UV light has a beneficial effect for many fish
actually not. Ultraviolet refers to all electromagnetic radiation with wavelengths in the range of 10 to 400 nanometers, or frequencies from 7.5E14 to 3E16 Hz. The UVA range is wavelengths from 315 to 400 nanometers. Wavelengths from about 345 to 400 nM are used for "Blacklight" effects (causing many fluorescent objects to glow) and are usually very slightly visible if isolated from more visible wavelengths. Shorter UVA wavelengths from 315 to 345 nM are used for suntanning.
UVB refers to wavelengths from 280 to 315 nanometers. These wavelengths are more hazardous than UVA wavelengths, and are largely responsible for sunburn. The ozone layer partially blocks these wavelengths.
UVB lasers are considered less hazardous than UVA lasers, since UVB is more easily absorbed by various fluids and tissues in the eye and cannot reach the retina in significant amounts. UVB also does not penetrate as deeply in the skin as UVA. However, the deadliest types of skin cancer (malignant melanomas) start in the epidermis, an upper layer of the skin. UVB is largely blamed for these cancers, although shorter UVA wavelengths are considered possibly cancer-causing.
UVC refers to shorter UV wavelengths, usually 200 to 280 nM. Even shorter wavelengths from 10 to 200 nM are usually considered separately as "Vacuum Ultraviolet" since they are absorbed by air, although these wavelengths are also considered a shorter range of UVC. Wavelengths in the UVC range, especially from the low 200's to about 275 nM, are especially damaging to exposed cells. Such shortwave UV is often used for germ killing purposes.
Shorter UVA wavelengths cause suntanning. It is widely believed that UV suntanning is not completely safe no matter what you do. Possible ill effects include skin cancers and accelerated aging of the skin.
UVB is much worse, inherently irritating. The skin is designed to take small amounts of UVB and recover, but nothing is perfect here. You probably want to avoid any significant UVB exposure unless a physician recommends UVB exposure for medical purposes.
UVC is even worse, but less penetrating since it is easily absorbed. The outermost portion of human skin is made of dead cells. This layer sometimes gives partial protection from UVC, especially shorter wavelengths. However, don't count on this. UVC is quite damaging to living cells and has nasty burning effects.
EYE HAZARDS
UVA has one nasty effect on eyes, known as "nuclear cataracts". This is a permanent "tanning" of the lens of the eye. "Nuclear" refers to this occuring mainly in the center of the lens of the eye. You need lots of heavy exposure to get this. Nuclear cataracts occur mainly among people who work outdoors.
Although incandescent blacklights emit wavelengths that cause nuclear cataracts, they are largely safe since you typically get less UV exposure from these than from sunlight. Even considering the fact that your pupils are wider under blacklight than under sunlight, there is so much less UV that it is safe to stare into the bulb from a few feet away for large amounts of time. (No warranty, so I don't recommend this.)
Fluorescent blacklights are worse, but are usually comparable to sunlight when you are a couple feet away. Although I have yet to hear of eye damage blamed on reasonable use of blacklights, those who are around them a lot may want UV blocking eyewear.
Persons who have had cataract surgery need extra protection, since the lens of the eye largely absorbs UV. The retina does not like strong exposure to UV.
It should be noted that UVA-related eye damage involves a process with at least two steps. The first step (excitation of tryptophan or related molecules) normally reverses itself. Damage requires excited molecules to receive additional UV before they de-excite. This means that UVA eye damage is not linearly proportional to the intensity of exposure, although it may be proportional to the duration of exposure of a given intensity. UVB is worse on eyes than UVA, and can burn the cornea and the conjunctiva. UVC is especially burning to the conjunctiva.
