Recent developments in the computational simulation of dry powder inhalers.

This article reviews recent developments in computational modeling of dry powder inhalers (DPIs). DPIs deliver drug formulations (sometimes blended with larger carrier particles) to a patient's lungs via inhalation. Inhaler design is complicated by the need for maximum aerosolization efficiency, which is favored by high levels of turbulence near the mouthpiece, with low extrathoracic depositional loss, which requires low turbulence levels near the mouth-throat region. In this article, we review the physical processes contributing to aerosolization and subsequent dispersion and deposition. We assess the performance characteristics of DPIs using existing simulation techniques and offer a perspective on how such simulations can be improved to capture the physical processes occurring over a wide range of length- and timescales more efficiently.

Beyond well-mixed: A simple probabilistic model of airborne disease transmission in indoor spaces.

We develop a simple model for assessing risk of airborne disease transmission that accounts for non-uniform mixing in indoor spaces and is compatible with existing epidemiological models. A database containing 174 high-resolution simulations of airflow in classrooms, lecture halls, and buses is generated and used to quantify the spatial distribution of expiratory droplet nuclei for a wide range of ventilation rates, exposure times, and room configurations. Imperfect mixing due to obstructions, buoyancy, and turbulent dispersion results in concentration fields with significant variance. The spatial non-uniformity is found to be accurately described by a shifted lognormal distribution. A well-mixed mass balance model is used to predict the mean, and the standard deviation is parameterized based on ventilation rate and room geometry. When employed in a dose-response function risk model, infection probability can be estimated considering spatial heterogeneity that contributes to both short- and long-range transmission.

Influence of expiratory flow pulsatility on the effectiveness of a surgical mask.

BACKGROUND: Expiratory events, such as coughs, are often pulsatile in nature and result in vortical flow structures that transport expiratory particles. The World Health Organization recommends wearing face masks to reduce the airborne transmission of diseases such as SARS-CoV-2 (COVID-19). However, masks are not perfect as flow leakage occurs around the mask, and their effectiveness under realistic (multi-pulse) coughing conditions is unknown. OBJECTIVE: To assess the influence of expiratory flow pulsatility on the effectiveness of a surgical face mask by quantifying and classifying the flow leakage around the mask. METHODS: A custom-built pulsatile expiratory flow simulator is used to generate single- and multi-pulsed coughing events. Flow visualization and particle image velocimetry are used to assess the penetration distance and volume of leakage flow at the top and sides of a surgical mask. RESULTS: Leakage flow velocity profiles at the top and sides of a surgical mask take the form of a wall jet and a free-shear jet, respectively. Multi-pulsed expiratory flow events are found to generate greater leakage flow around the mask than single-pulsed events. SIGNIFICANCE: For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough. IMPACT STATEMENT: The novelties of this study are: First, flow field measurements are used to quantify and classify the leakage flow fields around the top and sides of a surgical mask, providing a benchmark for quantitative modeling of leakage flow velocity profiles. Second, the influence of pulsatility on the effectiveness of surgical face masks is studied by quantifying the leakage volume. For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough, as multi-pulsed expiratory flow events are found to generate greater flow leakage around the mask than single-pulsed events.

Experimental evaluation of aerosol mitigation strategies in large open-plan dental clinics.

BACKGROUND: Aerosols are generated routinely during patient care in dentistry. Managing exposure risk requires understanding characteristics of aerosols created during procedures such as those performed using high-speed drills that operate at 200,000 revolutions per minute. METHODS: A trained dentist performed drilling procedures on a manikin's incisors (teeth nos. 8 and 9) using a high-speed drill and high-volume evacuator. The authors used high-speed imaging to visualize the formation and transport of aerosol clouds and particle sampling to measure aerosol concentration and size distribution at several locations. The authors studied several aerosol mitigation strategies. RESULTS: Aerosols produced during high-speed drilling were erratic and yielded high concentrations that were at least an order of magnitude above baseline. High-speed imaging showed aerosols initially travelled at 1 m per second. Owing to erratic behavior of aerosols, supplemental suction was not effective at collecting all aerosols; however, barriers were effective. CONCLUSIONS: Barriers are the most effective mitigation strategy. Other methods studied have limitations and risks. To the authors' knowledge, this article presents the first characterization of aerosols generated during high-speed drilling by a dentist. PRACTICAL IMPLICATIONS: With thorough preoperative planning and the use of this investigation's findings about effectiveness of mitigation strategies as a guide, dental offices may be able to return to prepandemic productivity.

On the utility of a well-mixed model for predicting disease transmission on an urban bus.

The transport of virus-laden aerosols from a host to a susceptible person is governed by complex turbulent airflow and physics related to breathing, coughing and sneezing, mechanical and passive ventilation, thermal buoyancy effects, surface deposition, masks, and air filtration. In this paper, we study the infection risk via airborne transmission on an urban bus using unsteady Reynolds-averaged Navier-Stokes equations and a passive-scalar model of the virus-laden aerosol concentration. Results from these simulations are directly compared to the widely used well-mixed model and show significant differences in the concentration field and number of inhaled particles. Specifically, in the limit of low mechanical ventilation rates, the well-mixed model will overpredict the concentration far from the infected passenger and substantially underpredict the concentration near the infected passenger. The results reported herein also apply to other enclosed spaces.

Disease transmission through expiratory aerosols on an urban bus.

Airborne respiratory diseases such as COVID-19 pose significant challenges to public transportation. Several recent outbreaks of SARS-CoV-2 indicate the high risk of transmission among passengers on public buses if special precautions are not taken. This study presents a combined experimental and numerical analysis to identify transmission mechanisms on an urban bus and assess strategies to reduce risk. The effects of the ventilation and air-conditioning systems, opening windows and doors, and wearing masks are analyzed. Specific attention is paid to the transport of submicron- and micron-sized particles relevant to typical respiratory droplets. High-resolution instrumentation was used to measure size distribution and aerosol response time on a campus bus of the University of Michigan under these different conditions. Computational fluid dynamics was employed to measure the airflow within the bus and evaluate risk. A risk metric was adopted based on the number of particles exposed to susceptible passengers. The flow that carries these aerosols is predominantly controlled by the ventilation system, which acts to uniformly distribute the aerosol concentration throughout the bus while simultaneously diluting it with fresh air. The opening of doors and windows was found to reduce the concentration by approximately one half, albeit its benefit does not uniformly impact all passengers on the bus due to the recirculation of airflow caused by entrainment through windows. Finally, it was found that well fitted surgical masks, when worn by both infected and susceptible passengers, can nearly eliminate the transmission of the disease.