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's 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 × 10 −10 and 3.52 × 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 × 10 −4 (at 95th percentile). The infectious viral loads in human feces are the most sensitive input parameter and contribute significantly to model uncertainty.

Uncertainties in estimating SARS-CoV-2 prevalence by wastewater-based epidemiology

“…Additionally, during the 2003 SARS outbreak, high concentrations of SARS were detected in sewer aerosols of an apartment, which resulted in a rapid spread of the disease [109] . To date, the potential occurrence of SARS-CoV-2 in sewer aerosols have been confirmed through dynamic modeling [110] and one such occasion was reported in China [111] . The occurrence and persistence of SARS-CoV-2 in sewer gas needs further research.…”

Time-Variant Positive Air Pressure in Drainage Stacks as a Pathogen Transmission Pathway of COVID-19

“…Aerosols generated by discharging wastewater through indoor drainage stacks are among the suspected sources of virus transmission [ 1 ]. Based on the spread of COVID-19 virus aerosols from the drainage stack to the toilet of an apartment through a faulty connection to the stack, health risk associated with faulty connections to a building drainage stack was estimated [ 2 ]. To reduce the risk of pathogenic transmission such as SARS and SARS-CoV-2, understanding of positive air pressure in a vertical drainage stack has been identified as an important factor [ 3 ].…”

What is the risk of acquiring SARS-CoV-2 from the use of public toilets?

“…Breathing in aerosolized faecal/urinary material during or after flushing offers an additional risk. This would affect anyone in the bathroom at the time or who enters the cubicle or toilet afterwards within a time interval dependent on particle settling (Birgand et al, 2020;Brönimann et al, 2020;Gerba et al, 1975;Jones et al, 2020;Knowlton et al, 2018;Lai et al, 2018;Li et al, 2020;Liu et al, 2020;Ma et al, 2020;McDermott et al, 2020;Sassi et al, 2018;Shi et al, 2021;Patel, 2020;. Bioaerosols may be produced from toilets that are flushed containing no waste, which suggests that the virus could remain in the toilet following prior use (Knowlton et al, 2018;Johnson et al, 2017).…”

“…Bioaerosols may be produced from toilets that are flushed containing no waste, which suggests that the virus could remain in the toilet following prior use (Knowlton et al, 2018;Johnson et al, 2017). The air may also be contaminated by (re)-aerosolized waste water from sewage outlets (Hu et al, 2020), via drains (Shi et al, 2021), spillages, leaks (Yuan et al, 2020;Kang et al, 2020) etc., or incorrectly installed plumbing (Gormley et al, 2021).…”

“…Bathroom air is likely to be filled with aerosols generated by multiple toilet flushes within a short time window (Shi et al, 2021). This means that the concentration of airborne virus would be significantly higher if large numbers of carriers converge on the bathroom (Fitzgerald et al, 2021).…”