Sunscreen (also known as sunblock, suncream, suntan lotion) is a topical lotion that helps protect the skin from the sun's ultraviolet radiation, and which reduces sunburn and other skin damage, ultimately leading to a lower risk of skin cancer.
The best sunscreens protect against both UVB (ultraviolet radiation with wavelength between 315-280 nm), which can cause sunburn, and UVA (380–315 nm), which damages the skin with more long-term effects, such as premature skin aging (UVA=Aging). Most sunscreens work by containing either an organic chemical compound that absorbs ultraviolet light (such as oxybenzone) or an opaque material that reflects light (such as titanium dioxide, zinc oxide), or a combination of both.
Many people apply sunscreen when participating in outdoor activities during the summer. However, some experts suggest wearing sunscreen throughout the year to prevent cumulative damage, and to lower the risk of skin cancer. It is recommended that sunscreen be applied 30 minutes before exposure to the sun.
Dosing for sunscreen can be calculated using the formula for body surface area and subsequently subtracting the area covered by clothing. The dose used in FDA sunscreen testing is 2 mg/cm2. From a sample calculation in a FDA monograph, if one assumes an "average" adult build of height 5'4" (163 cm) and weight 150 lb (68 kg) with a 32" (82 cm) waist, that adult wearing a bathing suit covering the groin area should apply 29 grams (approximately 1 fluid ounce) evenly to the uncovered body area.
Sunscreen inhibits the production of Vitamin D. Though excessive sun exposure has been conclusively linked to some forms of skin cancer, Vitamin D is known to prevent other more dangerous forms of cancer. Season, geographic latitude, time of day, cloud cover, smog, skin type, and sunscreen all have an effect on Vitamin D production in the skin, but fifteen minutes per day of direct exposure to the sun is a generally accepted guideline to follow for optimum Vitamin D production.
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