Quantifying the Risk of Indoor Drainage System in Multi-unit Apartment Building as a Transmission Route of SARS-CoV-2

Abstract

The COVID-19 pandemic has had a profound impact on human society. The isolation of SARS-CoV-2 from patients feces on human cell line raised concerns of possible transmission through human feces including exposure to aerosols generated by toilet flushing and through the indoor drainage system. Currently, routes of transmission, other than the close contact droplet transmission, are still not well understood. A quantitative microbial risk assessment was conducted to estimate the health risks associated with two aerosol exposure scenarios: 1) toilet flushing, and 2) faulty connection of a floor drain with the building main sewer pipe. SARS-CoV-2 data were collected from the emerging literature. The infectivity of the virus in feces was estimated based on a range of assumption between viral genome equivalence and infectious unit. The human exposure dose was calculated using Monte Carlo simulation of viral concentrations in aerosols under each scenario and human breathing rates. The probability of COVID-19 illness was generated using the dose-response model for SARS-CoV-1, a close relative of SARS-CoV-2, that was responsible for the SARS outbreak in 2003. The results indicate the median risks of developing COVID-19 for a single day exposure is 1.11 x 10-10 and 3.52 x 10-11 for toilet flushing and faulty drain scenario, respectively. The worst case scenario predicted the high end of COVID-19 risk for the toilet flushing scenario was 5.78 x 10-4 (at 95th percentile). The infectious viral loads in human feces are the most sensitive input parameter and contribute significantly to model uncertainty.

Analysis of COVID-19 clusters involving vertical transmission in residential buildings in Hong Kong

“…Prior studies have demonstrated the paths of vertical transmission between vertically aligned flats, and the transmission could be related to the stack effect. For COVID-19 clusters involving vertical transmission in buildings, virusladen aerosols are primarily transmitted via wastewater plumbing systems (Shi et al 2021;Wang et al 2022a). Drainpipes are not sealed as tightly as pipes for pressurised water supplies, which can lead to long-term and occasional leakages.…”

“…Drainpipes are not sealed as tightly as pipes for pressurised water supplies, which can lead to long-term and occasional leakages. Flushing a toilet can temporarily alter the pressure inside drainpipes, leading to the infiltration of virus-laden aerosols into or out of the drainpipes if pipe leakage occurs (Shi et al 2021;Wang et al 2022a). In addition, differences in temperature or humidity between the inner and outer environments of a leaky drainage stack can produce the typical or reverse stack effect, which produces vertical airflow in the drainage stack and promotes air exchange between flats sharing the same drainage stack (Mijorski and Cammelli 2016;Magyar 2022).…”