Role of pathogen-laden expiratory droplet dispersion and natural ventilation explaining a COVID-19 outbreak in a coach bus

The influencing mechanism of droplet transmissions inside crowded and poorly ventilated buses on infection risks of respiratory diseases is still unclear. Based on experiments of one-infecting-seven COVID-19 outbreak with an index patient at bus rear, we conducted CFD simulations to investigate integrated effects of initial droplet diameters(tracer gas, 5 mu m, 50 mu m and 100 mu m), natural air change rates per hour(ACH = 0.62, 2.27 and 5.66 h-1 related to bus speeds) and relative humidity(RH = 35% and 95%) on pathogen-laden droplet dispersion and infection risks. Outdoor pressure difference around bus surfaces introduces natural ventilation airflow entering from bus-rear skylight and leaving from the front one. When ACH = 0.62 h-1(idling state), the 30-min-exposure infection risk(TIR) of tracer gas is 15.3%(bus rear) -11.1%(bus front), and decreases to 3.1%(bus rear)-1.3%(bus front) under ACH = 5.66 h-1(high bus speed).The TIR of large droplets(i.e., 100 mu m/50 mu m) is almost independent of ACH, with a peak value(-3.1%) near the index patient, because over 99.5%/97.0% of droplets deposit locally due to gravity. Moreover, 5 mu m droplets can disperse further with the increasing ventilation. However, TIR for 5 mu m droplets at ACH = 5.66 h-1 stays relatively small for rear passengers(maximum 0.4%), and is even smaller in the bus middle and front(< 0.1%). This study verifies that differing from general rooms, most 5 mu m droplets deposit on the route through the long-and-narrow bus space with large-area surfaces(L-11.4 m). Therefore, tracer gas can only simulate fine droplet with little deposition but cannot replace 5-100 mu m droplet dispersion in coach buses.

Automated summary

This study investigates the influencing mechanism of crowded and poorly ventilated buses on infection risks of respiratory diseases. Experimental and CFD simulation results show that the initial droplet diameters, air change rates, and relative humidity have integrated effects on pathogen-laden droplet dispersion and infection risks. Large droplets are mainly affected by gravity, while the dispersion of 5 μm droplets is influenced by ventilation.