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Far-UVC light efficiently and safely inactivates airborne human coronaviruses
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The global spread of coronavirus disease (COVID-19) has escalated the need for public sanitation. Ultraviolet (UV) light is a well-established antimicrobial; however, the UV light commonly used for this purpose (~254 nm) poses significant health risks in public spaces due to its cancer-causing effects. Far-UVC light (~207-222 nm) has similar germicidal properties without health risks, as it does not penetrate human eye or skin cells. This group previously found low doses of far-UVC light (222 nm) to be an effective germicide against aerosolized H1N1 influenza virus. In this study, the researchers tested two viruses that are closely related to SARS-CoV-2, the virus that causes COVID-19. Because the sensitivity of a virus to UV is determined by its genome size and all coronaviruses have genomes of similar sizes, it is reasonable to expect the sensitivity of SARS-CoV-2 to be comparable to these viruses.The authors tested several doses of far-UVC on aerosolized viruses. They found that for both viruses tested, 99.9% of the virus was inactivated at a lower dose than the one needed to inactivate H1N1 virus. The authors also suggested a safe dose for an 8-hour work shift based on recommendations from the International Commission on Non-Ionizing Radiation Protection (ICNIRP). At this dose, 90% of aerosolized viruses would be deactivated in 8 minutes, 95% in 16 minutes, and 99% in 25 minutes. These results combined with the earlier results for H1N1 suggest that low doses of far-UVC light in heavily occupied spaces would reduce the spread of viral diseases, likely including COVID-19.
Lay Summary
A direct approach to limit airborne transmission of pathogens is to inactivate them within a short time of their production. Germicidal ultraviolet light (UV), typically at 254 nm, is effective in this context, but it is a health hazard to the skin and eyes. By contrast, far-UVC light (207-222 nm) efficiently kills pathogens without harm to exposed human cells or tissues. We previously demonstrated that 222-nm UV light efficiently kills airborne influenza virus (H1N1); here we extend the far-UVC studies to explore efficacy against human coronaviruses from subgroups alpha (HCoV-229E) and beta (HCoV-OC43). We found that low doses of, respectively 1.7 and 1.2 mJ/cm2 inactivated 99.9% of aerosolized alpha coronavirus 229E and beta coronavirus OC43. Based on these results for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure in public locations at the currently recommended exposure limit (3 mJ/cm2/hour) would result in 99.9% viral inactivation in ~ 25 minutes. Increasing the far- UVC intensity by, say, a factor of 2 would halve these disinfection times, while still maintaining safety. As all human coronaviruses have similar genomic size, a key determinant of radiation sensitivity, it is realistic to expect that far-UVC light will show comparable inactivation efficiency against other human coronaviruses, including SARS-CoV-2.
Visual Abstract
Keywords
aerosolized coronavirus, beta HCoV-OC43, alpha HCoV-229E, far-UVC light,
Figure 1
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Coronavirus disease 2019 (COVID–19) was first reported in December 2019 and then characterized as a pandemic by the World Health Organization on March 11, 2020. Despite extensive efforts to contain the spread of the disease, it has spread worldwide with over 1.9 million confirmed cases and over 130,000 confirmed deaths as of April 16, 2020 1. Transmission of SARS-CoV–2, the beta coronavirus causing COVID–19, is both through direct contact and airborne routes, and studies of SARS-CoV–2 stability have shown viability in aerosols for at least 3 hours 2. Given the rapid spread of the disease, including through asymptomatic carriers 3, it is of clear importance to explore practical mitigation technologies that can inactivate the airborne virus in public locations and thus limit airborne transmission.
Ultraviolet (UV) light exposure is a direct antimicrobial approach 4 and its effectiveness against different strains of airborne viruses has long been established 5. The most commonly employed type of UV light for germicidal applications is a low pressure mercury-vapor arc lamp, emitting around 254 nm; more recently xenon lamp technology has been used, which emits broad UV spectrum 6. However, while these lamps can be used to disinfect unoccupied spaces, operation of these conventional germicidal UV lamps in occupied public spaces - critical to prevent person-to-person transmission - is not possible since exposure to these wavelengths is a health hazard, causing both skin cancer and eye diseases 7–10.
By contrast far-UVC light (207 to 222 nm) has been shown to be as efficient as conventional germicidal UV light in killing microorganisms 11, but it does not have the human health issues associated with conventional germicidal UV light. In short (see below) the reason is that far-UVC has a range in biological materials of only a few micrometers, and thus it cannot reach living human cells in the skin or eyes. But because viruses (and bacteria) are extremely small, far-UVC light can still penetrate and kill them. Thus far-UVC light has about the same highly effective germicidal properties of UV light, but without the associated human health risks 12–15. Several groups have thus proposed that far-UVC light (207 or 222 nm), which can be generated using inexpensive excimer lamps, is a potential safe and efficient anti-microbial technology 12–18 which can be deployed in occupied public locations.
The physics-based mechanistic basis to this far-UVC approach 12 is that light in this wavelength range has a very limited penetration depth. Specifically, far-UVC light (207–222 nm) is very strongly absorbed by proteins through the peptide bond, and other biomolecules 19,20, so its ability to penetrate biological materials is 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 12–14. 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 7–10.
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/cm2) 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 subgroup 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.
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 TCID50 assay 28, and estimated the corresponding plaque forming units (PFU)/ml using the conversion PFU/ml = 0.7 TCID50 29. Fig. 1 shows the fractional survival of aerosolized coronaviruses HCoV–229E and HCoV-OC43 expressed as PFUUV/PFUcontrols 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 subgroups alpha and beta is consistent with a classical exponential UV disinfection model (R2 = 0.86 for HCoV–229E and R2 = 0.78 for HCoV-OC43). For the alpha coronavirus HCoV–229E, the inactivation rate constant (susceptibility rate) was k = 4.1 cm2/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 D90 = 0.56 mJ/cm2. Similarly, the susceptibility rate for the beta coronavirus HCoV-OC43 was k = 5.9 cm2/mJ (95% C. I. 3.8–7.1) which corresponds to an inactivation cross section of D90 = 0.39 mJ/cm2.
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. Figs. 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/cm2 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 cell lines for expression of the viral spike glycoprotein, whose functional subunit S2 is highly conserved among coronaviruses 31,32. In Figs. 2 and 3, green fluorescence (Alexa Fluor®–488 used as secondary antibody against anti-human coronavirus spike glycoprotein antibody) qualitatively indicates infection of cells with live virus, while the nuclei were counterstained with DAPI appearing as blue fluorescence. For both alpha HCoV–229E and beta HCoV-OC43, exposure to 222-nm light reduced the expression of the viral spike glycoprotein as indicated by a reduction in green fluorescence.
The severity of the 2020 COVID–19 pandemic warrants the rapid development and deployment of effective countermeasures to reduce 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 inactivate viruses and bacteria, without inducing biological damage in exposed human cells and tissue 11–16. 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.
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 cm2/mJ (95% C. I. 2.5–4.8), and k = 5.9 cm2/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/cm2 or 1.2 mJ/cm2 respectively produce 99.9% (3-log reduction) of aerosolized alpha HCoV–229E or beta HCoV-OC43. A summary of k values and the corresponding D90, D99, and D99.9 values we have obtained for coronaviruses is shown in Table 2, together with our earlier results for aerosolized H1N1 influenza virus 23.
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 cm2/mJ and 3.8 to 7.1 cm2/mJ respectively for the 229E and OC43 strains. As all human 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 33, which used an aerosolized murine beta coronavirus. The study reported a D88 of 0.599 mJ/cm2, which others 4 have used to estimate the D90 for the virus with 254 nm light as 0.6 mJ/cm2. 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 12–18, suggests that it will be feasible and safe to have the lamps providing continuous far-UVC exposure in public places to significantly reduce the probability of person-to-person transmission of coronavirus as well as other seasonal viruses such as influenza. For example, the current dose limit guideline for 222 nm light from the International Commission on Non- Ionizing Radiation Protection (ICNIRP) is 23 mJ/cm2 per 8-hour exposure 34. Interpreting this as an intensity of ~3 mJ/cm2/hour, and based on our results here for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure at this intensity 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. Increasing the intensity by, say, a factor of 2 would halve these disinfection times, while still maintaining safety 12–18.
