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Sunscreen (also known as sunblock or suntan lotion) is a lotion, spray, gel or other topical product that absorbs or reflects the sun's ultraviolet (UV) radiation and protects the skin.

Sunscreens contain one or more UV filters of which there are three main types :

• Organic chemical compounds that absorb ultraviolet light (such as oxybenzone, a suspected photocarcinogen)
• Inorganic particulates that reflect, scatter, and absorb UV light (such as titanium dioxide, zinc oxide), or a combination of both.

• Organic particulates that mostly absorb light like organic chemical compounds, but contain multiple chromophores, may reflect and scatter a fraction of light like inorganic particulates, and behave differently in formulations than organic chemical compounds. An example is Tinosorb M.

Medical organizations such as the American Cancer Society recommend the use of sunscreen because it prevents the squamous cell carcinoma and the basal cell carcinoma.

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Dosing

The dose used in FDA sunscreen testing is 2 mg/cm² of exposed skin.http://www.fda.gov/ohrms/dockets/dailys/00/Sep00/090600/c000573_10_Attachment_F.pdf Provided one assumes an "average" adult build of height 5 ft 4 in (163 cm) and weight 150 lb (68 kg) with a 32 in (82 cm) waist, that adult wearing a bathing suit covering the groin area should apply 29 g (approximately 1 oz) evenly to the uncovered body area. Considering only the face, this translates to about 1/4 to 1/3 of a teaspoon for the average adult face. Larger individuals should scale these quantities accordingly.

Contrary to the common advice that sunscreen should be reapplied every 2–3 hours, some research has shown that the best protection is achieved by application 15–30 minutes before exposure, followed by one reapplication 15–30 minutes after the sun exposure begins. Further reapplication is only necessary after activities such as swimming, sweating, or rubbing/wiping.

However, more recent research at the University of California, Riverside, indicates that sunscreen needs to be reapplied within 2 hours in order to remain effective. Not reapplying could even cause more cell damage than not using sunscreen at all, due to the release of extra free radicals from those sunscreen chemicals which were absorbed into the skin. Some studies have shown that people commonly apply only 1/2 to 1/4 of the amount recommended to achieve the rated SPF, and the effective SPF should be downgraded to a square or 4th root of the advertised value.

History

The first effective sunscreen may have been developed by chemist Franz Greiter in 1938. The product, called Gletscher Crème (Glacier Cream), subsequently became the basis for the company Piz Buin (named in honor of the place Greiter allegedly obtained the sunburn that inspired his concoction), which today is a well-known marketer of sunscreen products. SomeWho: date=February 2009 suggest that Gletscher Crème had a sun protection factor of 2.