key: cord-0851633-u7qmywqr authors: Wang, Qun; Li, Yuguo; Lung, David Christopher; Chan, Pak-To; Dung, Chung-Hin; Jia, Wei; Miao, Te; Huang, Jianxiang; Chen, Wenzhao; Wang, Zixuan; Leung, Kai-Ming; Lin, Zhang; Wong, Daniel; Tse, Herman; Wong, Sally Cheuk Ying; Choi, Garnet Kwan-Yue; Lam, Jimmy Yiu-Wing; To, Kelvin Kai-Wang; Cheng, Vincent Chi-Chung; Yuen, Kwok-Yung title: Aerosol transmission of SARS-CoV-2 due to the chimney effect in two high-rise housing drainage stacks date: 2021-08-02 journal: J Hazard Mater DOI: 10.1016/j.jhazmat.2021.126799 sha: e67ca546ef391efbe4c17fc977068802e6b20fb4 doc_id: 851633 cord_uid: u7qmywqr Stack aerosols are generated within vertical building drainage stacks during the discharge of wastewater containing feces and exhaled mucus from toilets and washbasins. Fifteen stack aerosol-related outbreaks of coronavirus disease 2019 (COVID-19) in high-rise buildings have been observed in Hong Kong and Guangzhou. Currently, we investigated two such outbreaks of COVID-19 in Hong Kong, identified the probable role of chimney effect-induced airflow in a building drainage system in the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We injected tracer gas (SF6) into the drainage stacks via the water closet of the index case and monitored tracer gas concentrations in the bathrooms and along the facades of infected and non-infected flats and in roof vents. The air temperature, humidity, and pressure in vertical stacks were also monitored. The measured tracer gas distribution agreed with the observed distribution of the infected cases. Phylogenetic analysis of the SARS-CoV-2 genome sequences demonstrated clonal spread from a point source in cases along the same vertical column. The stack air pressure and temperature distributions suggested that stack aerosols can spread to indoors through pipe leaks which provide direct evidence for the long-range aerosol transmission of SARS-CoV-2 through drainage pipes via the chimney effect. More than one year into the pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) had led to more than 3 million deaths worldwide by early 2021 (WHO, 2021). Following the detection of the virus in fecal and anal swabs of infected patients (e.g., Wang et al., 2020) and the isolation of viable viruses from human feces (Holshue et al., 2020) in the early phase of the pandemic, the possible fecal-oral route of transmission has been debated (e.g., Hindson, 2020) . Fecal aerosol transmission was shown to be probable in a high-rise is not as viable in wastewater when compared with SARS-Cov-1, and these SARS-CoV-2 outbreaks were due to respiratory mucus. SARS-CoV-1 can survive for 14 days in sewage at 4°C and 2 days at 20°C (Wang et al., 2005) . In contrast, while live SARS-CoV-2 has been found in stool samples from two patients (Wang et al., 2020) , no viable SARS-CoV-2 has been detected in wastewater (Foladori et al., 2020) . Thus, respiratory aerosol/droplets likely play a more important role in the SARS-CoV-2 pandemic. Therefore, an alternative nonfaecal source of the virus in the stack aerosol was suspected, i.e., washbasin-collected oral/nasal mucus. Second, what is the ventilation flow rate in the drainage pipes? Kang et al. (2020) hypothesized the role of the chimney effect in the spread of virus-containing aerosols. However, there were no measured data to support the chimney effect. The chimney effect, if present, is related to the air flows induced by stack pressure, wind pressure at exterior openings, and exhaust fans in bathrooms or kitchens (Li and Delsante, 2001) . Chimney effect-induced flows can also ventilate drainage pipes, including vertical stacks. Air temperature and humidity in the drainage stacks may also affect virus viability (Paul et al., Kensington Plaza is an 18-story housing block in Kowloon that was built in 1999. There are 90 flats total with 6 flats on each floor (Figure 1a ). The index case (case K1) resided in Flat C on the 5 th floor (Flat 5C) during the sub-clinical infectious period, and was confirmed to have COVID-19 on January 16, 2021. A compulsory testing notice was issued on January 17, 2021 by the government to everyone who stayed in Kensington Plaza for more than 2 hours within the previous 14 days (CHP, 2021b). Additional infected residents were identified through this active case finding, and a vertical transmission pattern was observed after the preliminary investigation performed by the CHP. As a result, a multi-disciplinary response team (MDRT) investigation, which included representatives from the CHP, Environmental Protection Department, and government experts, was conducted on January 22, 2021. A quarantine order was issued to all asymptomatic residents of -C flats of all floors of Kensington Plaza We used a tracer gas (SF6) as a surrogate for stack aerosol containing virus-laden droplets and monitor the tracer gas concentration in the bathrooms of the infected and some non- In Kensington Plaza, iButton sensors (DS1923-F5, iButtonLink, Whitewater, USA) were hung in the drainage stack at two points ( Figure 2a ) from the rooftop, which recorded the air temperature and relative humidity at 1-minute intervals throughout the measurement periods. The two measurement points were 18 m and 24 m from the roof, approximately at the level of the 13 th floor and 11 th floor, respectively. We also used the same sensor with a radiation shield to record the air temperature and relative humidity above the roof and inside the light well (near the 7 th and 13 th floors) at 5-minutes intervals. After completing the tracer gas measurements each day, we continuously monitored the differential pressure between the soil pipe and the bathroom using differential pressure sensors (P350, Pace Scientific, Boone, USA) and dataloggers (XR5-SE-M, Pace Scientific, Boone, USA) with a sampling frequency of 1 Hz. The experimental setups are shown in Figure 2c . The sensor tube was pulled into the branch pipe through the WCs in Flats 5C, 7C, 10C, 15C, and 17C to record the differential pressure on the corresponding floors. A positive reading