Acute exposure of the skin to UV radiation causes sunburn. The amount of UV required to produce sunburn depends on the absorption in the superficial layers (i.e. the thickness and amount of pigment) of the skin and on other genetic factors. The efficiency with which sunlight produces sunburn depends on the amount of UV-B radiation it contains. For example, more UV-B is present at high altitudes and more is present in noontime sun than at earlier or later hours. Chronic exposure of the skin to UV radiation also causes wrinkling, thinning, and loss of elasticity of the skin (photoaging); however, UV-A radiation may be more important than UV-B radiation in causing these changes.

Sunscreens are designed to protect against sunburn and can be highly effective in this regard. There is also evidence that they reduce the incidence of squamous cell carcinoma and precancerous lesions in the skin. Sunscreens can also provide protection against the photoaging and immunological effects of UV radiation, particularly if they contain chemicals that absorb both UV-B and UV-A radiation.

There is no evidence that getting a suntan will help prevent skin cancer. The UV exposure needed to acquire the tan adds to the skin cancer risk. The fact that one is able to tan well does, however, signify that the personal risk is lower (by a factor of 2 to 3) than for people who do not tan. Naturally dark-skinned people have a built-in protection of their skin against sunlight.

Basal and squamous cell carcinomas occur most often and with high frequency in lightskinned Caucasians living in sunny climates.

Fortunately, most of these skin cancers are readily treated and are rarely fatal. Cutaneous melanoma is much more dangerous, but occurs with a much lower frequency than the other types. Its relationship to UV radiation is not well understood, but exposure early in life seems to be an important factor in the subsequent development of melanoma. Lightskinned populations have the highest risk of developing melanoma. Although melanoma can occur in highly pigmented persons, such cancers are often not related to sun exposure.

UV irradiation causes skin cancers by altering critical genes that control cell division and cell death. Altered genes result from the ability of UV to make chemical alterations in DNA, the building block of genes. Some of the genes involved in skin cancer development have been identified. These include the p53 tumour suppressor gene (squamous and basal cell carcinomas), the PTCH gene (basal cell carcinomas), the p16 gene (melanomas), and a variety of genes involved in the repair of UVdamaged DNA (all types).