Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses

A direct approach to limit airborne viral transmissions is to inactivate them within a short time of their production. Germicidal ultraviolet light, typically at 254 nm, is effective in this context but, used directly, can be a health hazard to skin and eyes. By contrast, far-UVC light (207–222 nm) efficiently kills pathogens potentially without harm to exposed human tissues. We previously demonstrated that 222-nm far-UVC light efficiently kills airborne influenza virus and we extend those studies to explore far-UVC efficacy against airborne human coronaviruses alpha HCoV-229E and beta HCoV-OC43. Low doses of 1.7 and 1.2 mJ/cm2 inactivated 99.9% of aerosolized coronavirus 229E and OC43, respectively. As all human coronaviruses have similar genomic sizes, far-UVC light would be expected to show similar inactivation efficiency against other human coronaviruses including SARS-CoV-2. Based on the beta-HCoV-OC43 results, continuous far-UVC exposure in occupied public locations at the current regulatory exposure limit (~3 mJ/cm2/hour) would result in ~90% viral inactivation in ~8 minutes, 95% in ~11 minutes, 99% in ~16 minutes and 99.9% inactivation in ~25 minutes. Thus while staying within current regulatory dose limits, low-dose-rate far-UVC exposure can potentially safely provide a major reduction in the ambient level of airborne coronaviruses in occupied public locations.


Scientific Reports
| (2020) 10:10285 | https://doi.org/10.1038/s41598-020-67211-2 www.nature.com/scientificreports/ very limited compared with, for example, 254 nm (or higher) conventional germicidal UV light 21,22 . This limited penetration is still much larger than the size of viruses and bacteria, so far-UVC light is as efficient in killing these pathogens as conventional germicidal UV light [12][13][14] . However, unlike germicidal UV light, far-UVC light cannot penetrate either the human stratum corneum (the outer dead-cell skin layer), nor the ocular tear layer, nor even the cytoplasm of individual human cells. Thus, far-UVC light cannot reach or damage living cells in the human skin or the human eye, in contrast to the conventional germicidal UV light which can reach these sensitive cells [7][8][9][10] .
In summary far-UVC light is anticipated to have about the same anti-microbial properties as conventional germicidal UV light, but without producing the corresponding health effects. Should this be the case, far-UVC light has the potential to be used in occupied public settings to prevent the airborne person-to-person transmission of pathogens such as coronaviruses.
We have previously shown that a very small dose (2 mJ/cm 2 ) of far-UVC light at 222 nm was highly efficient in inactivating aerosolized H1N1 influenza virus 23 . In this work we explore the efficacy of 222 nm light against two airborne human coronaviruses: alpha HCoV-229E and beta HCoV-OC43. Both were isolated over 50 years ago and are endemic to the human population, causing 15-30% of respiratory tract infections each year 24 . Like SARS-CoV-2, the HCoV-OC43 virus is from the beta genus 25 .
Here we measured the efficiency with which far-UVC light inactivates these two human coronaviruses when exposed in aerosol droplets of sizes similar to those generated during sneezing and coughing 26 . As all coronaviruses have comparable physical and genomic size, a critical determinant of radiation response 27 , we hypothesized that both viruses would respond similarly to far-UVC light, and indeed that all coronaviruses will respond similarly.

Results
Inactivation of human coronaviruses after exposure to 222 nm light in aerosols infectivity assay. We used a standard approach to measure viral inactivation, assaying coronavirus infectivity in human host cells (normal lung cells), in this case after exposure in aerosols to different doses of far-UVC light. We quantified virus infectivity with the 50% tissue culture infectious dose TCID 50 assay 28 , and estimated the corresponding plaque forming units (PFU)/ml using the conversion PFU/ml = 0.7 TCID 50 29 . Figure 1 shows the fractional survival of aerosolized coronaviruses HCoV-229E and HCoV-OC43 expressed as PFU UV /PFU controls as a function of the incident 222-nm dose. Robust linear regression (Table 1) using iterated reweighted least squares 30 indicated that the survival of both genera alpha and beta is consistent with a classical exponential UV disinfection model (R 2 = 0.86 for HCoV-229E and R 2 = 0.78 for HCoV-OC43). For the alpha coronavirus HCoV-229E, the inactivation rate constant (susceptibility rate) was k = 4.1 cm 2 /mJ (95% confidence intervals (C.I.) 2.5-4.8) which corresponds to an inactivation cross-section (or the dose required to kill 90% of the exposed viruses) of D 90 = 0.56 mJ/cm 2 . Similarly, the susceptibility rate for the beta coronavirus HCoV-OC43 was k = 5.9 cm 2 /mJ (95% C.I. 3.8-7.1) which corresponds to an inactivation cross section of D 90 = 0.39 mJ/cm 2 .

