It has been generally thought that all UV can be harmful to the human body. In particular, the 254‐nm UV‐C, primary emission line of the conventional germicidal lamps (mercury lamps), is particularly harmful because it is near the maximum absorption wavelength of nucleic acids (260–275 nm), and therefore widely used for disinfection.
However, recently it came to be known that far uvc, short‐wavelength UV‐C, such as 207 or 222 nm, would be safer for the human body while it had a disinfection effect since these wavelengths penetrate only to the stratum corneum, the outermost layer of the skin comprised of dead cells. In a previous study, we reported that 222‐nm UV‐C did not induce detectable corneal damage in rats, whereas 254‐nm UV‐C induced corneal damage even from a slight exposure dose.
Furthermore, after the repetitive irradiation of 222‐nm UV‐C for to detect long‐term, delayed abnormalities of the eye and skin, none were observed in xeroderma pigmentosum complementation group A (Xpa)‐knockout mice. Irradiation at 222 nm appeared to be less hazardous to the cornea, as well as the skin, and this fact indicates that far‐UV‐C can be used in whole‐room germicidal applications even with persons present—unlike the more stringent precautions required for 254‐nm UV‐C disinfection.
UV‐C at 222 nm will become one of the important tools for prevention of infectious diseases such as COVID‐19 because very low doses of 222‐nm UV‐C efficiently inactivate human coronaviruses. At present, the threshold limit value (TLV®) at 222 nm recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) is 22 mJ cm−2 per day.
However, this value was determined based upon the threshold studies performed about 50 years ago. Considering the expected use of shorter wavelength UV‐C as UV germicidal irradiation (UVGI) in future, it will be all the more important to reassess the threshold corneal damage by UV and reconsider the TLV®. Therefore, after initially focusing on 222 nm UV‐C, we expanded our investigations to evaluate the wavelength dependence of the rat corneal damage by UV‐B and UV‐C radiation.
In this study, we revealed extremely less hazardous property of far‐UVC, short‐wavelength 207‐nm and 222‐nm UV‐C. Far‐UVC appears to be less hazardous to the cornea than had been previously considered and indicates that higher dose irradiation should be permissible.
It is reported that 222‐nm UVC shows sufficient inactivation of SARS‐CoV‐2 with small exposure doses. As well as SARS‐CoV‐2, there will be possibility that an unknown virus appears in future. Far‐UV‐C, particularly 222 nm UV‐C, will become more important tools against any virus, because UV photons have an inactivation effect regardless of the virus mutation.
At present, 222‐nm UV‐C is used at doses below the current ACGIH TLV®. However, if 222‐nm UV‐C could be available with higher exposure doses, its inactivation effects become higher and its application scene might become more widespread.
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