The same lightbulbs used in offices and public spaces can destroy coronaviruses and HIV, according to a new study from University of Toronto (U of T) Scarborough.
“We’re at a critical time where we need to use every single possible stop to get us out of this pandemic,” says Guzzo, an assistant professor in the department of biological sciences. “Every mitigation strategy that can be easily implemented should be used.”
“If you’re able to kill these spores, then you can reasonably say you should be able to kill most other viruses that you would commonly encounter in the environment,” says Guzzo, principal investigator at the Guzzo Lab.
Within 20 seconds of UV exposure, the spores’ growth dropped by 99%.
The researchers then created droplets containing coronaviruses or HIV, to mimic typical ways people encounter viruses in public, such as from coughing, sneezing and bleeding. The droplets were then exposed to UV light and placed in a culture to see if any of the virus remained active. With just 30 seconds of exposure, the virus’s ability to infect dropped by 93%.
Upon testing the viruses at different concentrations, they found samples with more viral particles were more resistant to the UV lights. But even with a viral load so high Guzzo calls it “the worst-case scenario,” infectivity dropped 88%.
“I was really surprised that UV could perform on the same level of those commonly used lab chemicals, which we regard as the gold standard,” she says. “That made me think, “Oh, my gosh, this is a legitimate tool that’s really underutilized.'”
Balance UV’s pros and cons with clever use, researchers say
Repeated exposure to UV light is key to catching those missed particles – fortunately, it’s as easy as flipping a switch. It’s also simpler to change a lightbulb than an air filtration system.
Guzzo notes that UV-LEDs are cheap and could be easy to retrofit in existing light fixtures, and that the bulbs are long-lasting and simple to maintain.
“You could disinfect in a way that wouldn’t be infringing on people’s enjoyment of that everyday ‘normal’ life that they long for,” Guzzo says.
The lights also benefit from automation. A standardized, germicidal dose of light can be delivered each time, while the process of wiping down spaces with disinfectants leaves room for human error. Chemicals and waste from these disinfectants also end up in watersheds and landfills as hands are washed and wipes thrown away.
But the lights aren’t harmless, and there’s a reason for wearing sunscreen and sunglasses—UV radiation damages nucleic acid, and repeated, prolonged exposure is harmful. That’s why Guzzo says the lights should be used when public spaces are empty, such as vacated buses that have finished their routes, or empty elevators traveling between floors. Escalator handrails could be continuously disinfected by putting UV lights in the underground part of the track, cleaning it with each rotation.
Safe Antivirus Technologies, Inc., a Toronto-based start-up company that partnered with Guzzo for the study, is developing unique UV-LED lighting modules. With motion sensors, the lights automatically switch to UV light when a room is empty, then turn back to regular light with movement.
Published in the Virology Journal, this study highlights UV-LEDs as a tool that could be used beyond the pandemic, ideally to help prevent another.
“Worldwide events like the COVID-19 pandemic, as terrible as they are, hopefully can still be learned from,” Guzzo says. “One thing we learned is that this is an underutilized tool we should think more about implementing.”
IMPORTANCE UV light is an effective tool to help stem the spread of respiratory viruses and protect public health in commercial, public, transportation, and health care settings. For effective use of UV, there is a need to determine the efficiency of different UV wavelengths in killing pathogens, specifically SARS-CoV-2, to support efforts to control the ongoing COVID-19 global pandemic and future coronavirus-caused respiratory virus pandemics. We found that SARS-CoV-2 can be inactivated effectively using a broad range of UVC wavelengths, and 222 nm provided the best disinfection performance. Interestingly, 222-nm irradiation has been found to be safe for human exposure up to thresholds that are beyond those effective for inactivating viruses. Therefore, applying UV light from KrCl* excimers in public spaces can effectively help reduce viral aerosol or surface-based transmissions.
INTRODUCTION
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an enveloped, nonsegmented positive-sense RNA virus (1), which is causing the ongoing COVID-19 global pandemic. It is transmitted primarily via respiratory droplets produced while talking, coughing, and sneezing (2). Indirect routes, such as airborne and surface-mediated transmission, are also possible, especially considering SARS-CoV-2 can stay viable in aerosols and on surfaces for up to 72 h (3). E
ffective disinfection procedures can help reduce viral transmission, especially in high-risk places, such as hospitals, other health care facilities, and public transportation systems.UV devices emitting UVC irradiation (200 to 280 nm), such as the low-pressure (LP) UV lamp and UV light-emitting diodes (LEDs), have been widely used for virus disinfection of water, air, and surfaces since the early 20th century (4–8).
Compared to other disinfection methods (e.g., heating and using chemical oxidants), UVC disinfection has several advantages, including rapid effectiveness, no chemical residual, and limited to no material degradation (6). One limitation of conventional UVC devices is that they are not safe for human exposure due to adverse effects on human skin and eyes (9, 10).
