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.
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