In conclusion, we have shown that very low doses of far-UVC light efficiently kill airborne human coronaviruses carried by aerosols. An exposure dose as low as 1.2 to 1.7 mJ/cm2 of 222-nm light inactivates 99.9% of the airborne human coronavirus tested from both subgroups beta and alpha, respectively. Together with previous safety studies 12–18 and our earlier studies with aerosolized influenza A (H1N1) 23, these results indicate that far-UVC light is a potentially powerful and practical approach for reduction of airborne viral transmission, without the human health hazards associated with conventional germicidal UVC lamps.
These results suggest the utility of deploying low dose rate far-UVC lights in highly occupied indoor public locations such as hospitals, transportation vehicles, airports and schools, potentially representing a safe and inexpensive tool to reduce the spread of airborne-mediated microbial diseases.
Far-UVC technology can potentially help limit seasonal influenza epidemics, transmission of measles and tuberculosis, as well as future pandemics.
Viral Strains. HCoV–229E (VR–740) and HCoV-OC43 (VR–1558) were propagated in human diploid lung MRC–5 fibroblasts (CCL–171) and WI–38 (CCL–75), respectively (all from ATCC, Manassas, VA). Both human cell lines were grown in MEM supplemented with 10% Fetal Bovine Serum (FBS), 2 mM L-alanyl-L-glutamine, 100 U/ml penicillin and 100 µg/ml streptomycin (Sigma-Aldrich Corp. St. Louis, MO, USA). 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 TCID50 by assessing cytopathic effects (CPE), which were scored at a bright field microscope (10x) as vacuolization of cytoplasm, cell rounding and sloughing.
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 8007x10, Topas Advanced Polymers, 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). Aerosolized viruses were efficiently transmitted through the system as evidenced from the control (zero exposure) showing clear virus integration (Figs. 2 and 3, top left panels).
Far-UVC Lamp and Dosimetry. The far-UVC source used in this study was a 12 W 222-nm KrCl excimer lamp module made by Ushio America (Item #9101711, Cypress, CA). The lamp is equipped with a proprietary optical filtering window to reduce lamp emissions outside of the 222 nm KrCl emission peak. The lamp was positioned 22 cm away from the exposure chamber window and directed at the center of the window. Optical power measurements were performed using an 818-UV/DB low-power UV enhanced silicon photodetector with an 843-R optical power meter (Newport, Irvine, CA). Dosimetry was performed prior to starting an experiment to measure the irradiation fluence within the chamber at the position of the aerosol.
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/cm2 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/cm2. 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/cm2. 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/cm2, 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 107–108 TCID50 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 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/cm2. 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.
TCID50. We used the 50% tissue culture infectious dose assay to determine virus infectivity 28. Briefly, 105 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 34ºC. Cytopathic effects (CPE) were scored at a bright field microscope (10x) as vacuolization of cytoplasm, cell rounding and sloughing. The TCID50 was calculated with the Reed and Muench method 28,35. 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 TCID50 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. Briefly, 2x105 host cells (MRC–5 cells for HCoV–229E and WI–38 for HCoV- OC43) were plated in each well of 48-well plates the day prior the experiment. After running through the irradiation chamber for 30 minutes, 150 µl of virus suspension collected from the BioSampler was overlaid on the monolayer of host cells. The cells were incubated with the virus for one hour, then washed three times with HBSS++, and then incubated overnight in fresh infection medium. Infected cells were then fixed in 100% ice cold methanol at 4°C for 5 minutes and labeled with anti-human coronavirus spike glycoprotein (40021-MM07, Sino Biologicals, Chesterbrook, PA) 1:200 in HBSS++ containing 1% bovine serum albumin (BSA; Sigma-Aldrich Corp. St. Louis, MO, USA) at room temperature for one hour with gentle shaking. Cells were washed three times in HBSS++ and labeled with goat anti-mouse Alexa Fluor®–488 (Life Technologies, Grand Island, NY) 1:800 in HBSS++ containing 1% BSA, at room temperature for 30 minutes in the dark with gentle shaking. Following three washes in HBSS++, the cells were stained with Vectashield containing DAPI (4’,6-diamidino–2- phenylindole) (Victor Laboratories, Burlingame, CA) and observed with the 10x objective of an Olympus IX70 fluorescent microscope equipped with a Photometrics PVCAM high-resolution, high- efficiency digital camera and Image-Pro Plus 6.0 software (Media Cybernetics, Bethesda, MD). For each 222-nm dose and virus subgroup, the representative results were repeated twice. For each sample, up to ten fields of view of merged DAPI and Alexa Fluor®–488 images were acquired.
Data Analysis. The surviving fraction (S) of the virus was calculated by dividing the fraction PFU/ml at each UV dose (PFUUV) by the fraction at zero dose (PFUcontrols): S = PFUUV/PFUcontrols. Survival values were calculated for each repeat experiment and natural log (ln) transformed to bring the error distribution closer to normal 36. Robust linear regression using iterated re-weighted least squares (IWLS) 37,38 was performed in R 3.6.2 software using these normalized ln[S] values as the dependent variable and UV dose (D, mJ/cm2) as the independent variable. Using this approach, the virus survival [S] was described by first-order kinetics according to the equation 4:
ln[S] = −k × D[Eq. 1]
where k is the UV inactivation rate constant or susceptibility factor (cm2/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 (R2). Analysis of residuals for autocorrelation and for heteroskedasticity was performed using the Durbin- Watson test 39 and Breusch-Pagan test (implemented by lmtest R package) 40, 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, D90, which is the UV dose that inactivates 90% of the exposed virus, was calculated as D90 = − ln[1 − 0.90]/k. Other D values were calculated similarly.
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- Marazzi, Algorithm, Routines, and S functions for Robust Statistics. (Wadsworth & Brooks/cole, 1993).
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Acknowledgements
This work was supported by the Shostack Foundation and by NIH grant R42-AI125006-03. We thank Dr. Alan W. Bigelow and Gary W. Johnson for initial design and construction of the aerosol chamber, and Dr. Gerhard Randers-Pehrson for his conceptual insights.
Author Contributions
M.B. performed the biological experiments and analyzed the data; D.W. performed the chamber setup and lamp dosimetry; I. S. performed the statistical analysis; D.J.B. supervised the studies and contributed conceptual advice. All authors contributed in preparation of the manuscript. The authors declare no competing interests.
Table 1: Linear regression parameters for normalized ln[S] [survival] values (Eq. 1) as the dependent variable and UV dose (D, mJ/cm2) as the independent variable. k is the UV inactivation rate constant or susceptibility factor (cm2/mJ). The linear regression was performed with the intercept term set to zero representing the definition of 100% relative survival at zero UV dose. The coronavirus inactivation cross section, D90 (the UV dose that inactivates 90% of the exposed virus) was calculated using D90 = − ln[1 − 0.90]/k.
|
Human coronavirus type |
k (cm2/mJ) |
k 95% confidence interval |
p value |
R2 |
D90 (mJ/cm2) |
|
|
Lower |
Upper |
|||||
|
HCoV-229E |
4.1 |
2.5 |
4.8 |
0.0003 |
0.86 |
0.56 |
|
HCoV-OC43 |
5.9 |
3.8 |
7.1 |
0.0001 |
0.78 |
0.39 |
Table 2: Estimated k, D99, and D99.9 values for exposure to 222 nm far-UVC light for alphacoronavirus
HCoV-229E, betacoronavirus HCoV-OC43, and influenza A (H1N1).
|
Species |
k (cm2/mJ) |
D90 (mJ/cm2) |
D99 (mJ/cm2) |
D99.9 (mJ/cm2) |
|
HCoV-229E |
4.1 |
0.56 |
1.1 |
1.7 |
|
HCoV-OC43 |
5.9 |
0.39 |
0.78 |
1.2 |
|
Influenza A (H1N1)* |
1.8 |
1.3 |
2.6 |
3.8 |
*D99, and D99.9 values for influenza A (H1N1) denote extrapolated values, as these doses were not used during testing 23.
Our previous work with H1N1 utilized the fluorescent focus assay 23, while the current work with coronaviruses used the TCID50 assay.
Table 3. Example of particle size distributions from humans during various activities are given 26 along with the averaged measured values for this work.
|
|
Particle Size Distribution |
||||
|
< 1.0 µm |
> 1.0 µm |
||||
|
Papineni et al. 26 |
Coughing |
83%-91% |
9%-16% |
||
|
Mouth Breathing |
83%-95% |
4%-16% |
|||
|
Nose Breathing |
83%-100% |
0-16% |
|||
|
Talking |
77%-88% |
11%-22% |
|||
|
This work |
0.3-0.5 µm |
0.5-0.7 µm |
> 0.7 µm |
||
|
83% |
12% |
5% |
|||
Kent Smith
commented on 04 May, 2020
What regulatory exceptions (FDA, etc) are required to install far-uvc lighting in at risk nursing homes?
