Editing Sunscreen (section)

=== Sun protection factor and labeling {{anchor | Sun Protection Factor | Sun protection factor}} ===
[[Image:UV and Vis Sunscreen.jpg|thumb |upright=1.35 | Two photographs showing the effect of applying sunscreens in visible light and in UVA. The photograph on the right was taken using [[ultraviolet photography]] shortly after application of sunscreen to half of the face.]]

The sun protection factor (SPF rating, introduced in 1974) is a measure of the fraction of sunburn-producing UV rays that reach the skin. For example, "SPF 15" means that {{frac|1|15}} of the burning radiation will reach the skin, assuming sunscreen is applied evenly at a thick [[#Dosage|dosage]] of 2 milligrams per square centimeter<ref>{{Cite news|url=https://www.surfnation.com.au/blogs/news/sunscreen-the-comprehensive-guide-to-sunscreen-in-australia|title=Sunscreen: The comprehensive guide to sunscreen in Australia|work=Surf Nation|access-date=2018-06-24|language=en|archive-date=November 27, 2020|archive-url=https://web.archive.org/web/20201127051116/https://www.surfnation.com.au/blogs/news/sunscreen-the-comprehensive-guide-to-sunscreen-in-australia|url-status=live}}</ref> (mg/cm<sup>2</sup>). It is important to note that sunscreens with higher SPF do not last or remain effective on the skin any longer than lower SPF and must be continually reapplied as directed, usually every two hours.<ref>{{cite web |url=http://www.aad.org/media-resources/stats-and-facts/prevention-and-care/sunscreens |title=Sunscreen FAQs |publisher=American Academy of Dermatology |access-date=July 22, 2014 |archive-url=https://web.archive.org/web/20140721190111/http://www.aad.org/media-resources/stats-and-facts/prevention-and-care/sunscreens |archive-date=July 21, 2014 |url-status=dead |df=mdy-all }}</ref>

The SPF is an imperfect measure of skin damage because invisible damage and skin malignant melanomas are also caused by [[ultraviolet A]] (UVA, wavelengths 315–400 or 320–400 [[nanometre|nm]]), which does not primarily cause reddening or pain. Conventional sunscreen blocks very little UVA radiation relative to the nominal SPF; broad-spectrum sunscreens are designed to protect against both UVB and UVA.<ref>{{cite journal | vauthors = Stege H, Budde M, Grether-Beck S, Richard A, Rougier A, Ruzicka T, Krutmann J | title = Sunscreens with high SPF values are not equivalent in protection from UVA induced polymorphous light eruption | journal = European Journal of Dermatology | volume = 12 | issue = 4 | pages = IV-VI | year = 2002 | pmid = 12118426 }}</ref><ref>{{cite journal | vauthors = Haywood R, Wardman P, Sanders R, Linge C | title = Sunscreens inadequately protect against ultraviolet-A-induced free radicals in skin: implications for skin aging and melanoma? | journal = The Journal of Investigative Dermatology | volume = 121 | issue = 4 | pages = 862–868 | date = October 2003 | pmid = 14632206 | doi = 10.1046/j.1523-1747.2003.12498.x | doi-access = free }}</ref><ref>{{cite journal | vauthors = Moyal DD, Fourtanier AM | title = Broad-spectrum sunscreens provide better protection from solar ultraviolet-simulated radiation and natural sunlight-induced immunosuppression in human beings | journal = Journal of the American Academy of Dermatology | volume = 58 | issue = 5 Suppl 2 | pages = S149–S154 | date = May 2008 | pmid = 18410801 | doi = 10.1016/j.jaad.2007.04.035 }}</ref> According to a 2004 study, UVA also causes [[DNA repair#DNA damage|DNA damage]] to cells deep within the skin, increasing the risk of [[malignant melanoma]]s.<ref>{{cite journal | vauthors = Berneburg M, Plettenberg H, Medve-König K, Pfahlberg A, Gers-Barlag H, Gefeller O, Krutmann J | title = Induction of the photoaging-associated mitochondrial common deletion in vivo in normal human skin | journal = The Journal of Investigative Dermatology | volume = 122 | issue = 5 | pages = 1277–1283 | date = May 2004 | pmid = 15140232 | doi = 10.1111/j.0022-202X.2004.22502.x | doi-access = free }}</ref> Even some products labeled "broad-spectrum UVA/UVB protection" have not always provided good protection against UVA rays.<ref>{{cite news |url=https://www.nbcnews.com/id/wbna12081374 |title=Sunscreen makers sued for misleading claims |agency=Associated Press |date=April 24, 2006 |access-date=January 5, 2015 |archive-date=June 8, 2019 |archive-url=https://web.archive.org/web/20190608221941/http://www.nbcnews.com/id/12081374/ |url-status=live }}</ref> [[Titanium dioxide]] probably gives good protection but does not completely cover the UVA spectrum, with early 2000s research suggesting that zinc oxide is superior to titanium dioxide at wavelengths 340–380&nbsp;nm.<ref>{{cite journal | vauthors = Pinnell SR, Fairhurst D, Gillies R, Mitchnick MA, Kollias N | title = Microfine zinc oxide is a superior sunscreen ingredient to microfine titanium dioxide | journal = Dermatologic Surgery | volume = 26 | issue = 4 | pages = 309–314 | date = April 2000 | pmid = 10759815 | doi = 10.1046/j.1524-4725.2000.99237.x | s2cid = 39864876 }}</ref>

