Disease transmission through expiratory aerosols on an urban bus

Abstract

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.

COVID‐19 Transmission during Transportation of 1st to 12th Grade Students: Experience of an Independent School in Virginia

“…Current federal mandates require masking during bus transportation; however, ventilation with open windows is recommended but not mandated. While there is little data on how ventilation on buses affects transmission of COVID‐19, the opening of doors or windows on a bus can reduce the number of aerosol droplets by half 17 . In the present study, the consistent spaced window openings on each school bus, regardless of weather, may have significantly contributed to the lack of transmission of SARS‐CoV‐2.…”

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

“…In this paper, we consider the smallest aerosols only, those with diameter less than 10 µ m. Under this assumption, aerosols are represented with the continuum field C = C ( x , t ), which is transported passively with the unsteady RANS (URANS) velocity field [ Zhang et al (2021) ]. Details on both the well-mixed model and CFD simulations are summarized herein.…”

“…A gradient limiter is used to ensure that the interpolation from cell centers to face centers produces a bounded and stable solution. Additional details, including a mesh refinement study, can be found in Zhang et al (2021) . The custom solver is available on GitHub ( https://github.com/zhihangz/covid-transmission ).…”

The impact of large mobile air purifiers on aerosol concentration in classrooms and the reduction of airborne transmission of SARS-CoV-2

“…Since the experiments conducted in this study suggest that majority of the aerosol particles have a size of less than 1 µm, the distribution of the aerosols at the scale of a classroom can be modeled using the Eulerian approach, by solving the transport equation accounting for convection-diffusion of the aerosol (Z. Zhang et al, 2021). The effective diffusive constant for the aerosol is the sum of the molecular and turbulent contributions, = ⁄ + ⁄ , where Pr = 0.7 and Prt = 0.85, are the laminar and turbulent Prandtl numbers, respectively, with values that are generally used for the modeling of aerosol transport in ventilation cases (Burgmann & Janoske, 2021).…”