Disease transmission through expiratory aerosols on an urbanbus

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