Edber
commented on 09 May, 2020
Dear Dr Buonanno et all.. In this paper in Visual Abstract Table : Far-UV light efficiency and safety Airbone Human Coronaviruses I noticed that at the left-hand side you said that the exposure time to the Far-UVC is 20 seconds, and under the table as the caption, you said that the exposure time is about 21 Minutes. Because this info is absolutely critical and of the most important in your very valuable research, I appreciate it very much if you please clarify for me what is the actual real Exposure Time? Thanks and congratulations on your research. Edber
View 1 reply
Jose Antonio
commented on 15 May, 2020
Are there currently lamps in production? Could represent a group of buyers
View 1 reply
Tres Knippa
replied on 20 June, 2020
I represent a company who can now produce 222 with various sizes of excimer tubes. We are installing them in fixtures next week and sending prototypes to both the EPA and UL for approval the following week. We should be in production shortly thereafter. Also developing an excimer tile which is 1 inch X 1 inch. Happy to discuss
View 1 reply
Kirsten L
replied on 24 June, 2020
What COMPANY? BRAND?? Hurry hurry hurry, these things should have been fast-tracked months ago!! I can't believe what a missed opportunity....:(
View 1 reply
tres knippa
replied on 24 June, 2020
Take a look and talk to Far UV Solutions.
View 1 reply
GEORGE
replied on 23 July, 2020
I've Googled Far UV Solutions & found nothing. Got any other more direct links?
View 1 reply
GEORGE
replied on 23 July, 2020
Cancel. I figured it out.
Ayo Bandara
commented on 21 May, 2020
Have these lamps been deemed safe to use by any medical body ?
WENDEL ADAMS
commented on 21 May, 2020
1) It is my understanding that UV light from 100-240 nm produces ozone which can be a respiratory hazard, especialy to the elderly or those already experiencing respiratory issues. Does the far-UVc light being at 222 nm produce hazardous ozone? 2) It is also my understanding that prolonged exposure to UV light can have other negative effects on the body such as premature aging of the skin and a weakening of the immune system, among others. How would this far-UVc light not have similar effects even if it is suggested that it can't prenetrate the skin/eyes and damage DNA?
View 2 replies
Manuela Buonanno
replied on 23 May, 2020
Hi Mr Adams, 1) During each experiment we run an ozone meter, which never registered ozone above ''noise'' level. 2) In principle, because the far-UVC light does not penetrate the surface layers of skin and eye we would not expect premature aging or weakening of the immune system in the long-term; however, we have not done these experiments yet. Manuela Buonanno
Ian Ashdown
replied on 16 June, 2020
According to Kowalski, W. 2009. Ultraviolet Germicidal Irradiation Handbook. Springer, only UV-C radiation in the range of 175 nm to 210 nm can convert atmospheric oxygen to ozone. Some low-pressure mercury-vapour germicidal lamps can produce small amounts of ozone due to an emission line at 185 nm, but this can be prevented by using sodium-barium or suitably doped fused quartz tubes. The article further notes that the 12-watt Ushio KrCl excimer lamp uses a proprietary optical filtering window to reduce lamp emissions outside of the 222 nm peak emission.
Ryan Roth
commented on 26 May, 2020
Hello, thank you for your research during these unique times. 1. How does this research compare to your work done in "Far-UVC light: A new tool to control the spread of airborne-mediated microbial disease" published 2.9.18? Is this an extension of the work performed for that paper or is it based on the same experiments? 2. Have you looked at other wavelengths including 210 or 230 to evaluate their effectiveness or is it your claim that all far UV-C wavelengths are equally effective?
View 1 reply
Manuela Buonanno
replied on 26 May, 2020
Hi Mr. Roth, 1) For this this work, we used the same aerosol chamber as the 2018 influenza A studies; here we investigated the effectiveness of the far-UVC light against two different viruses (alpha and beta human coronavirus). Therefore, this work is an extension of the 2018 study with influenza A. 2) In the study we show that the alpha and beta human coronaviruses are susceptible to 222 nm light. We speculate that they will be susceptible to killing from exposure to other wavelengths of the far-UVC spectrum, but not necessarily equally effective; we have not measured the actual effectiveness of 210 nm or 230 nm light yet.
Loren Hoboy
commented on 29 May, 2020
The units reported are mJ/cm2. How does this measurement convert to the radiant flux to be applied to a cubic cm or cubic meter of air contaminated with virus to achieve 1-log?
View 2 replies
David Welch
replied on 08 June, 2020
Hi Loren, We report radiant exposure in units of mJ/cm^2. To determine the radiant flux you would multiply by the area of the surface (in cm^2) and then divide by the duration of the exposure (in seconds), which gives the units of Watts for the radiant flux. -David Welch
Ian Ashdown
replied on 16 June, 2020
The units reported (mJ/cm2) are technically correct, as you are interested in the amount of energy incident upon the cross-sectional area of the aerosolized droplet from every direction. This is radiant fluence, aka radiant spherical exposure. (The radiant fluence rate is the amount of radiant flux incident upon the cross-sectional area, with units of uW/cm2). It is basically impossible to measure radiant fluence in an experimental setup like this, but it does not matter. The setup consisted of a Ushio excimer lamp with a 4.4 cm x 5.9 cm emission area positioned 22 cm away from a 26 cm x 25.6 cm window. At this distance, the radiation is reasonably parallel, and so measuring the irradiance at the window (90 uW/cm2) provides a reasonable estimate of the radiant fluence rate. There was also a specular reflector on the opposite side of the chamber with a far-UV reflectance of 15 percent (which is surprising -- the reflectance of specular aluminum at 250 nm typically varies between 50 and 90 percent), which the authors reasonably assumed increased the radiant fluence rate to 100 uW/cm2.
William Murphy
commented on 05 June, 2020
Dear Dr Buonanno et al. I have a specific need for your expertise. Do you consult? Thank you, Bill
Dr Devang Shah
ORCiDcommented on 05 June, 2020
Dear Mr. Buonanno and team members, I have been studying your work published in the last five years and very excited about your research. Many congratulations for great breakthrough helpful to Mankind. Great work ....Can you please let me know the reason behind not conducting the test directly on SARS-CoV-2 (the virus responsible for pandemic)? It may seem a very primary question to you, please reply if convenient as I could not get an answer for long.
View 1 reply
Manuela
replied on 08 June, 2020
Dear Dr. Shah, thank you for your interest in our research. Indeed, we are currently testing our hypothesis on SARS-CoV-2. Our laboratory is certified to conduct studies using BSL2 (Biosafety Level 2) pathogens such as HCoV-229E and HCoV-OC43. SARS-CoV-2 instead is a BSL3 level pathogen. We have an ongoing collaboration with a team in the BSL3 facility at Columbia University to assess far-UVC efficacy against SARS-CoV-2 as well. Best regards, Manuela Buonanno
View 2 replies
Dr. Devang A Shah
replied on 08 June, 2020
Dear Mr. Buonanno, Thanks a lot and I wish you all the best. I hope you may publish results for conventional UV C light also to increase the outreach. Thanking you, Devang Shah
Luke Ong
replied on 18 June, 2020
Dear Dr. Buonanno and team, Thank you for the work that you are doing. May I know when we may expect to read about the results from your tests on SARS-CoV-2?