Viral integration assay.
We investigated integration of the coronavirus in human lung host cells, again after exposure in aerosols to different doses of far-UVC light. Figures 2 and 3 show representative fluorescent 10x images of human lung cells MRC-5 and WI-38 incubated, respectively, with HCoV-229E (Fig. 2) and HCoV-OC43 (Fig. 3), which had been exposed in aerosolized form to different far-UVC doses. The viral solution was collected from the BioSampler after running through the aerosol chamber while being exposed to 0, 0.5, 1 or 2 mJ/cm 2 of 222-nm light. Cells were incubated with the exposed virus for one hour, the medium was replaced with fresh infection medium, and immunofluorescence was performed 24 hours later. We assessed the human

Discussion
The severity of the 2020 COVID-19 pandemic warrants the rapid development and deployment of effective countermeasures to reduce indoor person-to-person transmission. We have developed a promising approach using single-wavelength far-UVC light at 222 nm generated by filtered excimer lamps, which inactivates airborne viruses without inducing biological damage in exposed human cells and tissue [11][12][13][14][15][16][17][18] . The approach is based on the biophysically-based principle that far-UVC light, because of its very limited penetration in biological materials, can traverse and kill viruses and bacteria which are typically micrometer dimensions or smaller, but it cannot penetrate even the outer dead-cell layers of human skin, nor the outer tear layer on the surface of the human eye 12 .  Figure 2. Infection of human lung cells from irradiated aerosolized alpha HCoV-229E as function of dose of far-UVC light. Representative fluorescent images of MRC-5 normal human lung fibroblasts infected with human alphacoronavirus 229E exposed in aerosolized form. The viral solution was collected from the BioSampler after running through the aerosol chamber while being exposed to (a) 0, (b) 0.5, (c) 1 or (d) 2 mJ/ cm 2 of 222-nm light. Green fluorescence qualitatively indicates infected cells (Green = Alexa Fluor-488 used as secondary antibody against anti-human coronavirus spike glycoprotein antibody; Blue = nuclear stain DAPI). Images were acquired with a 10× objective; the scale bar applies to all the panels in the figure.
Scientific Reports | (2020) 10:10285 | https://doi.org/10.1038/s41598-020-67211-2 www.nature.com/scientificreports/ In this work we have used an aerosol irradiation chamber to test the efficacy of 222-nm far-UVC light to inactivate two aerosolized human coronaviruses, beta HCoV-OC43 and alpha HCoV-229E. As shown in Fig. 1, inactivation of the two human coronavirus by 222-nm light follows a typical exponential disinfection model, with an inactivation constant for HCoV-229E of k = 4.1 cm 2 /mJ (95% C.I. 2.5-4.8), and k = 5.9 cm 2 /mJ (95% C.I. 3.8-7.1) for HCoV-OC43. These values imply that 222 nm UV light doses of only 1.7 mJ/cm 2 or 1.2 mJ/cm 2 respectively produce 99.9% inactivation (3-log reduction) of aerosolized alpha HCoV-229E or beta HCoV-OC43. A summary of k values and the corresponding D 90 , D 99 , and D 99.9 values we have obtained for coronaviruses is shown in Table 2, together with our earlier results for aerosolized H1N1 influenza virus 23 . The relatively small difference in influenza A (H1N1) and human coronaviruses sensitivity to 222-nm light is likely attributable to differences in structure, genome size, and nucleic acid configuration 33 . It is also important to note that the previous results with H1N1 virus utilized a fluorescent focus assay to assess virus survival 23 in contrast to this work which used the TCID 50 assay. While both assays are widely used to accurately determine viral infectivity 34 , the former employs immunofluorescence to detect a specific viral antigen, instead of depending on cytopathic effects as in the TCID 50 assay. Because the assays differ in methods and principles, some variance is expected between these two techniques.
