Sunscreening agents

Sunscreen
Sunscreen on back under normal and UV light
Synonyms	Sun screen, sunblock, suntan lotion, sunburn cream, sun cream, block out
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Sunscreen, also known as sunblock and suntan lotion, is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet (UV) radiation and thus helps protect against sunburn. Diligent use of sunscreen can also slow or temporarily prevent the development of wrinkles and sagging skin.

Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e., those that reflect the sunlight) or chemical sunscreens (i.e., those that absorb the UV light).

Medical organizations such as the American Cancer Society recommend the use of sunscreen because it aids in the prevention of squamous cell carcinomas. Many sunscreens do not block UVA radiation, which does not primarily cause sunburn but can increase the rate of melanoma and photodermatitis. The use of broad-spectrum (UVA/UVB) sunscreens can address this concern.

Sunscreens are commonly rated and labeled with a sun protection factor (SPF) that measures the fraction of sunburn-producing UV rays that reach the skin. For example, "SPF 15" means that 1/15th of the burning radiation reaches the skin through the recommended thickness of sunscreen. Other rating systems indicate the degree of protection from non-burning UVA radiation.

Sunscreen use can help prevent melanoma and squamous cell carcinoma, two types of skin cancer. There is little evidence that it is effective in preventing basal cell carcinoma.

A 2013 study concluded that the diligent, everyday application of sunscreen can slow or temporarily prevent the development of wrinkles and sagging skin. The study involved 900 white people in Australia and required some of them to apply a broad-spectrum sunscreen every day for four and a half years. It found that people who did so had noticeably more resilient and smoother skin than those assigned to continue their usual practices.