View 1 reply
Manuela Buonanno
replied on 18 June, 2020
Hi Mr. Ong those studies are still ongoing. Best regards Manuela
Oriana Jovanovic
commented on 08 June, 2020
Hi, I have a comment regarding following and similar statements written in this paper: "Based on these results for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure in public locations at the currently recommended exposure limit (3 mJ/cm2/hour) would result in 99.9% viral inactivation in ~ 25 minutes." Dose of 3mJ/cm2 you are referring to is derived based on the calculated 8-h occupational hazard in the workplace. This said, it is a daily limit, not 23 mJ/cm2 per 8 h. Public locations are not open to public for only 8h per day. Thus, correct me if I am wrong, but UV dose you are referring to is, either related to intermittent far-UVC exposure in public locations or 8-h-per-day continuous UV irradiation. Thank you
View 1 reply
Manuela Buonanno
replied on 08 June, 2020
Hi Ms Jovanovic ~3 mJ/cm2/hour refers to the International Commission on Non-Ionizing Radiation Protection and the American Conference of Governmental Industrial Hygienists (ACGIH(R) regulatory limit as to the amount of 222 nm light to which the public can be exposed, which is 23 mJ/cm2 8-h-per-day continuous UV irradiation. Best regards, Manuela Buonanno
David Pfund
commented on 10 June, 2020
I assume the prescribed limit of 23 mJ/cm2 8-hr/day for continuous human exposure accounts for the typical rate of corneal shedding and replacement that would occur during the 16 hours of non-exposure. It may follow that intermittent/periodic exposure totaling 8 hours per day would require an equal amount of corneal self-repair. Accordingly, a similar daily limit might apply in cases of intermittent/periodic exposure. Notably, the findings of 1.2-1.7 mJ/cm2 for achieving 99.9% inactivation would result in exposures greater than 23mJ/cm2 after 13.5 to 19 hours of continuous, or additive intermittent, dosing. Although the likelihood of 24-hours of continuous exposure is highly unlikely, in the absence of a defined 24-hour exposure limit, it may follow that only exposures below 0.96 mJ/cm2 can definitively be deemed safe.
Ronald Emile Mignery
commented on 17 June, 2020
Are there materials that might fluoresce under 222nm UV at potentially dangerous longer UV wavelengths?
Joorge Ferreira
commented on 18 June, 2020
hi sorry for this question but I want to be quite sure to understand, according with your article the HCoV-299E can be inactivated in 1 second if I apply a 1.1mW/cm2 and will expect a 99% of effectiveness as well with the HCoV-OC43 applying 0.78mW/cm2, am I correct?
Hikaru Brianti
commented on 19 June, 2020
Hello, Anyone have some information about the use of UVC 275 to 287 nm interaction with the Covid-19 Virus?
Jorge Ferreira
commented on 19 June, 2020
you are reporting "from the International Commission on Non- Ionizing Radiation Protection (ICNIRP) is 23 mJ/cm2 per 8-hour exposure " that value is wrong, and hard to interpret leading to confusions, the ICNIRP(located on sheet 5 or page 174 from reference 34) has determined that the maximum exposure is 0.1uW/cm2 by a time of 8 hour leading to 28.8mJ/m2 (.0001mW/cm2*28800s where 28800 seconds are the 8 hours of exposition ), then is not a safe exposure 3mJ/cm2/h (0.8uj/cm2) that is eit times bigger than the maximum exposure of 0.1uW/cm according the ICNIRP. Also I have a concern about ozone gas an other poison gasses, in the same reference 34 sheet 16 page 185 says: "It is worthy of note that in addition to the direct hazard of UV exposure, very intense UVC sources (particularly of wavelengths less than 230 nm) may also produce hazardous concentrations of ozone and nitrogen oxides from the airand of phosgene gas in the presence of degreasers; thus,many UV germicidal lamps now have quartz-glass envelopes that block wavelengths below230 nm." so, does 222nm is a safe wavelength ?
View 1 reply
David Welch
replied on 24 June, 2020
Hi Jorge, The table that you are referring to in reference 34 gives the "effective irradiance" which requires multiplication of the radiant exposure by the relative spectral effectiveness. The effective irradiance value is based on the 3.0 mJ/cm^2 limit for 270 nm light. The value the table gives as 0.0001 mW/cm^2 *28800 s is 2.88 mJ/cm^2, not 28.8 as you have listed, which is good approximation. Note that the relative spectral effectiveness of 222 nm light, shown in Table 1 of that same reference, recommends a daily safe exposure of about 23 mJ/cm^2 for the entire 8-hour day. Regarding ozone, while photons with wavelengths less than about 240 nm can react to produce ozone, the probability of this is very small until the wavelength is less than about 200 nm. We have measured ozone concentration next to these lamps during operation and the values were not even close to hazardous levels. -David
View 1 reply
Jorge Ferreira
replied on 25 June, 2020
and what about phosgene gas and nitrogen oxides? there is no study that confirm that 222nm does not produce that kind of gasses. the unit or dosis or what ever you mean called 3 mJ/cm2/hour, does not exist. you should type like 0.833uW/cm^2 (and you can not expose more than one hour to this dosis according to ICNIRP) now considering your 23mJ/cm^2 we have: 23 mJ/cm^2 per 8h = 230J/m^2 in a interval of 8 hours this mean 230J/m^2 /28800 seconds = 0.007986W/m^2 from reference 34 again: Eeff= E(lamda)*S(lamda)*Delta Lamda S(lamda)=for 210<=lamda <=270 =0.959^(270-lamda)=(0.959^(270-222))=0.134058 E(lamda)= 0.007986W/m^2 Delta Lamda=10 Tipical bandwidth of an eximer lamp after filter(cannot be 1 the perfect one nanometer filter does not exist) Eff=( 0.007986W/m^2 )*0.134058*10= 0.01070W/m^2 from reference 34 : "Permissible exposure time in seconds for exposure to UVR incident upon the unprotected skin or eye may be computed by dividing 30 J/m^2 by the value of Eeff in W/m^2. The maximal exposure duration may also be determined using Table 2, which provides representative exposure durations corresponding to effective irradiances in W/m^2or uW/cm^2." so: 30 J/m^2 / 0.01070W/m^2 = 2803.73 secconds or 46 minutes for ALL DAY. so according to INCIRP you only can be exposed by 46 minutes as maximum per day.
View 1 reply
David Welch
replied on 25 June, 2020
Hi Jorge, You will note that in the application of that equation from reference 34 the value for E(lambda) has units of W/m^2 / nm, not simply W/m^2. If you multiply by a 10 nm bandwidth you need to be sure that you are also accounting for the need to distribute the spectral irradiance value over that same bandwidth, so you need to divide E(lambda) by 10 in this case you describe. This will give you a calculated safe exposure time close to 8 hours. -David
Gary Duncan
commented on 20 June, 2020
Since 222 nm light does not penetrate the eye or the skin, is the ICNIRP daily maximum dosage even relevant or material? It doesn’t look like their recommendations consider that factor at all.
View 1 reply
Kirsten L
replied on 24 June, 2020
Good point!!! I hope you are correct, and that this lightning is seen ubiquitous, as this disease is truly crippling us, ...& no vaccination will be coming soon I'm sure.
View 1 reply
Kirsten L
replied on 24 June, 2020
*SOON not seen, sorry
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Far-UVC light efficiently and safely inactivates airborne human coronaviruses
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Lay Summary
Lay Summary
created by Research Square
The global spread of coronavirus disease (COVID-19) has escalated the need for public sanitation. Ultraviolet (UV) light is a well-established antimicrobial; however, the UV light commonly used for this purpose (~254 nm) poses significant health risks in public spaces due to its cancer-causing effects. Far-UVC light (~207-222 nm) has similar germicidal properties without health risks, as it does not penetrate human eye or skin cells. This group previously found low doses of far-UVC light (222 nm) to be an effective germicide against aerosolized H1N1 influenza virus. In this study, the researchers tested two viruses that are closely related to SARS-CoV-2, the virus that causes COVID-19. Because the sensitivity of a virus to UV is determined by its genome size and all coronaviruses have genomes of similar sizes, it is reasonable to expect the sensitivity of SARS-CoV-2 to be comparable to these viruses.The authors tested several doses of far-UVC on aerosolized viruses. They found that for both viruses tested, 99.9% of the virus was inactivated at a lower dose than the one needed to inactivate H1N1 virus. The authors also suggested a safe dose for an 8-hour work shift based on recommendations from the International Commission on Non-Ionizing Radiation Protection (ICNIRP). At this dose, 90% of aerosolized viruses would be deactivated in 8 minutes, 95% in 16 minutes, and 99% in 25 minutes. These results combined with the earlier results for H1N1 suggest that low doses of far-UVC light in heavily occupied spaces would reduce the spread of viral diseases, likely including COVID-19.