The results suggest that both of the studied coronavirus strains have similar high sensitivity to far-UVC inactivation. Robust linear regression produced overlapping 95% confidence intervals for the inactivation rate constant, k, of 2.5 to 4.8 cm 2 /mJ and 3.8 to 7.1 cm 2 /mJ respectively for the 229E and OC43 strains. As all human Representative fluorescent images of WI-38 normal human lung fibroblasts infected with human betacoronavirus OC43 exposed in aerosolized form. The viral solution was collected from the BioSampler after running through the aerosol chamber while being exposed to (a) 0, (b) 0.5, (c) 1 or (d) 2 mJ/cm 2 of 222-nm light. Green fluorescence qualitatively indicates infected cells (Green = Alexa Fluor-488 used as secondary antibody against anti-human coronavirus spike glycoprotein antibody; Blue = nuclear stain DAPI). Images were acquired with a 10× objective; the scale bar applies to all the panels in the figure. www.nature.com/scientificreports/ coronaviruses have similar genomic sizes which is a primary determinant of UV sensitivity 27 , it is reasonable to expect that far-UVC light will show similar inactivation efficiency against all human coronaviruses, including SARS-CoV-2. The data obtained here are consistent with this hypothesis. It is useful to compare the performance of far-UVC light with conventional germicidal (peak 254 nm) UVC exposure. We are aware of only one such study 35 , which used an aerosolized murine beta coronavirus. The study reported a D 88 of 0.599 mJ/cm 2 , which others 4 have used to estimate the D 90 for the virus with 254 nm light as 0.6 mJ/cm 2 . This value is similar to those estimated in the current work (see Table 2), suggesting similar inactivation efficiency of 222 nm far-UVC and conventional germicidal 254 nm UVC for aerosolized coronavirus, and providing further support for the suggestion that all coronaviruses have similar sensitivities to UV light.
The sensitivity of the coronaviruses to far-UVC light, together with extensive safety data even at much higher far-UVC exposures [12][13][14][15][16][17][18] , suggests that it may be feasible and safe to have the lamps providing continuous low-dose far-UVC exposure in public places -potentially reducing the probability of person-to-person transmission of coronavirus as well as other seasonal viruses such as influenza. In fact there is a regulatory limit as to the amount of 222 nm light to which the public can be exposed, which is 23 mJ/cm 2 per 8-hour exposure 36,37 . Based on our results here for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure at this regulatory limit would result in 90% viral inactivation in approximately 8 minutes, 95% viral inactivation in approximately 11 minutes, 99% inactivation in approximately 16 minutes and 99.9% inactivation in approximately 25 minutes. Thus continuous airborne disinfection with far-UVC light at the currently regulatory limit would provide a major reduction in the ambient level of airborne virus in occupied indoor environments.
In conclusion, we have shown that very low doses of far-UVC light efficiently kill airborne human coronaviruses carried by aerosols. A dose as low as 1.2 to 1.7 mJ/cm 2 of 222-nm light inactivates 99.9% of the airborne human coronavirus tested from both genera beta and alpha, respectively. As all human coronaviruses have similar genomic size, a key determinant of radiation sensitivity 27 , it is likely that far-UVC light will show comparable inactivation efficiency against other human coronaviruses, including SARS-CoV-2.