indicated that the pressure inside the drainage stack was higher than that in the bathroom, i.e., air would leak from the drainage stack to the bathroom if leaks existed. In the Wai Lee Building, we hung an iButton sensor in the drainage pipe through the rooftop to record the temperature and relative humidity inside the drainage pipe at the level of the In Kensington Plaza, 14 residents from 6 flats tested positive for SARS-CoV-2 by RT-PCR (CHP, 2021a). Among them, four -C flats, on the 5 th , 6 th , 7 th , and 17 th floors, were affected with 10 cases involved ( Table 1) Table 1 . In the Wai Lee Building, nine residents from five flats were confirmed to have COVID-19. Three were from -04 flats, which are vertically aligned and connected by the same drainage stack. The index case was an 18-year-old male college student who lived in Flat 1404 (14 th floor, Flat 04, case W2). The incubation periods of the other eight cases all overlapped with the period of communicability of the index case. Details of the cases are summarized in Table 1 . The Wai Lee Building has fully enclosed light wells (Figure 1b Figure 6 ). The gas concentration in the adjacent single-stacked pipes also increased significantly. The increase in the tracer gas concentration at approximately 12:05 was due to the release of tracer gas during a pre-test. The concentration variation due to tracer gas dispersion and transportation outside the pipe system was ruled out by a separate test ( Figure S3 ) because the concentration inside the vent pipe of -05 flats was higher than that outside the vent pipe. The drainage stacks of the flats are connected to each other below ground ( Figure 2b ), which could affect the distribution of differential pressures presented in Section 3.5. The pipes also serve as transport paths between different flats. An upward chimney effect occurs when the interior air is lighter than the outdoor air, and a downward chimney effect occurs when the interior air is heavier. In Kensington Plaza, the air temperature in the drainage stack ranged from 18.5°C to 28.0°C and the relative humidity ranged from 95.5% to 100.0% (Figure 7a ). Such high humidity in drainage pipes has also been observed by Gormley The analysis of temperature and relative humidity inside the drainage pipe indicated that the chimney effect should be salient during the nighttime due to the large temperature difference. Therefore, we monitored the differential pressure between the inner pipe and the bathroom before and/or after the tracer gas measurement period. Figure 9a -d shows the variation in differential pressure on different floors in Kensington Plaza and the Wai Lee Building. In Kensington Plaza, negative differential pressure was recorded on the 10 th floor and positive differential pressure was recorded on the 5 th , 7 th , 15 th , and 17 th floors (Figure 9a and 9b) . The differential pressure results are consistent with the those of the tracer gas measurements in Kensington Plaza. Figure 2a shows a schematic of the drainage system of Kensington Plaza. In this building, flats below the 9 th floor shared the same vertical stack and flats above the 9 th The chimney effect only explained the transport of stack aerosols. The generation of stack aerosols is a complicated process. A significant number of aerosols are generated during WC flushing, and more than 99.5% of aerosols are less than 5 m in size (Gormley et al., 2021) . detection and diagnosis problem (i.e., diagnostics identifies the root cause and location of detected faults), which has been addressed for air-conditioning systems, such as via the use of artificial neural networks for fault diagnosis (e.g., Lee et al., 1996a) and for comparison of the normal/expected operation data with measured, possibly abnormal data using residuals (Lee et al., 1996b) . It is also necessary for infected individuals, and everyone due to the existence of asymptomatic individuals, to minimize the discharge of nasal or oral secretions J o u r n a l P r e -p r o o f into the washbasin (using tissue paper instead). However, this may not be practical as people generally spit into the washbasin while brushing their teeth. This will involve changes in spitting behavior when using private bathrooms and public toilets. It is also important for health authorities to recognize vertical transmission patterns early and to implement appropriate measures to prevent the further propagation of an outbreak through vertical drainage stacks. We cannot rule out the possibility of buoyancy flow transport in fully enclosed or semi-open light wells. Therefore, we recommend that residents close the bathroom windows when turning on the exhaust fan. Opening the toilet door or installing a louver at the toilet door can prevent the buildup of a strong negative pressure in the toilet, thereby minimizing any leaks from the drainage system into a bathroom. Moreover, transmission through contact by common surface touching cannot be ruled out. However, the clear vertical transmission pattern found in this study demonstrates that transmission via chimney effect-induced flows was the most likely route. There are several major limitations of this study. Only two outbreaks with a total of 23 infected cases were reported. Monitoring of tracer gas concentrations and buoyancy pressure was only conducted in a small number of infected and non-infected flats due to access difficulties. Each building was tested within 1-3 days due to the constraints of the quarantine periods and the need for full disinfection before access by our field test team. Not all patient specimens were available for sequencing in this study. 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This work was partially supported by the Research Grants Council of Hong Kong's Collaborative Research Fund (grant number C7025-16G). We are grateful to the Centre for Health Protection for providing the epidemiological data and arranging the field measurements. We would also like to thank Dr Vincent Cheung of the Environmental Protection Department for the on-site support.