Lay Summary
A direct approach to limit airborne transmission of pathogens is to inactivate them within a short time of their production. Germicidal ultraviolet light (UV), typically at 254 nm, is effective in this context, but it is a health hazard to the skin and eyes. By contrast, far-UVC light (207-222 nm) efficiently kills pathogens without harm to exposed human cells or tissues. We previously demonstrated that 222-nm UV light efficiently kills airborne influenza virus (H1N1); here we extend the far-UVC studies to explore efficacy against human coronaviruses from subgroups alpha (HCoV-229E) and beta (HCoV-OC43). We found that low doses of, respectively 1.7 and 1.2 mJ/cm2 inactivated 99.9% of aerosolized alpha coronavirus 229E and beta coronavirus OC43. Based on these results for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure in public locations at the currently recommended exposure limit (3 mJ/cm2/hour) would result in 99.9% viral inactivation in ~ 25 minutes. Increasing the far- UVC intensity by, say, a factor of 2 would halve these disinfection times, while still maintaining safety. As all human coronaviruses have similar genomic size, a key determinant of radiation sensitivity, it is realistic to expect that far-UVC light will show comparable inactivation efficiency against other human coronaviruses, including SARS-CoV-2.
Visual Abstract
Figure 1
Figure 2
Figure 3
Coronavirus disease 2019 (COVID–19) was first reported in December 2019 and then characterized as a pandemic by the World Health Organization on March 11, 2020. Despite extensive efforts to contain the spread of the disease, it has spread worldwide with over 1.9 million confirmed cases and over 130,000 confirmed deaths as of April 16, 2020 1. Transmission of SARS-CoV–2, the beta coronavirus causing COVID–19, is both through direct contact and airborne routes, and studies of SARS-CoV–2 stability have shown viability in aerosols for at least 3 hours 2. Given the rapid spread of the disease, including through asymptomatic carriers 3, it is of clear importance to explore practical mitigation technologies that can inactivate the airborne virus in public locations and thus limit airborne transmission.
Ultraviolet (UV) light exposure is a direct antimicrobial approach 4 and its effectiveness against different strains of airborne viruses has long been established 5. The most commonly employed type of UV light for germicidal applications is a low pressure mercury-vapor arc lamp, emitting around 254 nm; more recently xenon lamp technology has been used, which emits broad UV spectrum 6. However, while these lamps can be used to disinfect unoccupied spaces, operation of these conventional germicidal UV lamps in occupied public spaces - critical to prevent person-to-person transmission - is not possible since exposure to these wavelengths is a health hazard, causing both skin cancer and eye diseases 7–10.
By contrast far-UVC light (207 to 222 nm) has been shown to be as efficient as conventional germicidal UV light in killing microorganisms 11, but it does not have the human health issues associated with conventional germicidal UV light. In short (see below) the reason is that far-UVC has a range in biological materials of only a few micrometers, and thus it cannot reach living human cells in the skin or eyes. But because viruses (and bacteria) are extremely small, far-UVC light can still penetrate and kill them. Thus far-UVC light has about the same highly effective germicidal properties of UV light, but without the associated human health risks 12–15. Several groups have thus proposed that far-UVC light (207 or 222 nm), which can be generated using inexpensive excimer lamps, is a potential safe and efficient anti-microbial technology 12–18 which can be deployed in occupied public locations.
The physics-based mechanistic basis to this far-UVC approach 12 is that light in this wavelength range has a very limited penetration depth. Specifically, far-UVC light (207–222 nm) is very strongly absorbed by proteins through the peptide bond, and other biomolecules 19,20, so its ability to penetrate biological materials is 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 12–14. 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 7–10.
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/cm2) 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 subgroup 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.
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 TCID50 assay 28, and estimated the corresponding plaque forming units (PFU)/ml using the conversion PFU/ml = 0.7 TCID50 29. Fig. 1 shows the fractional survival of aerosolized coronaviruses HCoV–229E and HCoV-OC43 expressed as PFUUV/PFUcontrols 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 subgroups alpha and beta is consistent with a classical exponential UV disinfection model (R2 = 0.86 for HCoV–229E and R2 = 0.78 for HCoV-OC43). For the alpha coronavirus HCoV–229E, the inactivation rate constant (susceptibility rate) was k = 4.1 cm2/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 D90 = 0.56 mJ/cm2. Similarly, the susceptibility rate for the beta coronavirus HCoV-OC43 was k = 5.9 cm2/mJ (95% C. I. 3.8–7.1) which corresponds to an inactivation cross section of D90 = 0.39 mJ/cm2.
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. Figs. 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/cm2 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 cell lines for expression of the viral spike glycoprotein, whose functional subunit S2 is highly conserved among coronaviruses 31,32. In Figs. 2 and 3, green fluorescence (Alexa Fluor®–488 used as secondary antibody against anti-human coronavirus spike glycoprotein antibody) qualitatively indicates infection of cells with live virus, while the nuclei were counterstained with DAPI appearing as blue fluorescence. For both alpha HCoV–229E and beta HCoV-OC43, exposure to 222-nm light reduced the expression of the viral spike glycoprotein as indicated by a reduction in green fluorescence.
The severity of the 2020 COVID–19 pandemic warrants the rapid development and deployment of effective countermeasures to reduce 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 inactivate viruses and bacteria, without inducing biological damage in exposed human cells and tissue 11–16. 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.
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 cm2/mJ (95% C. I. 2.5–4.8), and k = 5.9 cm2/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/cm2 or 1.2 mJ/cm2 respectively produce 99.9% (3-log reduction) of aerosolized alpha HCoV–229E or beta HCoV-OC43. A summary of k values and the corresponding D90, D99, and D99.9 values we have obtained for coronaviruses is shown in Table 2, together with our earlier results for aerosolized H1N1 influenza virus 23.
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 cm2/mJ and 3.8 to 7.1 cm2/mJ respectively for the 229E and OC43 strains. As all human 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 33, which used an aerosolized murine beta coronavirus. The study reported a D88 of 0.599 mJ/cm2, which others 4 have used to estimate the D90 for the virus with 254 nm light as 0.6 mJ/cm2. 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 12–18, suggests that it will be feasible and safe to have the lamps providing continuous far-UVC exposure in public places to significantly reduce the probability of person-to-person transmission of coronavirus as well as other seasonal viruses such as influenza. For example, the current dose limit guideline for 222 nm light from the International Commission on Non- Ionizing Radiation Protection (ICNIRP) is 23 mJ/cm2 per 8-hour exposure 34. Interpreting this as an intensity of ~3 mJ/cm2/hour, and based on our results here for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure at this intensity 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. Increasing the intensity by, say, a factor of 2 would halve these disinfection times, while still maintaining safety 12–18.
In conclusion, we have shown that very low doses of far-UVC light efficiently kill airborne human coronaviruses carried by aerosols. An exposure dose as low as 1.2 to 1.7 mJ/cm2 of 222-nm light inactivates 99.9% of the airborne human coronavirus tested from both subgroups beta and alpha, respectively. Together with previous safety studies 12–18 and our earlier studies with aerosolized influenza A (H1N1) 23, these results indicate that far-UVC light is a potentially powerful and practical approach for reduction of airborne viral transmission, without the human health hazards associated with conventional germicidal UVC lamps.
These results suggest the utility of deploying low dose rate far-UVC lights in highly occupied indoor public locations such as hospitals, transportation vehicles, airports and schools, potentially representing a safe and inexpensive tool to reduce the spread of airborne-mediated microbial diseases.
Far-UVC technology can potentially help limit seasonal influenza epidemics, transmission of measles and tuberculosis, as well as future pandemics.
Viral Strains. HCoV–229E (VR–740) and HCoV-OC43 (VR–1558) were propagated in human diploid lung MRC–5 fibroblasts (CCL–171) and WI–38 (CCL–75), respectively (all from ATCC, Manassas, VA). Both human cell lines were grown in MEM supplemented with 10% Fetal Bovine Serum (FBS), 2 mM L-alanyl-L-glutamine, 100 U/ml penicillin and 100 µg/ml streptomycin (Sigma-Aldrich Corp. St. Louis, MO, USA). 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 TCID50 by assessing cytopathic effects (CPE), which were scored at a bright field microscope (10x) as vacuolization of cytoplasm, cell rounding and sloughing.
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 8007x10, Topas Advanced Polymers, 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). Aerosolized viruses were efficiently transmitted through the system as evidenced from the control (zero exposure) showing clear virus integration (Figs. 2 and 3, top left panels).
