POV: date=April 2008

Sunscreen (also known as sunblock or suntan lotion) is a lotion, spray 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)
• 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. However, several epidemiological studies indicate an increased risk of malignant melanoma for the sunscreen user. Despite these studies no medical association has published recommendations to not use sunblock. Different meta-analysis publications have concluded that the evidence is not yet sufficient to claim a positive correlation between sunscreen use and malignant melanoma.

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.

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, and rubbing.

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.