Keywords
Airborne SARS-CoV-2, Covid-19, Thermal inactivation, Electric heater, Winter, Air Pathogen Purifier
Introduction
Airborne transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is one of the main routes of pandemic spread (Bazant and Bush, 2021; Jarvis, 2020; Morawska and Milton, 2020; Tang et al., 2020; Yao et al., 2020). Airborne transmission of the virus increases in confined spaces, especially during winter. Therefore, the most effective methods to prevent pandemic spread are natural ventilation (opening windows and doors), air conditioning, and air purification devices (Morawska et al., 2020). However, natural ventilation may be unsuitable in winter due to energy costs. Currently, most purification devices use HEPA filters, UV-C, or both (Buonanno et al., 2020; Curtius et al., 2021; Darnell et al., 2004; Ma et al., 2021; Storm et al., 2020). Several studies have shown that pathogens are thermally inactivated in liquids or air (Aboud et al., 2019; Bertrand et al., 2012). Airborne pathogens’ inactivation depends on air temperature. For example, Escherichia coli was inactivated at 150 ˚C; Bacillus subtilis was partially inactive at 150 oC, and 99.9% inactivated at 360 oC (Jung et al., 2009). Aspergillus versicolor and Cladosporium cladosporioides were inactivated by 99.00% within 0.2 seconds at 350 and 400 °C (Jung et al., 2009). MS2 virus was inactivated by 99.99% in hot air at 250°C in 2 seconds (Grinshpun et al., 2010). Another study investigated the inactivation of airborne E. coli and MS2 virus at very high temperatures and showed that both pathogens were inactivated at a rate of 4.7 log10 in 0.41 seconds at 450 oC (Damit et al., 2013).
SARS-CoV-2 is a single-strand RNA-enveloped virus (Ramanathan et al., 2020). It contains four structural proteins: nucleocapsid (N), spike (S), envelope (E), and membrane (M) protein. N-protein starts unfolding at 35 oC and is denatured at 55 oC (Wang et al., 2004). Heat inactivation of SARS-CoV-2 occurs through the N-Protein denaturation.
Thermal inactivation of SARS-CoV-2 in suspensions, on surfaces, or in the air has been studied and modeled for different temperatures, humidity, and exposure times (Batéjat et al., 2021; Biryukov et al., 2021; Burton et al., 2021; Guillier et al., 2020; Seifer and Elbaum, 2021; Yap et al., 2020a). The inactivation of SARS-CoV-2 in N95 masks was investigated by exposing used N95 masks to hot air, demonstrating that SARS-CoV-2 in N95 respirators was inactivated by 99.9% at 70oC within 3 min (Yap et al., 2020b). Another study showed that SARS-CoV was ineffective in a liquid after exposure to 75oC for 45 minutes (Darnell et al., 2004). It has been shown that SARS-CoV-2 in serum became ineffective at 92oC for 15 minutes (Pastorino et al., 2020). In heat treatment of SARS-CoV-2, evaporation is a critical parameter and changes the virus inactivation half-life. Hence, the presence of the virus in a closed container or an open container affects its inactivation (Gamble et al., 2021). Heat inactivation of coronavirus has been studied in a fluidic system for different temperatures and exposure times, and complete inactivation (> Log10 reduction) was obtained at a temperature of 83.4 oC and exposure time of 1.03 s (Jiang et al., 2021). It has been shown that SARS-CoV and SARS-COV-2 inactivation rates with temperature were the same (Hessling et al., 2020). For both viruses, a five log10-reduction was obtained at temperatures of 60 oC, 80oC, and 100 oC in 32.5, 3.7, and 0.5 minutes, respectively. Both viruses were deactivated at 120oC with a five log10-decline in 5.4 seconds (Hessling et al., 2020). Reduction of SARS-CoV-2 viability through solar heating in a vehicle has also been shown (Wang et al., 2021). Two studies were conducted to examine the inactivation of airborne SARS-CoV-2 as it passed through a heater (Canpolat et al., 2022; Yu et al., 2020). In one experiment, nickel foam was used as a heater, and 99.8% of the virus was inactivated at 200oC nickel foam temperature (Yu et al., 2020). In the other study, a coiled resistance wire was used as a heater, and at heater output temperatures of 150±5 oC and 220±5oC, the virus inactivation rates were 99.900% and 99.999%, respectively (Canpolat et al., 2022). However, the thermal inactivation of airborne viruses by an electric heater in a confined space has not been investigated yet.
This study used two electric heaters with different power and airflow rates to inactivate SARS-CoV-2 in a 30 m3 test room, and their effectiveness was compared.
Materials and Methods
2.1 Preparation of SARS-CoV-2 SuspensionsThe experiments were performed in biosafety level 3 (BSL3) facilities of Antimikrop Research and Biocidal Analysis Laboratories, accredited by the Ministry of Health of Turkey. BSL3 virology laboratory is fully equipped with negative pressure vacuum systems, air-lock systems, HEPA filters, and biosafety cabinets with HEPA filters (http://www.antimikrop.com.tr/ana-sayfa). A stock suspension of the SARS-CoV-2 strain (Gen Bank No: MT955161.1) was used. SARS-CoV-2 virus stock was prepared by inoculating the Vero E6 cell line in Dulbecco’s modified Eagle’s medium with supplements (DMEM-10). Dulbecco’s modified Eagle’s medium containing supplements (10% fetal bovine serum, 2nM/ml L-glutamine, 100 U/ml penicillin, 100 mg/ml streptomycin, and 0.5 mg/ml fungizone (Amphotericin B)) was added to the flask, and the cells were incubated at 37oC for 72 h. When the cells were lysed >95% by the virus under the microscope, the supernatant was collected, clarified by centrifugation, and stored at -80oC. TCID50 titer was determined by the Spearman- Kärber method as described (Hubert, 1984).