By contrast far-UVC light (207 to 222 nm) has been shown to be as efficient as conventional germicidal UV light in killing microorganisms11, but studies to date12,13,14,15 suggest that these wavelengths do not cause the human health issues associated with direct exposure to conventional germicidal UV light. In short (see below) the reason is that far-UVC light has a range in biological materials of less than a few micrometers, and thus it cannot reach living human cells in the skin or eyes, being absorbed in the skin stratum corneum or the ocular tear layer. But because viruses (and bacteria) are extremely small, far-UVC light can still penetrate and kill them. Thus far-UVC light potentially has about the same highly effective germicidal properties of UV light, but without the associated human health risks12,13,14,15. Several groups have thus proposed that far-UVC light (207 or 222 nm), which can be generated using inexpensive excimer lamps, is a potential safe and efficient anti-microbial technology12,13,14,15,16,17,18 which can be deployed in occupied public locations.
The biophysically-based mechanistic basis to this far-UVC approach12 is that light in this wavelength range has a very limited penetration depth. Specifically, far-UVC light (207–222 nm) is very strongly absorbed by proteins through the peptide bond, and other biomolecules19,20, so its ability to penetrate biological materials is very limited compared with, for example, 254 nm (or higher) conventional germicidal UV light21,22. This limited penetration is still much larger than the size of viruses and bacteria, so far-UVC light is as efficient in killing these pathogens as conventional germicidal UV light12,13,14. However, unlike germicidal UV light, far-UVC light cannot penetrate either the human stratum corneum (the outer dead-cell skin layer), nor the ocular tear layer, nor even the cytoplasm of individual human cells. Thus, far-UVC light cannot reach or damage living cells in the human skin or the human eye, in contrast to the conventional germicidal UV light which can reach these sensitive cells7,8,9,10.