Transmission of pathogen-laden expiratory droplets in a coach bus

Abstract

J o u r n a l P r e -p r o o f  RH=95% induces less droplets suspended in air and more deposition fraction(85%-100%). Wet air, sitting at nonadjacent seats, supply to bus backward reduce infection risk. AbstractDroplet dispersion carrying viruses/bacteria in enclosed/crowded buses may induce transmissions of respiratory infectious diseases, but the influencing mechanisms have been rarely investigated. By conducting high-resolution CFD simulations, this paper investigates the evaporation and transport of solid-liquid mixed droplets (initial diameter 10μm and 50μm, 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. 50μm droplets tend to evaporate completely within 1.8s and 7s as RH=35% and 95% respectively, while 0.2s or less for 10μm droplets. Thus 10μm droplets diffuse farther with wider range than 50μm 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.J o u r n a l P r e -p r o o f

How human thermal plume influences near-human transport of respiratory droplets and airborne particles: a review

“…For sedentary occupants in enclosed spaces, thermal plume often has a significant and persistent influence on the transport of respiratory droplets and airborne particles, despite the co-existing influences of ventilation. This was demonstrated in a recent study where researchers simulated the transport of expiratory droplets with initial sizes of 10 and 50 μm in a coach bus environment (Yang et al 2020 ). The study showed that gravity, ventilation flows and upward body thermal plumes had concurrent effects on the dispersion and final deposition of the droplets generated by seated passengers in the enclosed environment.…”

“…An approximately linear growth of plume flow rate with height was observed Craven and Settles ( 2006 ) Studying the effects of human thermal plume on indoor airflows and particle transport Simulation Computational fluid dynamics Buoyancy driven by human thermal plume significantly pulled airflows from floor ventilation inlet toward the human body and altered the flow trajectories of airborne particles Salmanzadeh et al ( 2012 ) Evaluation of key variables in the intensity of human thermal plume Thermal manikin Dressing a thermal human manikin with various types of clothing under different conditions Increasing the ambient temperature could decrease the intensity of the human thermal plume. Long and loose clothing could reduce its velocity Licina et al ( 2014 ) Studying the influences of human thermal plume on particle transport and distribution after emitted by a laser printer in a ventilated room Simulation Computational fluid dynamics Particle concentrations were significantly higher near the breathing zone under the influence of thermal plume Ansaripour et al ( 2016 ) Investigating the interactions between human thermal plume and cough flow Simulation Computational fluid dynamics Human thermal plume could ascend the cough flow and elevate the droplets into upper atmosphere along the human boundary layer Yan et al ( 2019 ) Investigation of human exposure to indoor airborne microplastics Thermal manikin Using a breathing thermal manikin to simulate human respiration Thermal plume continuously transported microplastics from lower regions of the room into the breathing zone of the sedentary manikin Vianello et al ( 2019 ) Analyzing airborne transmission of expiratory droplets in a coach bus environment Simulation Computational fluid dynamics Gravity, ventilation flows and upward body thermal plumes had concurrent effects on the dispersion and final deposition of the droplets generated by seated passengers in the coach bus Yang et al ( 2...…”

Transmission risk of infectious droplets in physical spreading process at different times: A review

“…Xie [ 44 ] and Wei [ 21 ] both found in their respective studies that the larger the initial particle size, the longer the evaporation time required under the same environmental conditions. Yang et al [ 53 ] believed that droplets with the same initial particle size would evaporate for a longer time under high humidity because air with higher relative humidity has less potential to absorb water vapor. Another scholar [ 54 ] found that the relative humidity not only affects the speed of evaporation, but also affects the diameter of the final droplet nuclei due to the Kelvin effect; the final normalized diameter d e /d o of small droplets is smaller than that of large droplets.…”

Aerosol Transmission of SARS-CoV-2: Physical Principles and Implications

“…Exhaled droplets become completely dry at 50-70% relative humidity and their equilibrium water content increases, roughly exponentially, at higher relative humidity levels (42,59). Droplet drying, along with settling and entrainment in cough airflows, has been modeled by computational fluid dynamics (55,60,61) to make important predictions about virus transmission in confined settings. In some of these studies (60,61), an unrealistically high salt content was assumed [100 g/L NaCl, compared with <10 g/L salts in saliva (62)] so that the dry diameter and settling rate were (36).…”

“…If possible, air should not flow from any person toward other people, especially at face height. Aboveseat ventilators on coaches (60) and aircraft (19,106) may cause exactly that if used inappropriately. In public buildings, clean air may be obtained by recirculating through HEPA filters (19,102,107,108) or by ventilating with outside rather than recirculated air (103,105) or simply by opening windows (102), accepting that indoor air temperatures may then be colder than guideline limits in winter or hotter in summer.…”