After more than one and a half years of pandemic, cost-effective, reliable and fast technologies for disinfection and decontamination are in great demand. Many liquid chemical disinfectants are available for surface disinfection. But these are associated with a high consumption of chemicals and resources. Everyone's perception of hygiene has changed, and at the same time the issue of environmental protection has come back into focus. Are there no more sustainable alternatives to rid surfaces of germs?
UVC radiation has been used for years in medicine and the food industry for disinfection. That UVC light also renders SARS-CoV-2 harmless has been proven in various studies, which, however, did not depict realistic conditions as in the regular disinfection of a table. Therefore, the scientists Natalia Ruetalo, Ramona Businger and Michael Schindler from the Institute for Medical Virology and Epidemiology of Viral Diseases at the University Hospital in Tübingen created these conditions. The study showed that even a short irradiation of the SARS-CoV-2 viruses with UVC light completely inactivated them. They were no longer infectious.
UVC radiation for disinfection and decontamination of surfaces and aerosols is a long-established technology. Previous studies and investigations showed a very high effect against the SARS-CoV-2 virus, but not under real conditions. Also, the UV doses, i.e. how long and at what distance an appropriate surface should be irradiated in order to inactivate the viruses, were insufficiently defined in the previous studies.
In the meantime, everyone knows that the virus spreads mainly through aerosols. But how infectious is the exhaled virus on surfaces? How efficiently does UVC radiation act on these viruses? And, how long and with what intensity must a surface be irradiated?
Infectious viruses on surfaces
In order to test the efficiency of UVC disinfection and to find out the minimum dose for inactivating the viruses, the viruses were spotted into so-called 6-well plates and dried for two hours. The subsequent examination showed that even after drying, the viruses were basically still just as infectious as when "freshly" applied. This setup was chosen to mimic the situation in which an infected person exhales droplets that dry on surfaces and potentially stay infectious and hazardous over a prolonged period of time. The results allow the assumption that exhaled viruses remain infectious on surfaces for a certain time.
The dried viruses were then exposed to UVC radiation. Three UVC disinfection devices from Heraeus were used, the Soluva® pro UV disinfection chamber, the Soluva® pro HP handheld UV disinfection device and the battery-operated SOLUVA® Zone H handheld device, all with a wavelength of 254 nanometres (nm).
In the disinfection chamber, the samples were exposed to UVC radiation for sixty seconds. Using the Soluva® pro handheld UV disinfector, the plates were irradiated for two seconds, each at five and twenty centimetre intervals. In addition, the efficiency was tested with the handheld device at a distance of twenty centimetres with a fast and a slow movement over the virus samples to simulate a disinfection of surfaces with the handheld device.
With the cordless disinfection handheld Soluva® Zone H, it was tested which UV intensity can be achieved at which distance from the surface in which time period. At the same time, it was tested when at least a disinfection of 99.9999 percent, which corresponds to the scientific classification of log 6, is achieved. For this purpose, the dried virus samples were irradiated at a distance of 50 cm for different lengths of time from 20 to 2.5 seconds with and without a 97-percent UV filter, which corresponded to a UV dose of 14 mJ/cm2 to 0.21 mJ/cm2.
Subsequently, the irradiated viruses from all samples as well as non-irradiated counter-samples were reconstituted with in a nutrient solution and compared with a diluted version of the original, non-dried viruses.
The remarkable results showed that even with the rapid movement at a distance of twenty centimetres, which was supposed to simulate a disinfection of common surfaces like an office desk with the handheld device (zone HP), no more infected cells could be detected in the sample. The calculations showed that an infection reduction of at least 99.9999 percent was achieved for all irradiated samples, which corresponds to complete inactivation.
The study showed that the viruses were still very infectious on the surfaces even after a longer drying time. This suggests that smear infections may indeed play a role in the transmission of the virus. However, the virus samples were processed and examined under optimal conditions. A comparison with a longer drying time or a different nutrient solution could of course produce different results.
Using the Soluva Zone H handheld device, it could be shown that at a wavelength of 254 nanometres with an intensity of 3.5 mJ/cm2, a reduction of the viral load of over 99.9999 percent could already be achieved, but with half the intensity of 1.75 mJ/cm2, only a reduction of 93 percent could be achieved. The relationship between virus inactivation and UVC irradiation is therefore not linear.
SARS-CoV-2 particularly sensitive
Compared to a previous study, this one showed that the UVC dose needed to achieve a 99.9999 per cent reduction in viral load was many times lower - 16 mJ/cm2 at 254 nm as opposed to the previously reported 1048 mJ/cm2. Another study using UV LED light showed 99.7 per cent viral inactivation with three seconds of irradiation. However, extending the irradiation to three hundred seconds did not change the result. In comparison with other pathogens, such as adenoviruses or polioviruses, these investigations show that SARS-CoV-2 seems to react particularly sensitively to UVC light.
The scientists thus proved once again that UVC radiation is not only a very effective means in the fight against the SARS-CoV-2 virus, but also a fast, inexpensive and reliable technology for disinfecting surfaces.
bioRxiv preprint doi: https://doi.org/10.1101/2020.09.22.308098 ; this version posted July 15, 2021; last opened on 15 October 2021
Published: 19. October 2021