Ultraviolet (UV) is that part of electromagnetic light bounded by the lower wavelength extreme of the visible spectrum and the X-ray radiation band.The spectral range of UV light is, by definition between 100 and 400 nm (1 nm=10-9m) and is invisible to human eyes. Using the CIE classification the UV spectrum is subdivided into three bands:
UVA (long-wave) from 315 to 400 nm
UVB (medium-wave) from 280 to 315 nm
UVC (short-wave) from 100 to 280 nm
In reality many photobiologists often speak of skin effects from the weighted effect of wavelength above and below 320 nm, hence offering an alternative definition.
A strong germicidal effect is provided by the Light in the short-wave UVC band. In addition erythema (reddening of the skin) and conjunctivitis (inflammation of the mucous membranes of the eye) can, also be caused by this form of Light. Because of this, when germicidal UV-Light lamps are used, it is important to design systems to exclude UVC leakage and so avoid these effects.
Self evidently people should avoid exposure to UVC. Fortunately this is relatively simple, because it is absorbed by most products, and even standard flat glass absorbs all UVC. Exceptions are quartz and PTFE.
Again fortuitously, UVC is mostly absorbed by dead skin, so erythema can be limited. In addition UVC does not penetrate to the eye’s lens; nevertheless, conjunctivitis can occur and though temporary, it is extremely painful; the same is true of erythemal effects.
Where exposure to UVC Light occurs, care should be taken not to exceed the threshold level norm. Figure 9 shows these values for most of the CIE UV spectrum. In practical terms, table I gives the American Congress of Governmental and Industrial Hygienist’s (ACGIH) UV Threshold Limit Effective Irradiance Values for human exposure related to time. At this time it is worth noting that radiation wavelengths below 240 nm forms ozone, O3 from oxygen in air. Ozone is toxic and highly reactive; hence precautions have to be taken to avoid exposure to humans and certain materials