Airborne transmission of pathogen-laden expiratory droplets in open outdoor space

Virus-laden droplets dispersion may induce transmissions of respiratory infectious diseases. Existing research mainly focuses on indoor droplet dispersion, but the mechanism of its dispersion and exposure in outdoor environment is unclear. By conducting CFD simulations, this paper investigates the evaporation and transport of solid-liquid droplets in an open outdoor environment. Droplet initial sizes (dp=10m, 50m, 100m), background relative humidity (RH=35%, 95%), background wind speed (Uref=3m/s, 0.2m/s) and social distances between two people (D=0.5m, 1m, 1.5m, 3m, 5m) are investigated. Results show that thermal body plume is destroyed when the background wind speed is 3m/s (Froude number Fr~10). The inhalation fraction (IF) of susceptible person decreases exponentially when the social distance (D) increases from 0.5m to 5m. The exponential decay rate of inhalation fraction (b) ranges between 0.93 and 1.06 (IF=IF0e-b(D-0.5)) determined by the droplet initial diameter and relative humidity. Under weak background wind (Uref=0.2m/s, Fr~0.01), the upward thermal body plume significantly influences droplet dispersion, which is similar with that in indoor space. Droplets in the initial sizes of 10m and 50m disperse upwards while most of 100m droplets fall down to the ground due to large gravity force. Interestingly, the deposition fraction on susceptible person is ten times higher at Uref=3m/s than that at Uref=0.2m/s. Thus, a high outdoor wind speed does not necessarily lead to a smaller exposure risk if the susceptible person locating at the downwind region of the infected person, and people in outdoors are suggested to not only keep distance of greater than 1.5m from each other but also stand with considerable angles from the prevailing wind direction

Transmission and infection risk of various pathogen-laden expiratory droplets in a coach bus with COVID-19

The study about droplet transmission in crowded, poorly ventilated buses and the resulting infection risk(IR) remains rare. Based on a COVID-19 outbreak which the index patient located at bus rear, we performed CFD simulations to study the effect of initial droplet diameters and hourly ventilation rate(ACH) on droplet transmission and IR. The outdoor pressure differential creates the natural ventilation enters from theskylight at bus rear and exits from the front one. With increased ACH, the IR of tracer gas reduced quickly, from 11.1-15.3% under 0.62ACH to 1.3-3.1% under 5.66ACH. Furthermore, the IR of 100μm/50μm droplets was almost independent of ACH as most droplets were deposited due to gravity. Furthermore, 5μm droplets are more widely dispersed than larger droplets, and can spread further with increasing ACH with a low IR(≤0.4%). Unlike general rooms, most droplets are deposited on the route passing through the long-distance bus space(~9.46m). But the tracer gas will not deposit, so the tracer gas can only be used to simulate the fine droplet dispersion process in the long-distance bus. Our research results provide a reference for future research on droplet transmission in the bus environment, and also provide a guidance for epidemic prevention

Influence of natural ventilation design on the dispersion of pathogen-laden droplets in a coach bus

Natural ventilation is an energy-efficient design approach to reduce infection risk (IR), but its optimized design in a coach bus environment is less studied. Based on a COVID-19 outbreak in a bus in Hunan, China, the indoor-outdoor coupled CFD modeling approach is adopted to comprehensively explore how optimized bus natural ventilation (e.g., opening/closing status of front/middle/rear windows (FW/MW/RW)) and ceiling wind catcher (WCH) affect the dispersion of pathogen-laden droplets (tracer gas, 5 μm, 50 μm) and IR. Other key influential factors including bus speed, infector's location, and ambient temperature (Tref) are also considered. Buses have unique natural ventilation airflow patterns: from bus rear to front, and air change rate per hour (ACH) increases linearly with bus speed. When driving at 60 km/h, ACH is only 6.14 h−1 and intake fractions of tracer gas (IFg) and 5 μm droplets (IFd) are up to 3372 ppm and 1394 ppm with ventilation through leakages on skylights and no windows open. When FW and RW are both open, ACH increases by 43.5 times to 267.50 h−1, and IFg and IFd drop rapidly by 1–2 orders of magnitude compared to when no windows are open. Utilizing a wind catcher and opening front windows significantly increases ACH (up to 8.8 times) and reduces IF (5–30 times) compared to only opening front windows. When the infector locates at the bus front with FW open, IFg and IFd of all passengers are <10 ppm. More droplets suspend and further spread in a higher Tref environment. It is recommended to open two pairs of windows or open front windows and utilize the wind catcher to reduce IR in coach buses

