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 Vr, 0.8Vr 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.