Owing to consumer confusion over the real degree and duration of protection offered, labelling restrictions are enforced in several countries. In the [[European Union|EU]], sunscreen labels can only go up to SPF 50+ (initially listed as 30 but soon revised to 50).<ref>{{cite journal | url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32006H0647:EN:NOT | title=Commission Recommendation of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto | journal=Official Journal of the European Union | date=2006-09-22 | access-date=2009-09-25 | archive-date=February 22, 2014 | archive-url=https://web.archive.org/web/20140222055637/http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32006H0647:EN:NOT | url-status=live }}</ref> [[Australia]]'s [[Therapeutic Goods Administration]] increased the upper limit to 50+ in 2012.<ref>{{cite web | url=http://www.arpansa.gov.au/uvrg/rginfo_p13.cfm | title=UV Resource Guide - Sunscreens | publisher=Arpansa | date=2008-12-20 | access-date=2009-09-25 | url-status=dead | archive-url=https://web.archive.org/web/20091119074507/http://www.arpansa.gov.au/uvrg/rginfo_p13.cfm | archive-date=November 19, 2009 | df=mdy-all }}</ref><ref>{{cite web | url=http://www.cancer.org.au/preventing-cancer/sun-protection/preventing-skin-cancer/spf50sunscreen.html | title=SPF50+ Sunscreen | date=2013-02-01 | access-date=2014-02-06 | archive-date=February 7, 2014 | archive-url=https://web.archive.org/web/20140207205843/http://www.cancer.org.au/preventing-cancer/sun-protection/preventing-skin-cancer/spf50sunscreen.html | url-status=live }}</ref> In its 2007 and 2011 draft rules, the US [[Food and Drug Administration]] (FDA) proposed a maximum SPF label of 50, to limit unrealistic claims.<ref name="qa2007">{{cite web|url=https://www.fda.gov/cder/drug/infopage/sunscreen/qa.htm|title=Questions and Answers on the 2007 Sunscreen Proposed Rule|website=[[Food and Drug Administration]]|url-status=dead|archive-url=https://web.archive.org/web/20080921135529/https://www.fda.gov/CDER/drug/infopage/sunscreen/qa.htm|archive-date=September 21, 2008|df=mdy-all}}</ref><ref name="fda2011">{{cite web | url=https://www.fda.gov/Drugs/ResourcesForYou/Consumers/BuyingUsingMedicineSafely/UnderstandingOver-the-CounterMedicines/ucm258468.htm | title=Questions and Answers: FDA announces new requirements for over-the-counter (OTC) sunscreen products marketed in the U.S. | website=[[Food and Drug Administration]] | date=2011-06-23 | access-date=2012-04-10 | archive-date=April 23, 2019 | archive-url=https://web.archive.org/web/20190423132658/https://www.fda.gov/Drugs/ResourcesForYou/Consumers/BuyingUsingMedicineSafely/UnderstandingOver-the-CounterMedicines/ucm258468.htm | url-status=dead }}</ref><ref>{{cite journal |title=Revised Effectiveness Determination; Sunscreen Drug Products for Over-the-Counter Human Use |journal=[[Federal Register]] |date=June 17, 2011 |volume=76 |issue=117 |pages=35672–35678 |url=https://www.fda.gov/downloads/Drugs/DevelopmentApprovalProcess/DevelopmentResources/Over-the-CounterOTCDrugs/StatusofOTCRulemakings/UCM313542.pdf |access-date=November 21, 2013 |author=Department of Health and Human Services: Food and Drug Administration |archive-date=February 22, 2017 |archive-url=https://web.archive.org/web/20170222152537/https://www.fda.gov/downloads/Drugs/DevelopmentApprovalProcess/DevelopmentResources/Over-the-CounterOTCDrugs/StatusofOTCRulemakings/UCM313542.pdf |url-status=dead }}</ref> (As of August 2019, the FDA has not adopted the SPF 50 limit.<ref>{{cite journal|url=https://www.fda.gov/Drugs/DevelopmentApprovalProcess/DevelopmentResources/Over-the-CounterOTCDrugs/StatusofOTCRulemakings/ucm072134.htm|title=Status of OTC Rulemakings - Rulemaking History for OTC Sunscreen Drug Products| author = Center for Drug Evaluation and Research|journal=FDA|date=April 23, 2019|access-date=June 27, 2024|archive-date=November 9, 2020|archive-url=https://web.archive.org/web/20201109033819/https://www.fda.gov/drugs/status-otc-rulemakings/rulemaking-history-otc-sunscreen-drug-products|url-status=dead}}</ref>) Others have proposed restricting the active ingredients to an SPF of no more than 50, due to lack of evidence that higher dosages provide more meaningful protection.<ref name="onpoint">{{cite web | url=http://onpoint.wbur.org/2011/06/16/sunscreen | date=2011-06-16 | access-date=2012-04-10 | title=Sunscreen Takes Some Heat: New Dangers, New Rules | archive-date=July 5, 2012 | archive-url=https://web.archive.org/web/20120705232835/http://onpoint.wbur.org/2011/06/16/sunscreen | url-status=live }}</ref> Different sunscreen ingredients have different effectiveness against UVA and UVB.<ref>{{cite web|title=The Burning Facts|url=https://www.epa.gov/sites/production/files/documents/sunscreen.pdf|access-date=1 December 2017|date=2006|archive-date=November 12, 2020|archive-url=https://web.archive.org/web/20201112023944/https://www.epa.gov/sites/production/files/documents/sunscreen.pdf|url-status=live}}</ref>

