<P> UV rays also treat certain skin conditions . Modern phototherapy has been used to successfully treat psoriasis, eczema, jaundice, vitiligo, atopic dermatitis, and localized scleroderma . In addition, UV light, in particular UVB radiation, has been shown to induce cell cycle arrest in keratinocytes, the most common type of skin cell . As such, sunlight therapy can be a candidate for treatment of conditions such as psoriasis and exfoliative cheilitis, conditions in which skin cells divide more rapidly than usual or necessary . </P> <P> In humans, excessive exposure to UV radiation can result in acute and chronic harmful effects on the eye's dioptric system and retina . The risk is elevated at high altitudes and people living in high latitude countries where snow covers the ground right into early summer and sun positions even at zenith are low, are particularly at risk . Skin, the circadian and immune systems can also be affected . </P> <P> The differential effects of various wavelengths of light on the human cornea and skin are sometimes called the "erythemal action spectrum .". The action spectrum shows that UVA does not cause immediate reaction, but rather UV begins to cause photokeratitis and skin redness (with Caucasians more sensitive) at wavelengths starting near the beginning of the UVB band at 315 nm, and rapidly increasing to 300 nm . The skin and eyes are most sensitive to damage by UV at 265--275 nm, which is in the lower UVC band . At still shorter wavelengths of UV, damage continues to happen, but the overt effects are not as great with so little penetrating the atmosphere . The WHO - standard ultraviolet index is a widely publicized measurement of total strength of UV wavelengths that cause sunburn on human skin, by weighting UV exposure for action spectrum effects at a given time and location . This standard shows that most sunburn happens due to UV at wavelengths near the boundary of the UVA and UVB bands . </P> <P> Overexposure to UVB radiation not only can cause sunburn but also some forms of skin cancer . However, the degree of redness and eye irritation (which are largely not caused by UVA) do not predict the long - term effects of UV, although they do mirror the direct damage of DNA by ultraviolet . </P>

Which wavelengths of radiation are likely to cause photokeratitis