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===Skin===
UVUVA, UVB and UVC raysall can damage [[collagen]] fibers and thereby accelerate aging of the skin. In general, UVUVA is the least harmful, but can contribute to the aging of skin, DNA damage and possibly skin cancer. It penetrates deeply and does not cause [[sunburn]]. Because it does not cause reddening of the skin (erythema) it cannot be measured in the [[Sunscreen#Sun protection factor|SPF]] testing. There is no good clinical measurement of the blocking of UVUVA radiation, but it is important that [[sunscreen]] block both UVUVA and UVUVB.
UVUVA light is also known as "dark-light" and, because of its longer wavelength, can penetrate most windows. It also penetrates deeper into the skin than UVB light and is thought to be a prime cause of wrinkles.
UVUVB light in particular has been linked to [[skin cancer]]s such as [[melanoma]]. The radiation [[excitation|excites]] [[DNA]] molecules in skin cells, causing [[covalent bond]]s to form between adjacent [[thymine]] bases, producing thymidine dimers. Thymidine dimers do not base pair normally, which can cause distortion of the DNA helix, stalled replication, gaps, and misincorporation. These can lead to [[mutation]]s, which can result in [[cancer]]ous growths. The [[mutagen]]icity of UV radiation can be easily observed in [[bacterium|bacteria]] cultures.
This cancer connection is one reason for concern about [[ozone depletion]] and the ozone hole.
As a defense against UV radiation, the body tans when exposed to moderate (depending on [[human skin color|skin type]]) levels of radiation by releasing the brown pigment [[melanin]]. This helps to block UV penetration and prevent damage to the vulnerable skin tissues deeper down. Suntan lotion that partly blocks UV is widely available (often referred to as "sun block" or "[[sunscreen]]"). Most of these products contain an "SPF rating" that describes the amount of protection given. This protection applies only to UVUVB light. In any case, most dermatologists recommend against prolonged sunbathing.
===Eye===
High intensities of UVUVB light are hazardous to the eyes, and exposure can cause ''welder's flash'' ([[photokeratitis]] or [[arc eye]]) and may lead to [[cataract]]s, [[pterygium]][http://ajp.amjpathol.org/cgi/content/abstract/162/2/567] [http://ajp.amjpathol.org/cgi/content/abstract/167/2/489], and [[pinguecula]] formation.
[[Protective eyewear]] is beneficial to those who are working with or those who might be exposed to ultraviolet radiation, particularly short wave UV. Given that light may reach the eye from the sides, full coverage eye protection is usually warranted if there is an increased risk of exposure as in high altitude mountaineering. Mountaineers are exposed to higher than ordinary levels of UV radiation, both because there is less atmospheric filtering and because of reflection from snow and ice.
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