflorescence and extending crystallization times. PA forms denser peripheral arrangements under high evaporation rates and shows a fivefold increase in survivability under low evaporation rates. These findings highlight the critical impact of environmental conditions on pathogen persistence and disease spread from inanimate surfaces.

Toward an Understanding of Uric Acid Particle Dissolution: Surface and Bulk-Driven Water Activity

Uric acid particles contribute to kidney stones, and natural processes for the elimination of stones depend on solute-solvent interactions. The process of uric acid dissolution has previously been understood via the lens of solubility; however, for pure and mixed salt solutions, these approaches do not provide a comprehensive picture of nanoscale particle solution thermodynamics. Unlike solubility measurements, water activity measurements provide us with information about the chemical potential responsible for the migration of water molecules driving the dissolution of particles. In this work, we used in situ experimental tools to estimate water activity values for pure uric acid aqueous droplets at different stages of droplet growth. The process of cloud formation, i.e., water condensation on a solute particle resulting in aqueous droplet formation, was leveraged to compare the water affinity for nanosized uric acid particles with a well-studied inorganic salt, sodium chloride. Specifically, we investigated microscopic uric acid particles (nanoparticles <300 nm, amorphous and super micron particles >5 μm, crystalline) and the mechanism of water uptake. The growth of droplet volume (Growth Factor, GF) for uric acid particles is experimentally observed for supermicrometer crystalline particles (>1 μm) at subsaturated humidity conditions (<100% RH). In addition, the water activity of submicrometer-size uric acid particles is estimated under subsaturated and supersaturated humidity conditions. These measurements provide us with information about the volume growth of droplets as water condenses in particles exposed to different humidity conditions. Our observations under subsaturated humidity conditions show that the uric acid particles have limited volume growth (<1% change per volume and <10% change per volume for crystalline and amorphous measurement, respectively). From the experimental data, the affinity of uric acid solute with water as a solvent is quantified in terms of water activity.

Relative Humidity-Dependent Phase Transitions in Submicron Respiratory Aerosols

Respiratory viruses, such as influenza and severe acute respiratory syndrome coronavirus 2, represent a substantial public health burden and are largely transmitted through respiratory droplets and aerosols. Environmental factors such as relative humidity (RH) and temperature impact virus transmission rates, and a precise mechanistic understanding of the connection between these environmental factors and virus transmission would improve efforts to mitigate respiratory disease transmission. Previous studies on supermicrometer particles observed RH-dependent phase transitions and linked particle phase state to virus viability. Phase transitions in atmospheric aerosols are dependent on size in the submicrometer range, and actual respiratory particles are expelled over a large size range, including submicrometer aerosols that can transmit diseases over long distances. Here, we directly investigated the phase transitions of submicrometer model respiratory aerosols. A probe molecule, Nile red, was added to particle systems including multiple mucin/salt mixtures, a growth medium, and simulated lung fluid. For each system, the polarity-dependent fluorescence emission was measured following RH conditioning. Notably, the fluorescence measurements of mucin/NaCl and Dulbecco's modified Eagle's medium particles indicated that liquid-liquid phase separation (LLPS) also occurs in submicron particles, suggesting that LLPS can also impact the viability of viruses in submicron particles and thus affect aerosol virus transmission. Furthermore, the utility of fluorescence-based measurements to study submicrometer respiratory particle physicochemical properties in situ is demonstrated.

