Fig. 3. (a) The first electric heater at one meter high in the test room operated for 80 minutes. (b) The second electric heater on the ground operated for 75 minutes.
During the set-up of the experimental procedure, six control experiments were carried out to determine the TCID50 titer of SARS-CoV-2 in the remaining stock solution after the nebulization process. The mean log10 TCID50 titer of the virus stock solution was 7.50±0.30. At the beginning of the experiments, the test room’s temperature was 20\(\mp 1\)oC, at 40% humidity. At the end of all experiments, one m3 of air was drawn from the test room through a hose connected to a vacuum pump (MD8 Airscan, Sartorius, Göttingen, Germany). The inlet gelatin membrane filter (Sartorius, Göttingen, Germany) on the hose collected airborne virus while the vacuum pump drew air from the test room. Next, the filter was dissolved in phosphate-buffered saline (PBS) at 37 oC, and the TCID50 titer of the solution was determined by the Spearman- Kärber method as described (31). Briefly, Log10 dilutions of the harvest from the filter (10-1 to 10-6 dilutions) were transferred to 96-well plates containing Vero E6 cells and incubated at 37\(\mp 1\) oC, 5% CO2 conditions. After four days of incubation, the cytopathic effect was evaluated under an inverted microscope, and the virus TCID50 values of the gelatin filter were obtained in control and the test experiments.
In the first experiment, the small electric heater was operated in the test room for 80 min after the nebulization. The control experiment was performed under the same conditions, except the electric heater was off. In the second experiment, the large electric heater was operated in the test room for 75 min, and the control experiment was performed while the electric heater was off. Due to the temperature increase in the test room, the large electric heater was 5 min less operated than the small electric heater.
Results and Discussion
After running the small electric heater in the test room for 80 minutes, the airborne SARS-CoV-2 log10 TCID50value was log10T=2.63. After the control experiment, the log10 TCID50 value was log10C=5.00. Total log10- reduction in the infectivity of airborne SARS-CoV-2 in the test room was,LRS,total = log10C-log10T= 2.37 (99.57% ). At the end of the experiment, the room temperature was 40\(\mp 1\)oC at a relative humidity of 23%. Subindexes Sand L refer to small and large electric heaters.
The log10 TCID50 value of the airborne SARS-CoV-2 in the test room after running the large electric heater for 75 min was log10T=0.75, and for the control experiment, it was log10C=4.88. For the large electric heater,LRL,total =4.13 (99.99%). After the test experiment, the room’s temperature was 47\(\mp 1\) oC at a relative humidity of 19%.
The inactivation of airborne SARS-CoV-2 as a function of airflow temperature has been previously studied (Canpolat et al., 2022; Yu et al., 2020). The present study investigated the thermal inactivation of airborne SARS-CoV-2 in a 30 m3 test room. In this study, the small electric heater had an airflow rate of 44 m3/h and an air outlet temperature of 220oC. It was operated for 80 min in the test room, and 58.6 m3 of air passed through the heater. As a result, the total circulating air through the heater was 1.95 times the volume of the test room, and 99.57% of the airborne SARS-CoV-2 lost infectivity. The large electric heater had an airflow rate of 324 m3/h and an air outlet temperature of 150oC, and it was operated for 75 min in the test room. In this experiment, the total circulating air through the large electric heater was 13.5 times the volume of the test room, and the airborne SARS-CoV-2 was inactivated by 99.99%.
