Safely Killing SARS-CoV-2: 222 vs 254 nm UV-C

A study conducted by Hiroshima University researchers found that using Ultraviolet-C light with a wavelength of 222 nanometers which is safer to use around humans effectively kills SARS-CoV-2 – according to the authors, a first-published research to prove its efficacy against the virus that causes Covid-19.

UV-C light with a 222 nm wavelength, which doesn’t harm living cells in the human eye and skin, effectively kills the SARS-CoV-2. (Source: Hiroshima U.)

Other studies involving 222 nm, also known as far-UV-C, 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 HU researchers showed that 99.7 % of the SARS-CoV-2 viral culture was killed after a 30-second exposure to 222 nm UV-C irradiation at 0.1 mW/cm².

Tests were conducted using Ushio’s Care222 krypton-chloride excimer lamp. A 100 µl solution containing the virus (ca. 5 × 10⁶ TCID₅₀/mL) was spread onto a 9-cm sterile polystyrene plate. The researchers allowed it to dry in a biosafety cabinet at room temperature before placing the far-UV-C lamp 24 centimeters above the surface of the plates.

222 nm vs. 254 nm UVC

A wavelength of 222 nm UV-C cannot penetrate the outer, non-living layer of the human eye and skin so it won’t cause harm to the living cells beneath. This makes it a safer but equally potent alternative to the more damaging 254 nm UV-C germicidal lamps increasingly used in disinfecting healthcare facilities.

Since 254  nm UV-C harms exposed human tissues, it can only be used to sanitize empty rooms. But 222 nm UV-C can be a promising disinfection system for occupied public spaces including hospitals where nosocomial infections are a possibility.

The researchers, however, suggest further evaluation of the safety and effectiveness of 222 nm UV-C irradiation in killing SARS-CoV-2 viruses in real-world surfaces as their study only investigated its in-vitro efficacy. (Source: Hiroshima U.)

Reference: Hiroki Kitagawa et al.: Effectiveness of 222-nm ultraviolet light on disinfecting SARS-CoV-2 surface contamination, Am. J. Infect. Control, online September 04, 2020; DOI: 10.1016/j.ajic.2020.08.022

Link: Department of Infectious Diseases, Hiroshima University Hospital, Hiroshima, Japan

Further reading: Laser Diode Emits Deep UV Light, photonicsviews.com, 29th January 2020