Emerging far-UVC devices (emitting UVC irradiation in the wavelength range of 200 to 225 nm) like the krypton chloride (KrCl*) excimer, however, have been proposed to disinfect occupied public spaces, as recent studies reported that far-UVC light exposure results in no adverse effects on skin or eyes in mouse studies due to its very limited penetration into biological materials (11–14).There are only a few studies that document inactivation efficiencies of SARS-CoV-2 using UVC devices.
An average UV fluence of 1.2 to over 60 mJ/cm2 was required for 1-log inactivation (90%) of SARS-CoV-2 in aqueous solutions using LP UV lamps, reported in previous investigations (15–18), whereas 1.6 mJ/cm2 of UVC irradiation from a KrCl* excimer with a 222-nm bandpass filter was needed to achieve the same virus reduction (19). A few other studies also investigated UV inactivation effectiveness against SARS-CoV-2 in virus droplets and on surfaces using LP UV lamps and KrCl* excimers (17, 20, 21).
Despite these prior works, information on UVC inactivation of SARS-CoV-2 is still limited across UV wavelengths and compared to that of surrogate enveloped viruses, primarily due to the safety requirement of testing, which is limited to biosafety level 3 (BSL3) laboratories.
Thus, comparative studies, including reliable and accessible surrogates of SARS-CoV-2 with a lower BSL requirement, are needed for extensive assessment of UVC devices, sources, and wavelength disinfection performance.In this study, UVC inactivation of SARS-CoV-2 in thin-film aqueous solution was determined using five UVC devices with different emission spectra in a bench-scale collimated beam apparatus. The UV sensitivities of SARS-CoV-2 and its potential testing surrogates classified as BSL1 and BSL2 viruses, including human coronavirus (HCoV) 229E, murine hepatitis virus (MHV), and bacteriophage Phi6 (22), were compared, and recommendations for reliable UV testing surrogates of SARS-CoV-2 are made.
RESULTS AND DISCUSSION
All UVC devices tested in this study were very effective in inactivating SARS-CoV-2 in aqueous solution (see Fig. 2). Among all tested UVC devices, unfiltered and filtered KrCl* excimers exhibited the greatest performance, with inactivation rate constants (mean ± standard error [SE]) of 1.52 ± 0.17 and 1.42 ± 0.40 cm2/mJ, respectively.
These values are much higher than the value reported by Robinson et al. (19) (0.64 cm2/mJ). One possible explanation for such difference is that sample absorbance at 222 nm was much higher in the Robinson study (>30 cm−1) than this study (0.05 cm−1) (Fig. 1B), and UV absorption by constituents in the sample matrix (i.e., proteins and other constituents from the cell culture extracts) may affect the virus sensitivity to UV irradiation. Greater performance of KrCl* excimers compared to other UVC devices was also observed for nonenveloped viruses (e.g., MS2 coliphage and adenovirus) (4, 5, 23), enveloped bacteriophage Phi6 (22), and coronaviruses (22) in previous studies, suggesting such superior performance may be universal across virus types.
This is likely because KrCl* excimers were capable of inflicting greater viral protein and nucleic acid damage than the other UVC devices due to the higher protein absorbance at far-UVC wavelengths around the 222-nm wavelength emitted by these devices (Fig. 1B). The superior performance of the KrCl* excimer is particularly promising because far-UVC devices are safe to be applied in occupied public spaces—up to the daily allowable threshold limit value of 25 mJ/cm2 at 220 nm (24) or perhaps beyond—to disinfect viruses in respiratory secretions and airborne droplets, as well as on contaminated surfaces, to limit the presence and transmission of SARS-CoV-2 or other respiratory viruses.
Previous studies on aerosol and surface UV disinfection (25, 26) suggested that viruses in airborne droplets and on surfaces tend to be more susceptible to UVC irradiation. Recent work with UV 222-nm inactivation of SARS-CoV-2 on surfaces (17, 20, 21) and of other coronaviruses in air (12), however, show very similar inactivation compared to this study, suggesting data for inactivation generated using thin-film aqueous suspensions can represent inactivation of coronaviruses across various media.FIG 1
An average UV dose of 1.3 mJ/cm2 was required for 1-log inactivation of SARS-CoV-2 using the LP UV lamp, which is similar to the results from several previous studies (1.2 to 5.0 mJ/cm2 for 1-log inactivation) (15–17). Another study by Heilingloh et al. (18), however, suggested 1-log inactivation would require more than 60 mJ/cm2 using a LP UV source. This divergence from the UV doses reported in numerous other studies is likely due to the significant difference in the experimental setup for UV exposures and calculation for UV fluences.
The inactivation tests reported by Heilingloh et al. were performed in cell culture media in 24-well plates with the UV source placed only 3 cm above the bottom of the plate, which could lead to differences in UV intensity between each well. Also, no information was given on how the UV irradiance was measured, there was no report of the absorbance of the suspending media, and standardized procedures for UV fluence calculation (e.g., corrections for sample UV absorbance, depth of sample, UV beam reflection and divergence, and petri factor) were not followed.