Airborne transmission of the delta variant of SARS-CoV-2 in an auditorium

The Delta variant of SARS-CoV-2 has inflicted heavy burdens on healthcare systems globally, although direct evidence on the quantity of exhaled viral shedding from Delta cases is lacking. Literature remains inconclusive on whether existing public health guidance, formulated based on earlier evidence of COVID-19, should respond differently to more infectious viral strains. This paper describes a study on an outbreak of the Delta variant of COVID-19 in an auditorium, where one person contracted the virus from three asymptomatic index cases sitting in a different row. Field inspections were conducted on the configuration of seating, building and ventilation systems. Numerical simulation was conducted to retrospectively assess the exhaled viral emission, decay, airborne dispersion, with a modified Wells-Riley equation used to calculate the inhalation exposure and disease infection risks at the seat level. Results support the airborne disease transmission. The viral emission rate for Delta cases was estimated at 31 quanta per hour, 30 times higher than those of the original variant. The high quantity of viral plume exhaled by delta cases can create a risky zone nearby, which, for a mixing ventilation system, cannot be easily mitigated by raising mixing rates or introducing fresh air supply. Such risks can be reduced by wearing an N95 respirator, less so for social distancing. A displacement ventilation system, through which the air is supplied at the floor and returned from the ceiling, can reduce risks compared with a mixing system. The study has implications for ventilation guidelines and hygiene practices in light of more infectious viral strains of COVID-19

Assessment of exhaled pathogenic droplet dispersion and indoor-outdoor exposure risk in urban street with naturally-ventilated buildings

Outdoor droplet exposure risk is generally regarded much smaller than that indoor, but such indoor-outdoor assessment and comparison are still rare. By coupling indoor and outdoor environments, we numerically simulate the ventilation and dispersion of exhaled pathogenic droplets (e.g., diameter d = 10μm) within typical street canyon (outdoor, aspect ratio H/W = 1) and each room (indoor) of two eight-floor single-sided naturally-ventilated buildings. Inhaled fraction (IF) and suspended fraction (SF) between two face-to-face people are calculated to quantify and compare the human-to-human exposure risk in all 16 rooms (indoor) on eight floors and those at two outdoor sites. Numerical simulations are validated well by wind tunnel experiments. Results show that, the rooms in the 1st and 8th floors attain greater air change rate per hour (∼4.5–6.6h−1) and the lower exposure risk (IF∼2–4 ppm) than the 2nd-7th floors (air change rate per hour∼1.6–5.3h−1, IF∼4–11 ppm). Although inter-floor droplet dispersion exists, the room with index patient attains 2–4 order greater exposure risk than the other rooms without index patient. When the index patient stays outdoor, outdoor IF will change with locations, i.e. ∼55 ppm at leeward corner (even exceeding indoor IF∼2–11 ppm), and ∼7 ppm at middle street. Hence, the outdoor infection risk should not be ignored especially for people at leeward street corner where small vortex exists inducing local weak ventilation. Particularly, outdoor IF is decided by short-distance spraying droplet exposure (∼1m) and long-route airborne transmissions by the main recirculation through entire street canyon (∼50–100m)

COVID-19 transmission and control in land public transport: A literature review

Land public transport is an important link within and between cities, and how to control the transmission of COVID-19 in land public transport is a critical issue in our daily lives. However, there are still many inconsistent opinions and views about the spread of SARS-CoV-2 in land public transport, which limits our ability to implement effective interventions. The purpose of this review is to overview the literature on transmission characteristics and routes of the epidemic in land public transport, as well as to investigate factors affecting its spread and provide feasible measures to mitigate the infection risk of passengers. We obtained 898 papers by searching the Web of Science, Pubmed, and WHO global COVID database by keywords, and finally selected 45 papers that can address the purpose of this review. Land public transport is a high outbreak area for COVID-19 due to characteristics like crowding, inadequate ventilation, long exposure time, and environmental closure. Different from surface touch transmission and drop spray transmission, aerosol inhalation transmission can occur not only in short distances but also in long distances. Insufficient ventilation is the most important factor influencing long-distance aerosol transmission. Other transmission factors (e.g., interpersonal distance, relative orientation, and ambient conditions) should be noticed as well, which have been summarized in this paper. To address various influencing factors, it is essential to suggest practical and efficient preventive measures. Among these, increased ventilation, particularly the fresh air (i.e., natural ventilation), has proven to effectively reduce indoor infection risk. Many preventive measures are also effective, such as enlarging social distance, avoiding face-to-face orientation, setting up physical partitions, disinfection, avoiding talking, and so on. As research on the epidemic has intensified, people have broken down many perceived barriers, but more comprehensive studies on monitoring systems and prevention measures in land public transport are still needed

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, 5µm, 50µm and 100µm), natural air change rates per hour(ACH=0.62, 2.27 and 5.66h-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=0.62h-1(idling state), the 30-minute-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=5.66h-1(high bus speed).The TIR of large droplets(i.e., 100µm/50µm) 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, 5µm droplets can disperse further with the increasing ventilation. However, TIR for 5µm droplets at ACH=5.66h-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 5µm droplets deposit on the route through the long-and-narrow bus space with large-area surfaces(L~11.4m). Therefore, tracer gas can only simulate fine droplet with little deposition but cannot replace 5-100µm droplet dispersion in coach buses