Transmission of pathogen-laden expiratory droplets in a coach bus

Droplet dispersion carrying viruses/bacteria in enclosed/crowded buses may induce transmissions of respiratory infectious diseases, but the influencing mechanisms have been rarely investigated. By conducing high-resolution CFD simulations, this paper investigates the evaporation and transport of solid-liquid mixed droplets (initial diameter 10 mu m and 50 mu m solid to liquid ratio is 1:9) exhaled in a coach bus with 14 thermal manikins. Five air-conditioning supply directions and ambient relative humidity (RH = 35 % and 95 %) are considered. Results show that ventilation effectiveness, RH and initial droplet size significantly influence droplet transmissions in coach bus. 50 mu m droplets tend to evaporate completely within 1.8 s and 7 s as RH = 35 % and 95 % respectively, while 0.2 s or less for 10 mu m droplets. Thus 10 mu m droplets diffuse farther with wider range than 50 mu m droplets which tend to deposit more on surfaces. Droplet dispersion pattern differs due to various interactions of gravity, ventilation flows and the upward thermal body plume. The fractions of droplets suspended in air, deposited on wall surfaces are quantified. This study implies high RH, backward supply direction and passengers sitting at nonadjacent seats can effectively reduce infection risk of droplet transmission in buses. Besides taking masks, regular cleaning is also recommended since 85 %-100 % of droplets deposit on object surfaces.