Abstract
The coronaviruses have inflicted health and societal crises in recent decades. Both SARS CoV-1 and 2 are suspected to spread through outdoor routes in high-density cities, infecting residents in apartments on separate floors or in different buildings in many superspreading events, often in the absence of close personal contact. The viability of such mode of transmission is disputed in the research literature, and there is little evidence on the dose–response relationship at the apartment level. This paper describes a study to examine the viability of outdoor airborne transmission between neighboring apartments in high density cities. A first-principles model, airborne transmissionviaoutdoor route (ATOR), was developed to simulate airborne pathogen generation, natural decay, outdoor dispersion, apartment entry, and inhalation exposure of susceptible persons in neighboring apartments. The model was partially evaluated using a smoke tracer experiment in a mock-up high-density city site and cross-checking using the computational fluid dynamics (CFD) models. The ATOR model was used to retrospectively investigate the relationship between viral exposure and disease infection at an apartment level in two superspreading events in Hong Kong: the SARS outbreak in Amoy Gardens and the COVID-19 outbreak in Luk Chuen House. Logistic regression results suggested that the predicted viral exposure was positively correlated with the probability of disease infection at apartment level for both events. Infection risks associated with the outdoor route transmission of SARS can be reduced to <10%, if the quanta emission rate from the primary patient is below 30 q/h. Compared with the indoor route transmission, the outdoor route can better explain patterns of disease infection. A viral plume can spread upward and downward, driven by buoyancy forces, while also dispersing under natural wind. Fan-assistant natural ventilation in residential buildings may increase infection risks. Findings have implication for public health response to current and future pandemics and the ATOR model can serve as planning and design tool to identify the risk of airborne disease transmission in high-density cities.
Highlights
The coronaviruses, a group of RNA viruses of zoonotic origin (Woo et al, 2010), have inflicted global health and societal damages in recent decades
Air flow around buildings has been simulated to test whether it is feasible for the coronavirus transmission to occur between the neighboring apartments via an outdoor route
The airborne transmission via outdoor route (ATOR) model simulates the outdoor dispersion, transport, and reentry to another apartment of viruses shed from an infectious person living in the building
Summary
The coronaviruses, a group of RNA viruses of zoonotic origin (Woo et al, 2010), have inflicted global health and societal damages in recent decades. Research evidence is emerging on airborne transmission of coronaviruses, in which viral pathogens spread via droplet nuclei, the dried residua of larger respiratory droplets (Wells, 1955), alternatively known as microdroplets (WHO, 2020b) or aerosols (CDC, 2020). The WHO initially dismissed airborne transmission of COVID-19 as a major mode of transmission (WHO, 2020b). It reconsidered in October 2020, after receiving petitions from 200+ research scientists (Morawska and Milton, 2020), acknowledging that “more studies are underway to better understand the conditions in which aerosol transmission is occurring” (WHO, 2020c)
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