Editing Sunscreen (section)

== Application ==
SPF 30 or above must be used to effectively prevent UV rays from damaging skin cells. This is the amount that is recommended to prevent against skin cancer. Sunscreen must also be applied thoroughly and re-applied during the day, especially after being in the water. Special attention should be paid to areas like the ears and nose, which are common spots of skin cancer. Dermatologists may be able to advise about what sunscreen is best to use for specific skin types.<ref>{{cite web |title=Skin Cancer Foundation |url=https://www.skincancer.org/skin-cancer-prevention/ |access-date=December 12, 2021 |archive-date=December 12, 2021 |archive-url=https://web.archive.org/web/20211212032437/https://www.skincancer.org/skin-cancer-prevention/ |url-status=live }}</ref>

The dose used in FDA sunscreen testing is 2&nbsp;mg/cm<sup>2</sup> of exposed skin.<ref name= CTFA1998>{{cite web | url=https://www.fda.gov/ohrms/dockets/dailys/00/Sep00/090600/c000573_10_Attachment_F.pdf | title=Re: Tentative Final Monograph for OTC Sunscreen |  vauthors = Kavanaugh EW | publisher=Cosmetic, Toiletry, and Fragrance Association | date=1998-09-11 | access-date=2009-09-25 | archive-date=February 21, 2017 | archive-url=https://web.archive.org/web/20170221045451/https://www.fda.gov/ohrms/dockets/dailys/00/Sep00/090600/c000573_10_attachment_F.pdf | url-status=dead }}</ref> If one assumes an "average" adult build of height 5&nbsp;ft 4&nbsp;in (163&nbsp;cm) and weight 150&nbsp;lb (68&nbsp;kg) with a 32-inch (82-cm) waist, that adult wearing a bathing suit covering the groin area should apply approximately 30&nbsp;g (or 30&nbsp;ml, approximately 1&nbsp;oz) evenly to the uncovered body area. This can be more easily thought of as a "golf ball" size amount of product per body, or at least six teaspoonfuls. Larger or smaller individuals should scale these quantities accordingly.<ref>{{cite web |url=http://www.thefactsabout.co.uk/how-and-why-we-use-sunscreen/content/239 |title=How and why we use sunscreen |publisher=Cosmetic, Toiletry & Perfumery Association |access-date=May 11, 2016 |archive-date=September 18, 2016 |archive-url=https://web.archive.org/web/20160918071256/http://www.thefactsabout.co.uk/how-and-why-we-use-sunscreen/content/239 |url-status=dead }}</ref> Considering only the face, this translates to about 1/4 to 1/3 of a teaspoon for the average adult face.

Some studies have shown that people commonly apply only 1/4 to 1/2 of the amount recommended for achieving the rated sun protection factor (SPF), and in consequence the effective SPF should be downgraded to a 4th root or a square root of the advertised value, respectively.<ref name=squared>{{cite journal | vauthors = Faurschou A, Wulf HC | title = The relation between sun protection factor and amount of {{sic|nolink=y|reason=error in source|suncreen}} applied in vivo | journal = The British Journal of Dermatology | volume = 156 | issue = 4 | pages = 716–719 | date = April 2007 | pmid = 17493070 | doi = 10.1111/j.1365-2133.2006.07684.x | s2cid = 22599824 }}</ref> A later study found a significant exponential relation between SPF and the amount of sunscreen applied, and the results are closer to linearity than expected by theory.<ref name="DOI10.1111/j.1600-0781.2009.00408.x">{{cite journal | vauthors = Schalka S, dos Reis VM, Cucé LC | title = The influence of the amount of sunscreen applied and its sun protection factor (SPF): evaluation of two sunscreens including the same ingredients at different concentrations | journal = Photodermatology, Photoimmunology & Photomedicine | volume = 25 | issue = 4 | pages = 175–180 | date = August 2009 | pmid = 19614894 | doi = 10.1111/j.1600-0781.2009.00408.x | s2cid = 38250220 }}</ref>

Claims that substances in pill form can act as sunscreen are false and disallowed in the United States.<ref>{{cite web |title=Press Announcements - Statement from FDA Commissioner Scott Gottlieb, M.D., on new FDA actions to keep consumers safe from the harmful effects of sun exposure, and ensure the long-term safety and benefits of sunscreens |url=https://www.fda.gov/newsevents/newsroom/pressannouncements/ucm608499.htm |website=www.fda.gov |access-date=23 August 2018 |language=en |archive-date=November 14, 2020 |archive-url=https://web.archive.org/web/20201114162137/https://www.fda.gov/news-events/press-announcements/statement-fda-commissioner-scott-gottlieb-md-new-fda-actions-keep-consumers-safe-harmful-effects-sun |url-status=dead }}</ref>