Together with previous safety studies [12][13][14][15][16][17][18] and our earlier studies with aerosolized influenza A (H1N1) 23 , these results suggest the utility of continuous low-dose-rate far-UVC light in occupied indoor public locations such as hospitals, transportation vehicles, restaurants, airports and schools, potentially representing a safe and inexpensive tool to reduce the spread of airborne-mediated viruses. While staying within the current regulatory dose limits, low-dose-rate far-UVC exposure can potentially safely provide a major reduction in the ambient level of airborne coronaviruses including SARS-CoV-2. . The virus infection medium consisted of MEM or RPMI-1640 plus 2% heat inactivated FBS for HCoV-229E and HCoV-OC43, respectively. The viral strains were propagated by inoculation of flasks containing 24-hours old host cells, which were 80-90% confluent. After one hour incubation, the cell monolayer was washed and incubated in fresh infection medium for three or four days at 35 °C for HCoV-229E and at 33 °C for HCoV-OC43. The supernatant containing the working viral stock was then collected by centrifugation (300 g for 15 minutes). The virus titer was determined by 50% tissue culture infective dose TCID 50 by assessing cytopathic effects (CPE), which were scored at a bright field microscope (10×) as vacuolization of cytoplasm, cell rounding and sloughing.

Methods
Benchtop aerosol irradiation chamber. A one-pass, dynamic aerosol/virus irradiation chamber was used to generate, expose, and collect aerosol samples as previously described 23 . Viral aerosols were generated by adding a virus solution in a high-output extended aerosol respiratory therapy nebulizer (Westmed, Tucson, AZ) and operating using an air pump with an input flow rate of 11 L/min. Virus flowed into the chamber and was mixed with dry and humidified air to maintain humidity between approximately 50-70%. The relative humidity, temperature, and aerosol particle size distribution were monitored throughout operation. Aerosol was exposed to far-UVC light and finally collected using a BioSampler (SKC Inc., Eighty Four, PA).
The far-UVC lamp was positioned approximately 22 cm away from the UV exposure chamber and directed at the 26 cm × 25.6 cm × 254 μm UV-transmitting plastic window (TOPAS 8007 × 10, TOPAS Advanced Polymers Inc., Florence, KY). Consistent with our previous experiments using this chamber 23 , the flow rate through the system was 12.5 L/min. The volume of the UV exposure region was 4.2 L so each aerosol was exposed for approximately 20 seconds as it traversed the window. The entire irradiation chamber was contained in a biosafety level 2 cabinet and all air inputs and outputs were equipped with HEPA filters (GE Healthcare Bio-Sciences, Pittsburgh, PA) to prevent unwanted contamination from entering or exiting the system. Irradiation chamber performance. The custom irradiation chamber simulated the transmission of aerosolized viruses produced via human coughing and breathing. The chamber operated at an average relative humidity of 66% and an average temperature of 24 °C across all runs. The average particle size distribution was 83% between 0.3 μm and 0.5 μm, 12% between 0.5 μm and 0.7 μm, and 5% >0.7 μm (Table 3) The distance between the lamp and the irradiation chamber permitted a single lamp to uniformly irradiate the entire exposure window area. Measurements using the silicon photodetector indicated an exposure intensity of approximately 90 μW/cm 2 across the exposure area. The chamber is equipped with a reflective aluminum surface opposite of the exposure window. As in our previous work with this chamber 23 , the reflectivity of this surface was approximately 15%. We have therefore conservatively estimated the intensity across the entire exposure area to be 100 μW/cm 2 . With the lamp positioned 22 cm from the window and given the 20 seconds required for an aerosol particle to traverse the exposure window, we calculated the total exposure dose to a particle to be 2 mJ/ cm 2 . We used additional sheets of UV transmitting plastic windows to uniformly reduce the intensity across the exposure region to create different exposure conditions. While in our previous work with these sheets we measured a transmission closer to 65% 23 , for these tests we measured the 222-nm transmission of each sheet to be approximately 50%. This decrease in transmission is likely due to the photodegradation of the plastic over time 4 . The addition of one or two sheets of the plastic covering the exposure window decreases the exposure dose to 1 and 0.5 mJ/cm 2 , respectively. Experimental protocol. As previously described 23 , the virus solution in the nebulizer consisted of 1 ml of Modified Eagle's Medium (MEM, Life Technologies, Grand Island, NY) containing 10 7 -10 8 TCID 50 of coronavirus, 20 ml of deionized water, and 0.05 ml of Hank's Balanced Salt Solution with calcium and magnesium (HBSS ++ ). The irradiation chamber was operated with aerosolized virus particles flowing through the chamber and the bypass channel for 5 minutes prior to each sampling, in order to establish the desired RH value. Sample collection was initiated by changing air flow from the bypass channel to the BioSampler using the set of three way valves. The BioSampler was initially filled with 20 ml of HBSS ++ to capture the aerosol. During each sampling time, which lasted for 30 minutes, the inside of the irradiation chamber was exposed to 222-nm far-UVC light entering through the plastic window. Variation of the far-UVC dose delivered to aerosol particles was achieved by inserting additional UV-transparent plastic films as described above thereby delivering the three test doses of 0.5, 1.0 and 2.0 mJ/cm 2 . Zero-dose control studies were conducted with the excimer lamp turned off. After the sampling period was completed the solution from the BioSampler was used for the virus infectivity assays.
