Modelling aerosol transport and virus exposure with numerical simulations in relation to SARS-CoV-2 transmission by inhalation indoors (2005.12612v1)

Abstract: We provide research findings on the physics of aerosol dispersion relevant to the hypothesized aerosol transmission of SARS-CoV-2. We utilize physics-based modeling at different levels of complexity, and literature on coronaviruses, to investigate the possibility of airborne transmission. The previous literature, our 0D-3D simulations by various physics-based models, and theoretical calculations, indicate that the typical size range of speech and cough originated droplets (d < 20microns) allows lingering in the air for O(1h) so that they could be inhaled. Consistent with the previous literature, numerical evidence on the rapid drying process of even large droplets, up to sizes O(100microns), into droplet nuclei/aerosols is provided. Based on the literature and the public media sources, we provide evidence that the infected individuals could have been exposed to aerosols/droplet nuclei by inhaling them in significant numbers e.g. O(100). By 3D computational fluid dynamics (CFD) simulations, we give examples on the transport and dilution of aerosols (d<20microns) over distances O(10m) in generic environments. We study susceptible and infected individuals in generic public places by Monte-Carlo modeling. The model accounts for the locally varying aerosol concentration levels which the susceptible accumulate via inhalation. The introduced concept, 'exposure time' to virus containing aerosols is proposed to complement the traditional 'safety distance' thinking. We show that the exposure time to inhale O(100) aerosols could range from O(1s) to O(1min) or even to O(1h) depending on the situation. The Monte Carlo analysis provides clear quantitative insight to the exposure time in different public indoor environments.

• The paper characterizes aerosol dynamics, showing that droplets under 20 μm remain airborne for extended periods after rapid evaporation.
• The study employs CFD, LES, and Monte-Carlo models to simulate airflow patterns and aerosol dispersion in various indoor environments.
• The paper introduces the concept of 'exposure time,' offering quantitative metrics to guide ventilation improvements and distancing strategies.

Overview of "Modelling aerosol transport and virus exposure with numerical simulations in relation to SARS-CoV-2 transmission by inhalation indoors"

This paper presents a thorough investigation into the airborne transmission of the SARS-CoV-2 virus, particularly focusing on the dynamics of aerosol transport in indoor environments. The authors aim to deduce the physics behind aerosol dissemination and assess the risk of virus exposure through inhalation using computational fluid dynamics (CFD) models, large-eddy simulations (LES), and Monte-Carlo methods.

Key Findings and Methodologies

1. Aerosol Dynamics Characterization: The paper highlights that respiratory droplets from speaking and coughing, typically below 20 μm in size, can remain airborne for extended periods, potentially making them vectors for virus transmission. These droplets undergo rapid evaporation, effectively reducing to droplet nuclei/aerosols, which continue to suspend in the air.
2. Numerical Simulations: Using 0D-3D simulations and theoretical calculations, the authors explore how these aerosols propagate over distances typical of enclosed spaces like public stores. The CFD simulations model airflow patterns, aerosol dispersion, and potential exposure to these aerosols under various indoor conditions.
3. Exposure Time Concept: A novel concept introduced is the "exposure time" to aerosolized virus particles, designed to complement the conventional notion of "safety distance". The simulations predict that the time to inhale 100 aerosol particles can vary from a few seconds to an hour, depending on the environmental conditions and the cloud's dilution rate.
4. Monte-Carlo Modelling: Numerical results from Monte-Carlo models provide insights into simulations of gatherings in public places such as stores and work environments. These models assess scenarios of variable population densities and movement patterns, offering a probabilistic aspect to potential virus exposure.

Strong Numerical Results

• The paper offers quantitative benchmarks for aerosol transport, suggesting that droplets within certain size ranges linger long enough to be inhaled. For instance, droplets of around 20 μm can stay airborne for over an hour, significantly increasing the risk of transmission in poorly ventilated areas.
• The paper provides concrete CFD results showing that aerosol clouds can disperse over several meters, with dilution times sensitive to ambient turbulence intensity and airflow patterns.

Implications and Future Directions

1. Practical Applications: The findings inform public health policies by emphasizing the need for adequate ventilation and physical distancing in enclosed spaces to curb aerosol-based transmission. The research supports the redesign of indoor spaces to minimize elevated risk areas and optimize air flow dynamics to dilute viral aerosols effectively.
2. Theoretical Contributions: The development of the "exposure time" metric offers a new dimension for understanding infection spread dynamics beyond just physical distance measurements.
3. Further Research: There is continued need for modeling variations in droplet emission rates and sizes across different human activities. Future developments could involve enhanced simulation granularity and integration with real-world data to refine exposure risk assessments.

In summary, this paper contributes to the field by providing a comprehensive model of the physical processes underlying aerosol transmission of SARS-CoV-2 in indoor environments. The methodologies and results serve as a foundational framework for refining public health guidelines and advancing aerosol transmission models in pandemic response strategies.