At the end of the experiments with the small and large electric heater, the test room temperature was 40 oC, at 23% humidity, and 47 oC, at 19% humidity, respectively. Increased air temperature in the test room also reduces the infectivity of the viruses. The viruses may lose their infectivity either during passing through the electric heater or due to the increased room temperature. Therefore, we defined the total logarithmic reduction (LRtotal ) as a sum of reductions in viability due to the electric heater and an increase in the room’s air temperature. In order to achieve the heater’s effectiveness in reducing the viability of viruses (LREH ), the contribution of room temperature in reducing the viability of viruses (LRRT ) should be subtracted from theLRtotal . The room temperature and time dependence of the LRRT in the infectivity of SASR-CoV-2 can be expressed as (Hessling et al., 2020)
\(\text{LR}_{\text{RT}}=k\left(T\right).t=10^{-\frac{5574.7}{T}+15.928}\).t                                                                                   (1)
Where k(T) is the inactivation rate constant of SARS-CoV-2 in the first-order reaction model, T is the temperature in degrees Kelvin, and t is the time the virus was exposed to heat at the temperature of T . In the use of the small electric heater, the test room temperature increased from 293 oK (20oC) to 313 oK (40oC) in 80 minutes, and in the use of the large electric heater, the temperature increased from 293 oK to 320 oK (47 oC) in 75 minutes. We did not record the time-dependent temperature variation in the test room during the experiments; therefore, we can not directly calculate the temperature-dependent LRRT value using Eq.1. Here, we assumed that the temperature increases linearly with time and calculated the temperature rise per minute for both electric heaters to obtain LRRT. The temperature increase per minute for the small and large electric heaters are ΔT =0.25oK and ΔT = 0.36 oK, respectively. In that case, Eq.1 can be written in a discrete form as
\(\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\text{LR}_{\text{RT}}=\sum_{i=0}^{t}10^{-\frac{5574.7}{293+i*T}+15.928}\)                                           (2)
The temperature increased by ΔT in each time interval ofi = 0, 1, 2, …., t . Here, each time interval is one minute. For the small and the large electric heater experiments, the time t is 80 and 75, respectively, in Eq. 2. TheLRRT was calculated for the small and large electric heater using Eq. 2 and obtained asLRS,RT = 0.37, and LRL,RT=0.68, respectively. The LRS,total =2.37 (99.57% decrease in infectivity ), which is much higher than theLRS,RT =0.37 (57.14% decrease in infectivity). The small electric heater’s net contribution in the logarithmic reduction was LRS,EH =LRS,total - LRS,RT = 2.00 (99.00%). For the large electric heater,LRL,total =4.13 (99.99%) andLRL,RT = 0.68 (79.15%), and the net contribution of the large electric heater was, LRL,EH = 3.45 (99.96%). Potentially, viral loss also occurs during nebulization and air removal from the test room. However, since both processes were carried out under the same conditions in the control and test experiments, these losses have no effect when calculating the viral loss from the electric heater.
In addition to the air outlet temperature, the number of recirculations of all the air in the room within a given time may be an essential factor for the inactivation of airborne SARS-CoV-2 in the test room. In our first study (30), it was shown that the infectivity of the virus in the air passed through the electric heater decreased by 99.900% and 99.999%, at the electric heater’s outlet air temperatures of 150oC and 220 oC, respectively. Hence, we may define the log10 reduction of the infectivity (LRT ) as a function of the outlet air temperature of the electric heater and writeLR150 =3, and LR220 =5 and percentage reduction as PR150 =99.900% andPR220 =99.999%. In a room of volumeVr , if all the room air passes through the electric heater, the logarithmic reduction in airborne virus infectivity in the room should be equal to the LRT value. If all the air in the room passes through the electric heater ntimes, the logarithmic reduction in the airborne coronavirus infectivity (LRn,T ) and the percentage reduction (PRn,T ) for the room can be defined as
\(\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{\ \ \ \ \ \ \ \ \ \ }\text{LR}_{n,T}=n.\text{LR}_T\)                                                                              (3)
and
\(\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{PR}_{n,T}=\left(1-10^{-\text{LR}_{n,T}}\right).100\%\)                                                        (4)
\begin{equation} n=\frac{\text{t.}\Phi}{V_{r}}\nonumber \\ \end{equation}
The n depends on the operating time of the heater (t ), the electric heater’s air flow rate (Φ), and the room volume,Vr=30 m3 . The small electric heater with the outlet air temperature of T =220oC, RLT =5, t =80 min,\(\Phi=44\ m^{3}/h\), n =1.95, and LRS,n,T =9.75. The measured logarithmic reduction is due to the small electric heater, LRS,EH =2.00, and the ratio ofLRS,n,T /LRS,EH = 4.87. For the large electric heater with the outlet air temperature ofT =150 oC,LRL,T = 3, t = 75 min,\(\Phi=324\ m^{3}/h\), n =13.88 andLRL,n,T = 41.64. The measured logarithmic reduction after the experiment using the large electric heater isLRL,EH =3.45, and the ratio ofLRL,n,T /LRL,EH =12.07.