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

Airborne transmission plays a significant role in the transmission of respiratory diseases such as COVID-19, for which the respiratory aerosol droplets are responsible for the transportation of potentially infectious pathogens. However, the aerosol physicochemical dynamics during the exhalation process are not well understood. The representativeness of respiratory droplet surrogates of exhaled aerosol and suspension media for aerosols currently used for laboratory studies remains debated. Here, we compare the evaporation kinetics and equilibrium thermodynamics of surrogate respiratory aerosol droplets including sodium chloride, artificial saliva (AS) and Dulbecco's modified Eagle's medium (DMEM) by using the Comparative Kinetics Electrodynamic Balance. The potential influences of droplet composition on aerosol hygroscopic response and phase behavior, and the influence of mucin are reported. The equilibrium hygroscopicity measurement was used to verify and benchmark the prediction of evaporation kinetics of complex solutions using the Single Aerosol Particle Drying Kinetics and Trajectory model. We show that the compositionally complex culture media which differs from sodium chloride and artificial saliva (mucin-free solutions). The DMEM evaporation dynamics contained three distinctive phases when drying at a range of humidities, including a semi-dissolved phase when evaporating at the environmental humidity range. The effect of mucin on droplet evaporation and phase behavior at low RH were compared between AS and DMEM solutions. In both cases, mucin delayed the crystallization time of the droplets, but it promoted phase change (from homogenous to semi-dissolved/spherical with inclusions) to occur at higher water activities.

Toward Standardized Aerovirology: A Critical Review of Existing Results and Methodologies

Understanding the airborne survival of viruses is important for public health and epidemiological modeling and potentially to develop mitigation strategies to minimize the transmission of airborne pathogens. Laboratory experiments typically involve investigating the effects of environmental parameters on the viability or infectivity of a target airborne virus. However, conflicting results among studies are common. Herein, the results of 34 aerovirology studies were compared to identify links between environmental and compositional effects on the viability of airborne viruses. While the specific experimental apparatus was not a factor in variability between reported results, it was determined that the experimental procedure was a major factor that contributed to discrepancies in results. The most significant contributor to variability between studies was poorly defined initial viable virus concentration in the aerosol phase, causing many studies to not measure the rapid inactivation, which occurs quickly after particle generation, leading to conflicting results. Consistently, studies that measured their reference airborne viability minutes after aerosolization reported higher viability at subsequent times, which indicates that there is an initial loss of viability which is not captured in these studies. The composition of the particles which carry the viruses was also found to be important in the viability of airborne viruses; however, the mechanisms for this effect are unknown. Temperature was found to be important for aerosol-phase viability, but there is a lack of experiments that directly compare the effects of temperature in the aerosol phase and the bulk phase. There is a need for repeated measurements between different research groups under identical conditions both to assess the degree of variability between studies and also to attempt to better understand already published data. Lack of experimental standardization has hindered the ability to quantify the differences between studies, for which we provide recommendations for future studies. These recommendations are as follows: measuring the reference airborne viability using the "direct method"; use equipment which maximizes time resolution; quantify all losses appropriately; perform, at least, a 5- and 10-min sample, if possible; report clearly the composition of the virus suspension; measure the composition of the gas throughout the experiment. Implementing these recommendations will address the most significant oversights in the existing literature and produce data which can more easily be quantitatively compared.

Dynamics and Viability of Airborne Respiratory Syncytial Virus under Various Indoor Air Conditions

The factors governing the viability of airborne viruses embedded within respiratory particles are not well understood. This study aimed to investigate the relative humidity (RH)-dependent viability of airborne respiratory syncytial virus (RSV) in simulated respiratory particles suspended in various indoor air conditions. We tested airborne RSV viability in three static indoor air conditions, including sub-hysteresis (RH < 39%), hysteresis (39% < RH < 65%), and super-hysteresis (RH > 65%) air as well as in three dynamic indoor air conditions, including the transitions between the static conditions. The dynamic conditions were hysteresis → super-hysteresis → hysteresis, sub-hysteresis → hysteresis, and super-hysteresis → hysteresis. We found that after 45 min of particle aging in static conditions, the viability of RSV in sub-hysteresis, hysteresis, and super-hysteresis air was 0.72% ± 0.06%, 0.03% ± 0.006%, and 0.27% ± 0.008%, respectively. After 45 min of aging in dynamic conditions, the RSV viability decreased for particles that remained in a liquid (deliquesced) state during aging when compared with particles in a solid (effloresced) state. The decreased viability of airborne RSV for deliquesced particles is consistent with prolonged exposure to elevated aqueous solutes. These results represent the first measurements of the survival of airborne RSV over particle aging time, with equal viability in low, intermediate, and high RHs at 5 and 15 min and a V-shaped curve after 45 min.

