Sunscreen, also known as sunblock or suntan lotion, is a lotion, spray, gel, foam (such as an expanded foam lotion or whipped lotion), stick 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 help to slow or temporarily prevent the development of wrinkles, dark spots and sagging skin.
Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e., zinc oxide and titanium dioxide, which stay on the surface of the skin and mainly deflect the UV light) or chemical sunscreens (i.e., UV organic filters, which 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. The routine use of sunscreens may also reduce the risk of melanoma. However, many sunscreens do not block UVA (UVA) radiation, yet protection from UVA is important for the prevention of skin cancer.To provide a better indication of their ability to protect against skin cancer and other diseases associated with UVA radiation (such as phytophotodermatitis), the use of broad-spectrum (UVA/UVB) sunscreens has been recommended.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 ​1⁄15 of the burning radiation runs the skin through the recommended thickness of sunscreen. Other rating systems indicate the degree of protection from non-burning UVA radiation.
Sunscreens are designed to remain effective at original strength for up to three years, and are typical of doubtful value after that period. Some sunscreens include an expiration date—a date indicating when they are no longer expected to be effective.