Thermal Inactivation of Aerosolized SARS-CoV-2

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread worldwide with its different variants. The transmission efficiency of the new variants is much higher than the existing ones. Therefore, developing new preventive measures based on the transmission routes of the virus is needed to limit the spread. The possible transmission routes include direct contact with surfaces contaminated with droplets secreted by patients and airborne viral transmission from person to person. Thermal inactivation is a preventive measure that applies high temperature to objects or fluids, as has been reported previously. However, inactivation data of aerosolized SARS-CoV-2 exposed to heat for a short time at high temperatures are not in the literature yet. We evaluated the inactivation of the aerosolized virus while passing through an electric heater. The virus inactivation test experiments were conducted at two temperatures of the heater ’s outlet air, 150±5 o C, and 220±5 o C, at an air flow rate of 0.6 m 3 /h (10 L/min) and heat exposure time of 1.44 s. The loss in viability of the virus at 150 o C and 220 o C was measured as 99.900% and 99.999%, respectively. The results indicate that the high-temperature inactivation of SARS-CoV-2 may potentially reduce aerosolized viral indoors.


Introduction
The new variance of SARS-CoV-2 has a 43% to 90% higher reproduction number than the existing variants 1 . Possible transmission routes include airborne particles, respiratory droplets, and contact with a contaminated surface. Interrupting the chain of the transmission routes is vital to limit the spread of the virus. One of the inactivation methods is high-temperature exposure of contaminated surfaces or liquids, which has been reported previously [2][3][4] . Heat inactivation of SARS-CoV-2 has mainly been used to sterilize contaminated personal protective equipment such as masks and gloves in hospitals and contaminated equipment and liquids in laboratories before reuse 5-7 . SARS-CoV in liquid was inactivated in 45 minutes and 75 minutes at a temperature of 75 o C and 56 o C, respectively 3 . A 4 log10 TCID50 reduction was observed with a heat treatment protocol of 60 o C -60 min and 92 o C -15 min 8 . Using the SARS-CoV and SARS-CoV-2 data from different studies, the time required for 5 log10-reduction was estimated as 32.5, 3.7, and 0.5 minutes for temperatures of 60 o C, 80 o C, and 100 o C 4 . A 6 log10 TCID50 reduction was obtained within a fluidic system within 1.03 s at a temperature of 83.4 o C 9 . Nevertheless, dry heat inactivation of aerosolized SARS-CoV-2 has not been investigated yet.
During the Covid-19 pandemic, exposure to indoor aerosolized SARS CoV-2 has become one of the primary challenges 10,11 . Heat inactivation of the aerosolized SARS-CoV-2 is one way to reduce the spread of 2019 coronavirus disease (COVID-19). In the present study, the inactivation of SARS-CoV-2 at high air temperatures of 150 o C and 220 o C has been investigated using an experimental setup, while minimizing potential hazards. 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 37 o C for 72 h. The supernatant was collected, clarified by centrifugation, and stored at -80 o C. TCID50 titer was determined by the Spearman-Kärber method as described 13 .  A detailed illustration of the electric heater is shown schematically in Fig. 2. The body of the heater was made of a metal sheet with a thickness of one mm. In the heater, an electric heating coil was located perpendicular to the air flow and used as a thermal energy source. The heater's airway cross-section area is 4x4 cm 2 with a length of 25 cm. The heating coil occupies 15 cm of the heater. The thermocouple was located at the outlet of the heater to measure the outlet temperature. Outside of the heater was isolated using a fiberglass slab.

Fig.2 Detailed schematic presentation of the heater.
In the control experiment, the heater was off, the compressor and the nebulizer were on for five minutes. The control experiments were repeated twice. In the test experiments, the heater's outlet temperature was set to 150±5 o C and 220±5 o C. This was accomplished by varying the current through the electric heating coil. The air flow rate was set to 0.6 m 3 /h in all the experiments. In the test experiments, the compressor and the heater were turned on, and then the outlet air temperature was set to one of the above temperatures. Then, the nebulizer was turned on. After five minutes, the nebulizer was turned off, then the heater and the compressor were turned off. After the experiments, the gelatin filter inside the inline polycarbonate filter holder was dissolved in distilled water to harvest the virus.

Results and discussion
The viral presence of the stock virus was 7.5 log10 TCID50. The control experiments were performed twice, and the average viral load was 5.5 log10TCID50. In setting the heater outlet air temperature to 150±5 o C, the viral load was reduced to 2.5 log10TCID50. The reduction in the viability of the virus is 3 log10 or 99.9%. At the higher temperature of 220±5 o C, the virus load viability was 0.5 log10TCID50, reducing the infectivity of the virus to 5 log10 or 99.999%.

Conclusion
Our results show that high-temperature is very effective in inactivating aerosolized SARS-CoV-2 within a second. It can be implemented primarily during winter, just by increasing the heater's temperature to 150 o C or above for a fraction of a second to provide 3 log10 reductions in the viral load of SARS-CoV-2 in air. It has the potential to be used in houses, hospitals, shopping centers, HVAC (heating, ventilating, and air conditioning) systems, and public transport vehicles during winter.