If all air in the room had passed through the electric heaters ntimes, the ratio LRn,T/LREH would equal one. The ratio is 4.87 for the small electric heater and 12.07 for the large electric heater. Let’s assume that for everyVr m3 of air that passes through the device, x fraction of it is the air that passes more than once, and 1-x fraction of the room’s air volume passes through the heater once. In that case, we can re-write Eq. (3 ) and Eq. (4) as
\(\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{ \ \ }\text{LR}_{n,T,x}=(1-x).\text{n.LR}_T\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\)(5)
\(\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{\ \ \ }\text{PR}_{n,T,x}=\left(1-10^{-\text{LR}_{n,EHx}}\right).100\%\)                        (6)
Where LRn,T,x= LRL,EH for the small electric heater and LRn,T,x= LRS,EH for the large electric heater. During each circulation of the air volume of Vr through the electric heater, xVr m3 of the air volume a second time passes through the electric heater, and the same amount of the air volume does not pass through the heater. We did not count the LR of the air that second time passed through the heater in the circulation of the air volume of Vrin Eq. (5) and Eq. (6). We can calculate the x value using Eq. (5) as
\(\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 1-x=\frac{\text{LR}_{n,T,x}}{\text{n.}\text{LR}_T}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\) (7)
For the small electric heater LRn,T,x =2.0, LRT =5, n =1.95, and x =0.80. For the large electric heater, LRn,T,x =3.45, LRT =3, n =41.64, and x =0.97. These results show that, for the small electric heater, for each circulation of the air volume of Vr0.8Vr of the air volume passes through the heater a second time, and 0.2Vr volume of the air passes the first time through the electric heater. Here we can call the coefficient 1-x in Eq. (5) the “air circulation efficiency” of the electric heater, and its value is 0.2 for the small electric heater and 0.03 for the large electric heater. This means that for the circulation of all the air in the room, the volume of air that must pass through the device is V =Vr/(1-x) . The air volume is V=5Vr for the small electric heater and V= 33.3Vr for the large electric heater. The small electric heater is more efficient than the large one at circulating all the air in the room.
For the small electric heater, n =1.95 and 1/(1-x) =5, and since n<1/(1-x ), it means that all the air in the room did not pass through the electric heater, and the expectation is LRS,EH<LRT . The results are consistent with the estimation since LRS,EH = 2.0, and LRT =5. The fact that n =41.64 and 1/(1-x) = 33.3 and n>1/(1-x) for the large electric heater indicates that all the air in the room passed through the electric heater, and must be LRL,EH>LRT . Therefore, the measured LRL,EH =3.45, and LRT =3 are consistent with the expected result.
Two electric heaters with powers of 1.5 kW and 3 kW and an airflow rate of 44 m3/h and 324 m3/h were used in the SARS-CoV-2 test experiments in a 30 m3 room, leading to the room temperature increase of 20 oC and 27 oC while simultaneously reducing the infectivity of airborne SARS-CoV-2.
The x value is smaller for the small electric heater than the large electric heater, indicating that the small electric heater is better at circulating all air room than the large electric heater. Hence, having more small electric heaters, such as 3-4, locating different corners of the room may be more effective in reducing the infectivity of the airborne SARS-CoV-2.
The developed electric heater has the potential to be used to heat interior spaces while also reducing the infectivity of SARS-CoV-2 in homes, shopping centers, restaurants, classrooms, rooms in hospitals, offices, and public transport vehicles such as trains, metro, and tramways during winter. We propose a method to evaluate the efficacy of two electric heaters in reducing the infectiousness of airborne SARS-CoV-2 in a test room. As a result, we defined the 1-x parameter named “air circulation efficiency” to measure the efficiency of an electric heater at circulating air in the room. The air circulating efficiency parameter may depend on the volume of the room, the airflow rate, the device inactivation rate, the number of devices in the room, and their locations. Therefore, more experiments should be performed for the optimization of reducing the infectivity of airborne viruses in a confined space. Furthermore, this 1-x parameter can be used for air purification devices such as UV-C and HEPA filters.
There are some limitations of the study, such as a lack of monitoring temperature of the test room and repetition of the test experiments due to limited sources. One other limitation is the air extraction from only one location in the test room to measure the airborne virus infectivity.
Conclusion
The developed electric heater uses the same energy to heat an enclosed space and reduce the viability of airborne SARS-CoV-2. It has the potential to inactivate SARS-CoV-2 and other airborne pathogens during the winter months. Therefore, further experiments with different viruses and bacteria are needed. Nevertheless, the electric heater can potentially prevent the airborne spread of the pandemic indoors, besides heating in winter. The dual function of the electric heater gives it an edge over air purifiers such as UV-C or HEPA filters for winter use.