Far-UVC Lamp and Dosimetry. The far-UVC source used in this study was a 12 W 222-nm KrCl excimer lamp module made by Ushio America (Item #9101711, Cypress, CA). The lamp is equipped with a proprietary optical filtering window to reduce lamp emissions outside of the 222 nm KrCl emission peak. The lamp was positioned 22 cm away from the exposure chamber window and directed at the center of the window. Optical power measurements were performed using an 818-UV/DB low-power UV enhanced silicon photodetector with an 843-R optical power meter (Newport, Irvine, CA). Dosimetry was performed prior to starting an experiment to measure the irradiation fluence within the chamber at the position of the aerosol.
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/cm2 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/cm2. 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/cm2. 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/cm2, 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 107–108 TCID50 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 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/cm2. 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.
TCID50. We used the 50% tissue culture infectious dose assay to determine virus infectivity 28. Briefly, 105 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 34ºC. Cytopathic effects (CPE) were scored at a bright field microscope (10x) as vacuolization of cytoplasm, cell rounding and sloughing. The TCID50 was calculated with the Reed and Muench method 28,35. 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 TCID50 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. Briefly, 2x105 host cells (MRC–5 cells for HCoV–229E and WI–38 for HCoV- OC43) were plated in each well of 48-well plates the day prior the experiment. After running through the irradiation chamber for 30 minutes, 150 µl of virus suspension collected from the BioSampler was overlaid on the monolayer of host cells. The cells were incubated with the virus for one hour, then washed three times with HBSS++, and then incubated overnight in fresh infection medium. Infected cells were then fixed in 100% ice cold methanol at 4°C for 5 minutes and labeled with anti-human coronavirus spike glycoprotein (40021-MM07, Sino Biologicals, Chesterbrook, PA) 1:200 in HBSS++ containing 1% bovine serum albumin (BSA; Sigma-Aldrich Corp. St. Louis, MO, USA) at room temperature for one hour with gentle shaking. Cells were washed three times in HBSS++ and labeled with goat anti-mouse Alexa Fluor®–488 (Life Technologies, Grand Island, NY) 1:800 in HBSS++ containing 1% BSA, at room temperature for 30 minutes in the dark with gentle shaking. Following three washes in HBSS++, the cells were stained with Vectashield containing DAPI (4’,6-diamidino–2- phenylindole) (Victor Laboratories, Burlingame, CA) and observed with the 10x objective of an Olympus IX70 fluorescent microscope equipped with a Photometrics PVCAM high-resolution, high- efficiency digital camera and Image-Pro Plus 6.0 software (Media Cybernetics, Bethesda, MD). For each 222-nm dose and virus subgroup, the representative results were repeated twice. For each sample, up to ten fields of view of merged DAPI and Alexa Fluor®–488 images were acquired.
Data Analysis. The surviving fraction (S) of the virus was calculated by dividing the fraction PFU/ml at each UV dose (PFUUV) by the fraction at zero dose (PFUcontrols): S = PFUUV/PFUcontrols. Survival values were calculated for each repeat experiment and natural log (ln) transformed to bring the error distribution closer to normal 36. Robust linear regression using iterated re-weighted least squares (IWLS) 37,38 was performed in R 3.6.2 software using these normalized ln[S] values as the dependent variable and UV dose (D, mJ/cm2) as the independent variable. Using this approach, the virus survival [S] was described by first-order kinetics according to the equation 4:
ln[S] = −k × D[Eq. 1]
where k is the UV inactivation rate constant or susceptibility factor (cm2/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 (R2). Analysis of residuals for autocorrelation and for heteroskedasticity was performed using the Durbin- Watson test 39 and Breusch-Pagan test (implemented by lmtest R package) 40, 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, D90, which is the UV dose that inactivates 90% of the exposed virus, was calculated as D90 = − ln[1 − 0.90]/k. Other D values were calculated similarly.
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Acknowledgements
This work was supported by the Shostack Foundation and by NIH grant R42-AI125006-03. We thank Dr. Alan W. Bigelow and Gary W. Johnson for initial design and construction of the aerosol chamber, and Dr. Gerhard Randers-Pehrson for his conceptual insights.
Author Contributions
M.B. performed the biological experiments and analyzed the data; D.W. performed the chamber setup and lamp dosimetry; I. S. performed the statistical analysis; D.J.B. supervised the studies and contributed conceptual advice. All authors contributed in preparation of the manuscript. The authors declare no competing interests.
Table 1: Linear regression parameters for normalized ln[S] [survival] values (Eq. 1) as the dependent variable and UV dose (D, mJ/cm2) as the independent variable. k is the UV inactivation rate constant or susceptibility factor (cm2/mJ). The linear regression was performed with the intercept term set to zero representing the definition of 100% relative survival at zero UV dose. The coronavirus inactivation cross section, D90 (the UV dose that inactivates 90% of the exposed virus) was calculated using D90 = − ln[1 − 0.90]/k.
|
Human coronavirus type |
k (cm2/mJ) |
k 95% confidence interval |
p value |
R2 |
D90 (mJ/cm2) |
|
|
Lower |
Upper |
|||||
|
HCoV-229E |
4.1 |
2.5 |
4.8 |
0.0003 |
0.86 |
0.56 |
|
HCoV-OC43 |
5.9 |
3.8 |
7.1 |
0.0001 |
0.78 |
0.39 |
Table 2: Estimated k, D99, and D99.9 values for exposure to 222 nm far-UVC light for alphacoronavirus
HCoV-229E, betacoronavirus HCoV-OC43, and influenza A (H1N1).
|
Species |
k (cm2/mJ) |
D90 (mJ/cm2) |
D99 (mJ/cm2) |
D99.9 (mJ/cm2) |
|
HCoV-229E |
4.1 |
0.56 |
1.1 |
1.7 |
|
HCoV-OC43 |
5.9 |
0.39 |
0.78 |
1.2 |
|
Influenza A (H1N1)* |
1.8 |
1.3 |
2.6 |
3.8 |
*D99, and D99.9 values for influenza A (H1N1) denote extrapolated values, as these doses were not used during testing 23.
Our previous work with H1N1 utilized the fluorescent focus assay 23, while the current work with coronaviruses used the TCID50 assay.
Table 3. Example of particle size distributions from humans during various activities are given 26 along with the averaged measured values for this work.
|
|
Particle Size Distribution |
||||
|
< 1.0 µm |
> 1.0 µm |
||||
|
Papineni et al. 26 |
Coughing |
83%-91% |
9%-16% |
||
|
Mouth Breathing |
83%-95% |
4%-16% |
|||
|
Nose Breathing |
83%-100% |
0-16% |
|||
|
Talking |
77%-88% |
11%-22% |
|||
|
This work |
0.3-0.5 µm |
0.5-0.7 µm |
> 0.7 µm |
||
|
83% |
12% |
5% |
|||
Kent Smith
commented on 04 May, 2020
What regulatory exceptions (FDA, etc) are required to install far-uvc lighting in at risk nursing homes?
Edber
commented on 09 May, 2020
Dear Dr Buonanno et all.. In this paper in Visual Abstract Table : Far-UV light efficiency and safety Airbone Human Coronaviruses I noticed that at the left-hand side you said that the exposure time to the Far-UVC is 20 seconds, and under the table as the caption, you said that the exposure time is about 21 Minutes. Because this info is absolutely critical and of the most important in your very valuable research, I appreciate it very much if you please clarify for me what is the actual real Exposure Time? Thanks and congratulations on your research. Edber
View 1 reply
David Welch
replied on 11 May, 2020
Hi Edber, In these tests using the aerosol chamber the exposure time is only 20 seconds, but note that the intensity of this exposure is relatively high (100 uW/cm^2). The calculation of 25 minutes to inactivate 99.9% of virus reflects operation of the lamps at an intensity that is within current ACGIH/INCIRP daily exposure limits for 222 nm light. -David Welch
View 1 reply
Loren Hoboy
replied on 08 June, 2020
David, I received your reply "We report radiant exposure in units of mJ/cm^2. To determine the radiant flux you would multiply by the area of the surface (in cm^2) and then divide by the duration of the exposure (in seconds), which gives the units of Watts for the radiant flux. -David Welch" . I am not looking at surface treatment. I am trying to determine mW/cm3 to deactivate virus in a given air volume. I assume we need to deliver a certain amount of energy to disrupt the bongs. I want to understand the dosage of 270nm and/or 225nm required to treat 1-cm3 or 1-m3 of virus ladened air down to 1-log (or 5-log) for human breathing contaminated air (droplet exhale). Can you extrapolate this information from your test data? If you can tell me the air volume exposed at 20 seconds at the 100 uW, that could possibly begin to answer the question. Thank you.
