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.

FIG. 1.

Perspective view of the urban bus interior.

FIG. 2.

Schematic and pictures of sampling configuration for the evaluation of size distribution from the smoke generator.

FIG. 3.

Smoke generator emitting aerosol size distribution and concentrations: (a) EEPS—nanorange and (b) OPS—microrange.

FIG. 4.

Schematic of the experiment setup.

FIG. 5.

Total concentrations with and without windows open: (a) nanosized aerosols and (b) microsized aerosols. Case A-3.

FIG. 6.

Nano-sized aerosol maximum concentration comparisons under window open and closed conditions: sampling at (a) the driver seat, (b) front seat, and (c) middle seat.

FIG. 7.

Response time comparison between window closed and open conditions: sampling at (a) the driver seat, (b) front seat, and (c) middle seat.

FIG. 8.

Numerical grid in front of the cabin with the close-up of the HVAC supply vent.

FIG. 9.

Contour of inhaled particles on the center plane of the bus, t = 15 min. Black stars indicate the probe locations.

FIG. 10.

Time histories of concentration with different grid resolutions. (a) Front probe, (b) middle probe, and (c) rear probe.

FIG. 11.

Flow field details on the center plane of the bus with windows closed and HVAC at the maximum rate. (a) Contour of aerosol concentration and velocity vectors, t = 15 min. (b) Contour of vorticity and velocity vectors, t = 15 min.

FIG. 12.

Contours of inhaled particles for different HVAC rates with the infected passenger standing in the front of the bus at t = 15 min. The white contour lines represent the critical number of inhaled particles N_b,crit = 50. (a) Maximum HVAC rate. (b) 50% of the maximum rate. (c) 10% of the maximum rate.

FIG. 13.

Contours of inhaled particles for different HVAC rates with the infected passenger standing in the middle of the bus at t = 15 min. The white contour lines represent the critical number of inhaled particles N_b,crit = 50. (a) Maximum HVAC rate. (b) 50% of the maximum rate. (c) 10% of the maximum rate.

FIG. 14.

Contours of inhaled particles for different scenarios of face coverings at t = 15 min. The white contour lines represent the critical number of inhaled particles N_b,crit = 50. (a) Nobody wears a mask, (b) everyone wears a surgical mask, and (c) everyone wears a handmade mask.

FIG. 15.

Flow field details on the center plane of the bus with windows open. (a) Contour of aerosol concentration and velocity vectors, t = 15 min. (b) Contour of vorticity and velocity vectors, t = 15 min.

FIG. 16.

Contours of inhaled particles with different setup for the windows and doors at t = 15 min. The white contour lines represent the critical number of inhaled particles N_b,crit = 50. (a) Bus is enclosed, (b) windows are open, and (c) doors are open at each stop.