Generative design for COVID-19 and future pathogens using stochastic multi-agent simulation

We propose a generative design workflow that integrates a stochastic multi-agent simulation with the intent of helping building designers reduce the risk posed by COVID-19 and future pathogens. Our custom simulation randomly generates activities and movements of individual occupants, tracking the amount of virus transmitted through air and surfaces from contagious to susceptible agents. The stochastic nature of the simulation requires that many repetitions be performed to achieve statistically reliable results. Accordingly, a series of initial experiments identified parameter values that balanced the trade-off between computational cost and accuracy. Applying generative design to a case study based on an existing office space reduced the predicted transmission by around 10% to 20% compared with a baseline set of layouts. Additionally, a qualitative examination of the generated layouts revealed design patterns that may reduce transmission. Stochastic multi-agent simulation is a computationally expensive yet plausible way to generate safer building designs.

Recent development in antiviral surfaces: Impact of topography and environmental conditions

The transmission of viruses is largely dependent on contact with contaminated virus-laden communal surfaces. While frequent surface disinfection and antiviral coating techniques are put forth by researchers as a plan of action to tackle transmission in dire situations like the Covid-19 pandemic caused by SARS-CoV-2 virus, these procedures are often laborious, time-consuming, cost-intensive, and toxic. Hence, surface topography-mediated antiviral surfaces have been gaining more attention in recent times. Although bioinspired hydrophobic antibacterial nanopatterned surfaces mimicking the natural sources is a very prevalent and successful strategy, the antiviral prospect of these surfaces is yet to be explored. Few recent studies have explored the potential of nanopatterned antiviral surfaces. In this review, we highlighted surface properties that have an impact on virus attachment and persistence, particularly focusing and emphasizing on the prospect of the nanotextured surface with enhanced properties to be used as antiviral surface. In addition, recent developments in surface nanopatterning techniques depending on the nano-scaled dimensions have been discussed. The impacts of environments and surface topology on virus inactivation have also been reviewed.

Physicochemical characterization of porcine respiratory aerosol and considerations for future aerovirology

Understanding the mechanisms which inactivate airborne viruses is a current challenge. The composition of human respiratory aerosol is poorly understood and needs to be adequately investigated for use in aerovirology studies. Here, the physicochemical properties of porcine respiratory fluid (PRF) from the trachea and lungs were investigated both in bulk solutions and in aerosols. The mass ratio of Na:K in PRF compared with cell culture media (Dulbecco's Modified Eagle Medium, DMEM), which is frequently used in aerovirology studies, was significantly lower (∼2:1 vs ∼16:1). PRF contained significantly more potassium and protein than DMEM. PRF aerosols of all samples were similarly hygroscopic to human respiratory aerosol. PRF particles could nucleate with spatially separated crystals, indicating that the protein matrix was sufficiently viscous to prevent the complete coalescence of aqueous salts prior to efflorescence. The effects of these differences in compositions on the viability of viruses are currently not well understood. The virus suspensions in aerovirology studies need to be reconsidered to adequately reflect a real-world expiration scenario.