View 2 replies
Ian Ashdown
replied on 16 June, 2020
"I am trying to determine mW/cm3 to deactivate virus in a given air volume." What you are asking for here is related to "radiant energy density," an SI unit expressed in joules per cubic meter. You could have "radiant flux density," but this is not a recognized SI unit. I believe what you are asking for is "radiant fluence," aka "radiant spherical exposure," which is defined as "the quotient of the radiant energy of all radiation incident on the outer surface of an infinitely small sphere centred at the given point by the area of the diametrical cross-section of that sphere." This is the appropriate radiometric quantity for describing the irradiation of microorganisms, and it is expressed in Joules per square meter.
Jorge Ferreira
replied on 18 June, 2020
you should read this article DOI: 10.1023/A:1013951313398 Mathematical Modeling of Ultraviolet Germicidal Irradiation for Air Disinfection, there is all you need to know.
Jose Antonio
commented on 15 May, 2020
Are there currently lamps in production? Could represent a group of buyers
View 1 reply
Tres Knippa
replied on 20 June, 2020
I represent a company who can now produce 222 with various sizes of excimer tubes. We are installing them in fixtures next week and sending prototypes to both the EPA and UL for approval the following week. We should be in production shortly thereafter. Also developing an excimer tile which is 1 inch X 1 inch. Happy to discuss
View 1 reply
Kirsten L
replied on 24 June, 2020
What COMPANY? BRAND?? Hurry hurry hurry, these things should have been fast-tracked months ago!! I can't believe what a missed opportunity....:(
View 1 reply
tres knippa
replied on 24 June, 2020
Take a look and talk to Far UV Solutions.
View 1 reply
GEORGE
replied on 23 July, 2020
I've Googled Far UV Solutions & found nothing. Got any other more direct links?
View 1 reply
GEORGE
replied on 23 July, 2020
Cancel. I figured it out.
Ayo Bandara
commented on 21 May, 2020
Have these lamps been deemed safe to use by any medical body ?
WENDEL ADAMS
commented on 21 May, 2020
1) It is my understanding that UV light from 100-240 nm produces ozone which can be a respiratory hazard, especialy to the elderly or those already experiencing respiratory issues. Does the far-UVc light being at 222 nm produce hazardous ozone? 2) It is also my understanding that prolonged exposure to UV light can have other negative effects on the body such as premature aging of the skin and a weakening of the immune system, among others. How would this far-UVc light not have similar effects even if it is suggested that it can't prenetrate the skin/eyes and damage DNA?
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Manuela Buonanno
replied on 23 May, 2020
Hi Mr Adams, 1) During each experiment we run an ozone meter, which never registered ozone above ''noise'' level. 2) In principle, because the far-UVC light does not penetrate the surface layers of skin and eye we would not expect premature aging or weakening of the immune system in the long-term; however, we have not done these experiments yet. Manuela Buonanno
Ian Ashdown
replied on 16 June, 2020
According to Kowalski, W. 2009. Ultraviolet Germicidal Irradiation Handbook. Springer, only UV-C radiation in the range of 175 nm to 210 nm can convert atmospheric oxygen to ozone. Some low-pressure mercury-vapour germicidal lamps can produce small amounts of ozone due to an emission line at 185 nm, but this can be prevented by using sodium-barium or suitably doped fused quartz tubes. The article further notes that the 12-watt Ushio KrCl excimer lamp uses a proprietary optical filtering window to reduce lamp emissions outside of the 222 nm peak emission.
Ryan Roth
commented on 26 May, 2020
Hello, thank you for your research during these unique times. 1. How does this research compare to your work done in "Far-UVC light: A new tool to control the spread of airborne-mediated microbial disease" published 2.9.18? Is this an extension of the work performed for that paper or is it based on the same experiments? 2. Have you looked at other wavelengths including 210 or 230 to evaluate their effectiveness or is it your claim that all far UV-C wavelengths are equally effective?
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Manuela Buonanno
replied on 26 May, 2020
Hi Mr. Roth, 1) For this this work, we used the same aerosol chamber as the 2018 influenza A studies; here we investigated the effectiveness of the far-UVC light against two different viruses (alpha and beta human coronavirus). Therefore, this work is an extension of the 2018 study with influenza A. 2) In the study we show that the alpha and beta human coronaviruses are susceptible to 222 nm light. We speculate that they will be susceptible to killing from exposure to other wavelengths of the far-UVC spectrum, but not necessarily equally effective; we have not measured the actual effectiveness of 210 nm or 230 nm light yet.
Loren Hoboy
commented on 29 May, 2020
The units reported are mJ/cm2. How does this measurement convert to the radiant flux to be applied to a cubic cm or cubic meter of air contaminated with virus to achieve 1-log?
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David Welch
replied on 08 June, 2020
Hi Loren, We report radiant exposure in units of mJ/cm^2. To determine the radiant flux you would multiply by the area of the surface (in cm^2) and then divide by the duration of the exposure (in seconds), which gives the units of Watts for the radiant flux. -David Welch
Ian Ashdown
replied on 16 June, 2020
The units reported (mJ/cm2) are technically correct, as you are interested in the amount of energy incident upon the cross-sectional area of the aerosolized droplet from every direction. This is radiant fluence, aka radiant spherical exposure. (The radiant fluence rate is the amount of radiant flux incident upon the cross-sectional area, with units of uW/cm2). It is basically impossible to measure radiant fluence in an experimental setup like this, but it does not matter. The setup consisted of a Ushio excimer lamp with a 4.4 cm x 5.9 cm emission area positioned 22 cm away from a 26 cm x 25.6 cm window. At this distance, the radiation is reasonably parallel, and so measuring the irradiance at the window (90 uW/cm2) provides a reasonable estimate of the radiant fluence rate. There was also a specular reflector on the opposite side of the chamber with a far-UV reflectance of 15 percent (which is surprising -- the reflectance of specular aluminum at 250 nm typically varies between 50 and 90 percent), which the authors reasonably assumed increased the radiant fluence rate to 100 uW/cm2.
William Murphy
commented on 05 June, 2020
Dear Dr Buonanno et al. I have a specific need for your expertise. Do you consult? Thank you, Bill
Dr Devang Shah
ORCiDcommented on 05 June, 2020
Dear Mr. Buonanno and team members, I have been studying your work published in the last five years and very excited about your research. Many congratulations for great breakthrough helpful to Mankind. Great work ....Can you please let me know the reason behind not conducting the test directly on SARS-CoV-2 (the virus responsible for pandemic)? It may seem a very primary question to you, please reply if convenient as I could not get an answer for long.
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Manuela
replied on 08 June, 2020
Dear Dr. Shah, thank you for your interest in our research. Indeed, we are currently testing our hypothesis on SARS-CoV-2. Our laboratory is certified to conduct studies using BSL2 (Biosafety Level 2) pathogens such as HCoV-229E and HCoV-OC43. SARS-CoV-2 instead is a BSL3 level pathogen. We have an ongoing collaboration with a team in the BSL3 facility at Columbia University to assess far-UVC efficacy against SARS-CoV-2 as well. Best regards, Manuela Buonanno
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Dr. Devang A Shah
replied on 08 June, 2020
Dear Mr. Buonanno, Thanks a lot and I wish you all the best. I hope you may publish results for conventional UV C light also to increase the outreach. Thanking you, Devang Shah
Luke Ong
replied on 18 June, 2020
Dear Dr. Buonanno and team, Thank you for the work that you are doing. May I know when we may expect to read about the results from your tests on SARS-CoV-2?
