UV forms part of the electromagnetic spectrum and the UV wavelength range is from about 10 to 400 nm, placing it between X-rays and the visible part of the spectrum. Though frequently referred to as “non-ionizing radiation,” the shortest ultraviolet wavelengths do bring about some ionization. The UV portion of the spectrum has been sub-divided for convenience.
The term “vacuum ultraviolet” is reserved for wavelengths below 200 nm, because in this region, UV is strongly attenuated by air. It is usual to refer to the region between 200 and 300 nm as “far ultraviolet” and that between 300 and 400 nm as “near ultraviolet.” Alternative sub-divisions are often quoted in the scientific literature, thus, UV-C is used for wavelengths in the range 100 to 280 nm, UV-B for 280 to 315 nm, and UV-A for 315 to 400 nm. It is only UV-C that is able to inactivate microorganisms directly. However, it is still possible to employ the longer wavelengths to lethal effect in association with photocatalyst.
It is important to point out that UV is harmful to humans and, in any application, serious consideration must be given to protecting personnel from exposure to it. The eyes are particularly susceptible and the condition arising from exposure to UV, referred to as “welder’s eye,” is both painful and ultimately sight threatening. Exposure of skin to UV results in erythe-ma, or delayed reddening and, at sufficiently high doses, UV can have profound effects on the immune system that can lead to severe and potentially lethal consequences. However, all such harmful effects can be completely avoided by careful design of containment measures to eliminate stray UV through the use of shields and non-reflective surfaces.