18 September 2020
First proof that safer, 222nm UV-C effectively kills SARS-CoV-2
A study by Hiroshima University has found that using UV-C light with a wavelength of 222nm which is safer to use around humans effectively kills SARS-CoV-2 — the first research to prove its efficacy against the virus that causes COVID-19 (Hiroki Kitagawa et al, American Journal of Infection Control, DOI: 10.1016/j.ajic.2020.08.022).
Other studies involving 222nm UV-C (far-UVC) have so far only looked at its potency in eradicating seasonal coronaviruses that are structurally similar to the SARS-CoV-2 but not on the COVID-19-causing virus itself.
An in-vitro experiment by Hiroshima University showed that 99.7% of the SARS-CoV-2 viral culture was killed after a 30s exposure to 222nm UVC irradiation at 0.1mW/cm2.
Tests were conducted using Ushio’s Care222 krypton-chloride excimer lamp. A 100 microliter solution containing the virus (about 5x106TCID50/mL) was spread onto a 9cm sterile polystyrene plate. The researchers allowed it to dry in a biosafety cabinet at room temperature before placing the far-UVC lamp 24cm above the surface of the plates.
222nm versus 254nm UV-C
UV-C light with a wavelength of 222nm cannot penetrate the outer, non-living layer of the human eye and skin so it will not cause harm to the living cells beneath. This makes it a safer but equally potent alternative to the more damaging 254nm UV-C germicidal lamps that are increasingly used in disinfecting healthcare facilities.
Since 254nm UVC harms exposed human tissue, it can only be used to sanitize empty rooms. But 222nm UVC can be a promising disinfection system for occupied public spaces including hospitals where nosocomial infections are a possibility.
However, the researchers propose further evaluation of the safety and effectiveness of 222nm UV-C irradiation in killing SARS-CoV-2 viruses in real-world surfaces, as their study only investigated its in-vitro efficacy.
The far-UVC research is one of the four COVID-19 studies conducted by Hiroshima University scientists that received funding from the Japan Agency for Medical Research and Development (AMED). Drs Hiroki Kitagawa, Toshihito Nomura and Hiroki Ohge of Hiroshima University Hospital’s Department of Infectious Diseases, and Dr Takemasa Sakaguchi of the Hiroshima University Graduate School of Biomedical and Health Sciences were behind the study (under grant number JP20he0922003). Many laboratories in the university are conducting research on the novel coronavirus as part of the ‘Hiroshima University CoV-Peace-Project’.