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Manuela Buonanno
replied on 18 June, 2020
Hi Mr. Ong those studies are still ongoing. Best regards Manuela
Oriana Jovanovic
commented on 08 June, 2020
Hi, I have a comment regarding following and similar statements written in this paper: "Based on these results for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure in public locations at the currently recommended exposure limit (3 mJ/cm2/hour) would result in 99.9% viral inactivation in ~ 25 minutes." Dose of 3mJ/cm2 you are referring to is derived based on the calculated 8-h occupational hazard in the workplace. This said, it is a daily limit, not 23 mJ/cm2 per 8 h. Public locations are not open to public for only 8h per day. Thus, correct me if I am wrong, but UV dose you are referring to is, either related to intermittent far-UVC exposure in public locations or 8-h-per-day continuous UV irradiation. Thank you
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Manuela Buonanno
replied on 08 June, 2020
Hi Ms Jovanovic ~3 mJ/cm2/hour refers to the International Commission on Non-Ionizing Radiation Protection and the American Conference of Governmental Industrial Hygienists (ACGIH(R) regulatory limit as to the amount of 222 nm light to which the public can be exposed, which is 23 mJ/cm2 8-h-per-day continuous UV irradiation. Best regards, Manuela Buonanno
David Pfund
commented on 10 June, 2020
I assume the prescribed limit of 23 mJ/cm2 8-hr/day for continuous human exposure accounts for the typical rate of corneal shedding and replacement that would occur during the 16 hours of non-exposure. It may follow that intermittent/periodic exposure totaling 8 hours per day would require an equal amount of corneal self-repair. Accordingly, a similar daily limit might apply in cases of intermittent/periodic exposure. Notably, the findings of 1.2-1.7 mJ/cm2 for achieving 99.9% inactivation would result in exposures greater than 23mJ/cm2 after 13.5 to 19 hours of continuous, or additive intermittent, dosing. Although the likelihood of 24-hours of continuous exposure is highly unlikely, in the absence of a defined 24-hour exposure limit, it may follow that only exposures below 0.96 mJ/cm2 can definitively be deemed safe.
Ronald Emile Mignery
commented on 17 June, 2020
Are there materials that might fluoresce under 222nm UV at potentially dangerous longer UV wavelengths?
Joorge Ferreira
commented on 18 June, 2020
hi sorry for this question but I want to be quite sure to understand, according with your article the HCoV-299E can be inactivated in 1 second if I apply a 1.1mW/cm2 and will expect a 99% of effectiveness as well with the HCoV-OC43 applying 0.78mW/cm2, am I correct?
Hikaru Brianti
commented on 19 June, 2020
Hello, Anyone have some information about the use of UVC 275 to 287 nm interaction with the Covid-19 Virus?
Jorge Ferreira
commented on 19 June, 2020
you are reporting "from the International Commission on Non- Ionizing Radiation Protection (ICNIRP) is 23 mJ/cm2 per 8-hour exposure " that value is wrong, and hard to interpret leading to confusions, the ICNIRP(located on sheet 5 or page 174 from reference 34) has determined that the maximum exposure is 0.1uW/cm2 by a time of 8 hour leading to 28.8mJ/m2 (.0001mW/cm2*28800s where 28800 seconds are the 8 hours of exposition ), then is not a safe exposure 3mJ/cm2/h (0.8uj/cm2) that is eit times bigger than the maximum exposure of 0.1uW/cm according the ICNIRP. Also I have a concern about ozone gas an other poison gasses, in the same reference 34 sheet 16 page 185 says: "It is worthy of note that in addition to the direct hazard of UV exposure, very intense UVC sources (particularly of wavelengths less than 230 nm) may also produce hazardous concentrations of ozone and nitrogen oxides from the airand of phosgene gas in the presence of degreasers; thus,many UV germicidal lamps now have quartz-glass envelopes that block wavelengths below230 nm." so, does 222nm is a safe wavelength ?
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David Welch
replied on 24 June, 2020
Hi Jorge, The table that you are referring to in reference 34 gives the "effective irradiance" which requires multiplication of the radiant exposure by the relative spectral effectiveness. The effective irradiance value is based on the 3.0 mJ/cm^2 limit for 270 nm light. The value the table gives as 0.0001 mW/cm^2 *28800 s is 2.88 mJ/cm^2, not 28.8 as you have listed, which is good approximation. Note that the relative spectral effectiveness of 222 nm light, shown in Table 1 of that same reference, recommends a daily safe exposure of about 23 mJ/cm^2 for the entire 8-hour day. Regarding ozone, while photons with wavelengths less than about 240 nm can react to produce ozone, the probability of this is very small until the wavelength is less than about 200 nm. We have measured ozone concentration next to these lamps during operation and the values were not even close to hazardous levels. -David
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Jorge Ferreira
replied on 25 June, 2020
and what about phosgene gas and nitrogen oxides? there is no study that confirm that 222nm does not produce that kind of gasses. the unit or dosis or what ever you mean called 3 mJ/cm2/hour, does not exist. you should type like 0.833uW/cm^2 (and you can not expose more than one hour to this dosis according to ICNIRP) now considering your 23mJ/cm^2 we have: 23 mJ/cm^2 per 8h = 230J/m^2 in a interval of 8 hours this mean 230J/m^2 /28800 seconds = 0.007986W/m^2 from reference 34 again: Eeff= E(lamda)*S(lamda)*Delta Lamda S(lamda)=for 210<=lamda <=270 =0.959^(270-lamda)=(0.959^(270-222))=0.134058 E(lamda)= 0.007986W/m^2 Delta Lamda=10 Tipical bandwidth of an eximer lamp after filter(cannot be 1 the perfect one nanometer filter does not exist) Eff=( 0.007986W/m^2 )*0.134058*10= 0.01070W/m^2 from reference 34 : "Permissible exposure time in seconds for exposure to UVR incident upon the unprotected skin or eye may be computed by dividing 30 J/m^2 by the value of Eeff in W/m^2. The maximal exposure duration may also be determined using Table 2, which provides representative exposure durations corresponding to effective irradiances in W/m^2or uW/cm^2." so: 30 J/m^2 / 0.01070W/m^2 = 2803.73 secconds or 46 minutes for ALL DAY. so according to INCIRP you only can be exposed by 46 minutes as maximum per day.
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David Welch
replied on 25 June, 2020
Hi Jorge, You will note that in the application of that equation from reference 34 the value for E(lambda) has units of W/m^2 / nm, not simply W/m^2. If you multiply by a 10 nm bandwidth you need to be sure that you are also accounting for the need to distribute the spectral irradiance value over that same bandwidth, so you need to divide E(lambda) by 10 in this case you describe. This will give you a calculated safe exposure time close to 8 hours. -David
Gary Duncan
commented on 20 June, 2020
Since 222 nm light does not penetrate the eye or the skin, is the ICNIRP daily maximum dosage even relevant or material? It doesn’t look like their recommendations consider that factor at all.
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Kirsten L
replied on 24 June, 2020
Good point!!! I hope you are correct, and that this lightning is seen ubiquitous, as this disease is truly crippling us, ...& no vaccination will be coming soon I'm sure.
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Kirsten L
replied on 24 June, 2020
*SOON not seen, sorry
David Welch
replied on 11 May, 2020
Hi Edber, In these tests using the aerosol chamber the exposure time is only 20 seconds, but note that the intensity of this exposure is relatively high (100 uW/cm^2). The calculation of 25 minutes to inactivate 99.9% of virus reflects operation of the lamps at an intensity that is within current ACGIH/INCIRP daily exposure limits for 222 nm light. -David Welch
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Loren Hoboy
replied on 08 June, 2020
David, I received your reply "We report radiant exposure in units of mJ/cm^2. To determine the radiant flux you would multiply by the area of the surface (in cm^2) and then divide by the duration of the exposure (in seconds), which gives the units of Watts for the radiant flux. -David Welch" . I am not looking at surface treatment. I am trying to determine mW/cm3 to deactivate virus in a given air volume. I assume we need to deliver a certain amount of energy to disrupt the bongs. I want to understand the dosage of 270nm and/or 225nm required to treat 1-cm3 or 1-m3 of virus ladened air down to 1-log (or 5-log) for human breathing contaminated air (droplet exhale). Can you extrapolate this information from your test data? If you can tell me the air volume exposed at 20 seconds at the 100 uW, that could possibly begin to answer the question. Thank you.
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Ian Ashdown
replied on 16 June, 2020
"I am trying to determine mW/cm3 to deactivate virus in a given air volume." What you are asking for here is related to "radiant energy density," an SI unit expressed in joules per cubic meter. You could have "radiant flux density," but this is not a recognized SI unit. I believe what you are asking for is "radiant fluence," aka "radiant spherical exposure," which is defined as "the quotient of the radiant energy of all radiation incident on the outer surface of an infinitely small sphere centred at the given point by the area of the diametrical cross-section of that sphere." This is the appropriate radiometric quantity for describing the irradiation of microorganisms, and it is expressed in Joules per square meter.
Jorge Ferreira
replied on 18 June, 2020
you should read this article DOI: 10.1023/A:1013951313398 Mathematical Modeling of Ultraviolet Germicidal Irradiation for Air Disinfection, there is all you need to know.