Based on the data presented herein, no statistically significant difference in UV inactivation performance was observed between the LP UV lamp and the UV LED 270 (i.e., LED with a peak emission wavelength of 270 nm) (P = 0.16) (Fig. 2). Viral genome damage is likely to be the primary inactivation mechanism for these UVC devices (23), and SARS-CoV-2 should have similar sensitivities to UV irradiation from these devices due to similar levels of nucleic acid absorbance at their peak emission wavelengths (i.e., 254 and 270 nm, respectively) (Fig. 1B). UV LED 282 provided the lowest inactivation rate constant among all tested UVC devices. Viral genomes tend to absorb less UV irradiation in the wavelength range emitted from UV LED 282 (4, 27) (Fig. 1B), which leads to less genome damage. While viral proteins should be slightly more sensitive to UV irradiation from around the 282-nm wavelength (28), this previous observation did not appear to enhance the effectiveness of the 282-nm LED in the current study.FIG 2
The inactivation rate constants of SARS-CoV-2 were compared with the values of potential enveloped virus surrogates: HCoV 229E, MHV, and bacteriophage Phi6 (Fig. 3). These three viruses were selected as candidates of SARS-CoV-2 surrogates for UV inactivation tests due to their molecular similarities (i.e., all are enveloped RNA viruses) and lower biosafety requirements (BSL1 for Phi6 and BSL2 for HCoV 229E and MHV).
All virus surrogates were previously tested in the identical collimated beam apparatus, except that the quartz lid was not applied for non-BSL3 organisms. The inactivation rate constants were also calculated following the same data analysis method (22). Among the three candidates, MHV exhibited the greatest similarities in inactivation rate constants across UVC devices compared to SARS-CoV-2. No statistically significant differences in the rate constant values (P > 0.05) were observed for all tested UVC devices, except for the unfiltered KrCl* excimer (P = 0.008), for which the inactivation rate of MHV was only 26% lower than the value for SARS-CoV-2 (Fig. 3). These results suggest that MHV can serve as a reliable UV surrogate of SARS-CoV-2 testing across UVC wavelengths when a lower biosafety requirement is needed.
HCoV 229E could also serve as a viable surrogate of SARS-CoV-2, especially for testing unfiltered KrCl* excimer (Fig. 3). Considering SARS-CoV-2, MHV, and HCoV 229E are all coronaviruses, evidence suggests that coronaviruses in general have similar sensitivities to UVC irradiation across wavelengths due to their similar molecular structures. This is further supported by comparing the UV inactivation rate constants of other coronaviruses, such as HCoV-OC43. UV inactivation rate constants of 0.77, 0.64, and 0.43 cm2/mJ were reported by Gerchman et al. (29) using UV LEDs with peak emission at 267, 279, and 286 nm, respectively, which are similar to the values we observed using UV LED 270 and UV LED 282 (0.93 and 0.53 cm2/mJ) (Fig. 2).
Although significantly lower inactivation rate constants were observed for bacteriophage Phi6 (P < 0.05) (Fig. 3), it can still serve as a conservative virus surrogate where use of coronaviruses is not feasible (e.g., lack of mammalian cell culture facilities). Compared to nonenveloped viruses, use of enveloped viruses like bacteriophage Phi6 is particularly desirable in surface and aerosol disinfection tests to best represent any interactions between the viral envelope and its surrounding environment that may affect viral sensitivity to UVC irradiation (30–32). Other bacteriophages, such as T1 and T7, although nonenveloped double-stranded DNA phages, exhibit similar sensitivities to SARS-CoV-2 across UVC wavelengths (27) and could also serve as UV disinfection surrogates.FIG 3
This research defines the fundamental inactivation rate constants of SARS-CoV-2 for UVC devices with peak emission wavelengths of 222 to 282 nm. These devices can be used to effectively inactivate SARS-CoV-2, among which far-UVC devices like the KrCl* excimer provided the best disinfection performance, with the added benefit of limited safety requirements when applied in occupied spaces. MHV is recommended as a reliable UV testing surrogate of SARS-CoV-2 due to its similar UV sensitivities across UVC wavelengths, but other T phages could also serve as surrogates. While these inactivation data align well with those from previous studies of UV disinfection of coronaviruses in aerosols and dried on surfaces, future work should continue to evaluate UV inactivation of SARS-CoV-2 in aqueous and other media relative to surrogates such as MHV or bacteriophage and expand these comparisons to other disinfectants important to minimizing the transmission of respiratory viruses.
REFERENCE LINK: https://journals.asm.org/doi/10.1128/AEM.01532-21
More information: Arvin T. Persaud et al, A UV-LED module that is highly effective at inactivating human coronaviruses and HIV-1, Virology Journal (2022). DOI: 10.1186/s12985-022-01754-w
