The cells of three different organ systems can be directly exposed to UV radiation -- the eyes, the skin, and the immune system. Acute exposure of the eyes to UV radiation causes photokeratitis (snow blindness) and chronic exposure contributes to cataract formation. In the skin, UV irradiation causes sunburn, photoaging, and skin cancer.

Moderate exposure to sunlight in the course of everyday life is essential for health. UV-B radiation is involved in the formation of vitamin D, which is necessary for growth and maintenance of bones and teeth. There is some evidence suggesting that maintaining normal vitamin D levels protects against the development of colon, breast, and prostate cancers. However, very moderate exposure is all that is required, and excessive exposure confers no added benefit.

Skin cancer is found in almost all animals that have been studied in the long-term, for example, cattle, goats, sheep, cats, dogs, guinea pigs, rats, and mice. Direct effects of UV-B radiation on body parts that are covered by thick hair are negligible. However, even furred animals usually have exposed skin around the mouth, nostrils and on other parts of the body. These parts, unless they are heavily pigmented, can be damaged by radiation.