Virus infectivity assays. TCID 50 . We used the 50% tissue culture infectious dose assay to determine virus infectivity 28 . Briefly, 10 5 host cells were plated in each well of 96-well plates the day prior the experiment. Cells were washed twice in HBSS ++ and serial 1:10 dilutions in infection medium of the exposed virus from the BioSampler was overlaid on cells for two hours. The cells were then washed twice in HBSS ++ , covered with fresh infection medium, and incubated for three or four days at 34 °C. Cytopathic effects (CPE) were scored at a bright field microscope (10×) as vacuolization of cytoplasm, cell rounding and sloughing. The TCID 50 was calculated with the Reed and Muench method 28,38 . To confirm the CPE scores, the samples were fixed in 100% methanol for five minutes and stained with 0.1% crystal violet. The results are reported as the estimate of plaque forming units (PFU)/ml using the conversion PFU/ml = 0.7 TCID 50 by applying the Poisson distribution 29 .
Immunofluorescence. To assess whether increasing doses of 222-nm light reduced the number of infected cells, we performed a standard fluorescent immunostaining protocol to detect a viral antigen in the host human cells 23   Data analysis. The surviving fraction (S) of the virus was calculated by dividing the fraction PFU/ml at each UV dose (PFU UV ) by the fraction at zero dose (PFU controls ): S = PFU UV /PFU controls . Survival values were calculated for each repeat experiment and natural log (ln) transformed to bring the error distribution closer to normal 39 . Robust linear regression using iterated re-weighted least squares (IWLS) 40,41 was performed in R 3.6.2 software using these normalized ln[S] values as the dependent variable and UV dose (D, mJ/cm 2 ) as the independent variable. Using this approach, the virus survival [S] was described by first-order kinetics according to the equation 4 : where k is the UV inactivation rate constant or susceptibility factor (cm 2 /mJ). The regression was performed with the intercept term set to zero representing the definition of 100% relative survival at zero UV dose, separately for each studied virus strain. The data at zero dose, which by definition represent ln[S] = 0, were not included in the regression. Uncertainties (95% confidence intervals, CI) for the k parameter for each virus strain were estimated by bootstrapping for each regression method because bootstrapping may result in more realistic uncertainty estimates, compared with the standard analytic approximation based on asymptotic normality, in small data sets such as those used here (n = 3 HCoV-229E and n = 4 for HCoV-OC43). Goodness of fit was assessed by coefficient of determination (R 2 ). Analysis of residuals for autocorrelation and for heteroskedasticity was performed using the Durbin-Watson test 42 and Breusch-Pagan test (implemented by lmtest R package) 43 , respectively. Parameter estimates (k) for each virus strain were compared with each other based on the 95% CIs and directly by t-test, using the sample sizes, k values, and their standard errors. The virus inactivation cross section, D 90 , which is the UV dose that inactivates 90% of the exposed virus, was calculated as D 90 = − ln[1 − 0.90]/k. Other D values were calculated similarly.