Viral Preservation with Protein-Supplemented Nebulizing Media in Aerosols

The outbreak of SARS-CoV-2 has emphasized the need for a deeper understanding of infectivity, spread, and treatment of airborne viruses. Bacteriophages (phages) serve as ideal surrogates for respiratory pathogenic viruses thanks to their high tractability and the structural similarities tailless phages bear to viral pathogens. However, the aerosolization of enveloped SARS-CoV-2 surrogate phi6 usually results in a >3-log₁₀ reduction in viability, limiting its usefulness as a surrogate for aerosolized coronavirus in "real world" contexts, such as a sneeze or cough. Recent work has shown that saliva or artificial saliva greatly improves the stability of viruses in aerosols and microdroplets relative to standard dilution/storage buffers like suspension medium (SM) buffer. These findings led us to investigate whether we could formulate media that preserves the viability of phi6 and other phages in artificially derived aerosols. Results indicate that SM buffer supplemented with bovine serum albumin (BSA) significantly improves the recovery of airborne phi6, MS2, and 80α and outperforms commercially formulated artificial saliva. Particle sizing and acoustic particle trapping data indicate that BSA supplementation dose-dependently improves viral survivability by reducing the extent of particle evaporation. These data suggest that our viral preservation medium may facilitate a lower-cost alternative to artificial saliva for future applied aerobiology studies. IMPORTANCE We have identified common and inexpensive lab reagents that confer increased aerosol survivability on phi6 and other phages. Our results suggest that soluble protein is a key protective component in nebulizing medium. Protein supplementation likely reduces exposure of the phage to the air-water interface by reducing the extent of particle evaporation. These findings will be useful for applications in which researchers wish to improve the survivability of these (and likely other) aerosolized viruses to better approximate highly transmissible airborne viruses like SARS-CoV-2.

The Effectiveness of Ultraviolet-C (UV-C) Irradiation on the Viability of Airborne Pseudomonas aeruginosa

Pseudomonas aeruginosa (Pa) is the predominant bacterial pathogen in people with cystic fibrosis (CF) and can be transmitted by airborne droplet nuclei. Little is known about the ability of ultraviolet band C (UV-C) irradiation to inactivate Pa at doses and conditions relevant to implementation in indoor clinical settings. We assessed the effectiveness of UV-C (265 nm) at up to seven doses on the decay of nebulized Pa aerosols (clonal Pa strain) under a range of experimental conditions. Experiments were done in a 400 L rotating sampling drum. A six-stage Andersen cascade impactor was used to collect aerosols inside the drum and the particle size distribution was characterized by an optical particle counter. UV-C effectiveness was characterized relative to control tests (no UV-C) of the natural decay of Pa. We performed 112 tests in total across all experimental conditions. The addition of UV-C significantly increased the inactivation of Pa compared with natural decay alone at all but one of the UV-C doses assessed. UV-C doses from 246-1968 µW s/cm² had an estimated effectiveness of approximately 50-90% for airborne Pa. The effectiveness of doses ≥984 µW s/cm² were not significantly different from each other (p-values: 0.365 to ~1), consistent with a flattening of effectiveness at higher doses. Modelling showed that delivering the highest dose associated with significant improvement in effectiveness (984 µW s/cm²) to the upper air of three clinical rooms would lead to lower room doses from 37-49% of the 8 h occupational limit. Our results suggest that UV-C can expedite the inactivation of nebulized airborne Pa under controlled conditions, at levels that can be delivered safely in occupied settings. These findings need corroboration, but UV-C may have potential applications in locations where people with CF congregate, coupled with other indoor and administrative infection control measures.

Nanomechanics and Morphology of Simulated Respiratory Particles

The impact of respiratory particle composition on the equilibrium morphology and phase is not well understood. Furthermore, the effects of these different phases and morphologies on the viability of viruses embedded within these particles are equally unknown. Physiologically relevant respiratory fluid analogues were constructed, and their hygroscopic behavior was measured using an ensemble technique. A relationship between hygroscopicity and protein concentration was determined, providing additional validation to the high protein content of respiratory aerosol measured in prior works (>90%). It was found that the salt component of the respiratory particles could crystallize as a single crystal, multiple crystals, or would not crystallize at all. It was found that dried protein particles at indoor-relevant climatic conditions could exist separately in a glassy (∼77% of particles) or viscoelastic state (∼23% of particles). The phase state and morphology of respiratory particles may influence the viability of embedded pathogens. We recommend that pathogen research aiming to mimic the native composition of respiratory fluid should use a protein concentration of at least 90% by solute volume to improve the representativity of the pathogen's microenvironment.

