Hiroshima University researchers say Ultraviolet-C light with a wavelength of 222 nm effectively kills SARS-CoV-2. Other studies involving 222-nm UV-C have so far only examined how well it kills seasonal coronaviruses structurally similar to the SARS-CoV-2 but not on the COVID-19-causing virus itself.
Where 222 nm sits on the light spectrum.
Operation at 222 nm is said to be safer than emitting light at higher
UH study results. Click image to enlarge.
UV-C wavelengths because such wavelengths cannot penetrate the outer, non-living layer of the human eye and skin. Although intense UV light at lower wavelengths can still be problematic for humans, the 222-nm range seems to be a safer alternative to the UV-C germicidal lamps used in disinfecting healthcare facilities which emit only 254-nm UV-C.
UV mercury lamps also have several other disadvantages: They can be fragile and thus risk mercury leakage if they break. Also, they take time to warm-up time and cannot exhibit maximum efficacy at low operating temperatures. In contrast, UV light-emitting diodes (UV-LED) can be configured to emit target wavelengths, not just the 254-nm seen from mercury lamps. Thus it looks as though it should be practical to devise UV-C-emitting LEDs that work at 222 nm.
The in vitro experiment by HU researchers showed that 99.7% of the SARS-CoV-2 viral culture was killed after a 30-sec exposure to 222 nm UV-C at 0.1 mW/cm2. The study is published in the American Journal of Infection Control. The tests used a Care222TM krypton-chloride excimer lamp. A 100 microliter 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-UVC lamp 24 cm above the surface of the plates.
The researchers, however, also say there’s a need for tests on real-world surfaces as their study only investigated in vitro efficacy.