Evidence for a semisolid phase state of aerosols and droplets relevant to the airborne and surface survival of pathogens

Ambient humidity can influence the survival of pathogens in respiratory aerosols and droplets, although the mechanism and optimum humidity level for public health remain unclear. Here, we present evidence for a humidity-dependent, semisolid state of aerosols and droplets relevant to pathogen survival. These observations indicate that a semisolid state may protect pathogens from inactivation by hindering disinfection reactions at intermediate-to-low humidity levels. The formation of the semisolid state was dependent on the composition of the aerosols, which suggests that the humidity for optimum pathogen destruction will depend on the composition of respiratory particles released from an infected host. These observations can be used to help interpret laboratory studies and inform public health recommendations.

The phase state of respiratory aerosols and droplets has been linked to the humidity-dependent survival of pathogens such as SARS-CoV-2. To inform strategies to mitigate the spread of infectious disease, it is thus necessary to understand the humidity-dependent phase changes associated with the particles in which pathogens are suspended. Here, we study phase changes of levitated aerosols and droplets composed of model respiratory compounds (salt and protein) and growth media (organic–inorganic mixtures commonly used in studies of pathogen survival) with decreasing relative humidity (RH). Efflorescence was suppressed in many particle compositions and thus unlikely to fully account for the humidity-dependent survival of viruses. Rather, we identify organic-based, semisolid phase states that form under equilibrium conditions at intermediate RH (45 to 80%). A higher-protein content causes particles to exist in a semisolid state under a wider range of RH conditions. Diffusion and, thus, disinfection kinetics are expected to be inhibited in these semisolid states. These observations suggest that organic-based, semisolid states are an important consideration to account for the recovery of virus viability at low RH observed in previous studies. We propose a mechanism in which the semisolid phase shields pathogens from inactivation by hindering the diffusion of solutes. This suggests that the exogenous lifetime of pathogens will depend, in part, on the organic composition of the carrier respiratory particle and thus its origin in the respiratory tract. Furthermore, this work highlights the importance of accounting for spatial heterogeneities and time-dependent changes in the properties of aerosols and droplets undergoing evaporation in studies of pathogen viability.

Drying of virus-containing particles: modelling effects of droplet origin and composition

BACKGROUND AND PURPOSE

Virus-containing aerosol droplets emitted by breathing, speech or coughing dry rapidly to equilibrium with ambient relative humidity (RH), increasing in solute concentration with effects on virus survival and decreasing in diameter with effects on sedimentation and respiratory uptake. The aim of this paper is to model the effect of ionic and macromolecular solutes on droplet drying and solute concentration.

METHODS

Deliquescence-efflorescence concepts and Kohler theory were used to simulate the evolution of solute concentrations and water activity in respiratory droplets, starting from efflorescence data on mixed NaCl/KCl aerosols and osmotic pressure data on respiratory macromolecules.

RESULTS

In NaCl/KCl solutions total salt concentrations were shown to reach 10-13 M at the efflorescence RH of 40-55%, depending on the K:Na ratio. Dependence on K:Na ratio implies that the evaporation curves differ between aerosols derived from saliva and from airway surfaces. The direct effect of liquid droplet size through the Kelvin term was shown to be smaller and restricted to the evolution of breath emissions. Modelling the effect of proteins and glycoproteins showed that salts determine drying equilibria down to the efflorescence RH, and macromolecules at lower RH.

CONCLUSION

Differences in solute composition between airway surfaces and saliva are predicted to lead to different drying behaviour of droplets emitted by breathing, speech and coughing. These differences may influence the inactivation of viruses.