[[Image:erythemal action spectrum.svg|thumb|upright=1.35 | UV sunlight spectrum (on a summer day in the Netherlands), along with the CIE Erythemal action spectrum. The effective spectrum is the product of the former two.]]

The SPF can be measured by applying sunscreen to the skin of a volunteer and measuring how long it takes before sunburn occurs when exposed to an artificial sunlight source. In the US, such an ''[[in vivo]]'' test is required by the FDA. It can also be measured ''[[in vitro]]'' with the help of a specially designed [[spectrometer]]. In this case, the actual [[transmittance]] of the sunscreen is measured, along with the degradation of the product due to being exposed to sunlight. In this case, the transmittance of the sunscreen must be measured over all wavelengths in sunlight's UVB–UVA range (290–400&nbsp;nm), along with a table of how effective various wavelengths are in causing sunburn (the ''erythemal action spectrum'') and the standard intensity [[spectrum]] of sunlight (see the figure). Such ''in vitro'' measurements agree very well with ''in vivo'' measurements.{{Attribution needed|date=September 2016}}

Numerous methods have been devised for evaluation of UVA and UVB protection. The most-reliable spectrophotochemical methods eliminate the subjective nature of grading [[erythema]].<ref>{{cite journal | vauthors = Moyal D | title = How to measure UVA protection afforded by sunscreen products. | date = June 2008 | volume = 3 | issue = 3 | pages = 307–13 | doi = 10.1586/17469872.3.3.307 | url = http://www.medscape.com/viewarticle/576849 | journal = Expert Review of Dermatology | access-date = August 17, 2016 | archive-date = July 13, 2017 | archive-url = https://web.archive.org/web/20170713165717/http://www.medscape.com/viewarticle/576849 | url-status = live | url-access = subscription }}</ref>

The ultraviolet protection factor (UPF) is a similar scale developed for rating fabrics for [[sun protective clothing]]. According to recent testing by ''[[Consumer Reports]]'', UPF ~30+ is typical for protective fabrics, while UPF ~20 is typical for standard summer fabrics.<ref>{{cite journal |url=http://www.consumerreports.org/cro/magazine/2014/07/what-to-know-about-sunscreen-before-buying-it/ |title=What to Know About Sunscreen Before Buying It |journal=Consumer Reports |date=May 2014 |access-date=December 20, 2014 |archive-date=October 12, 2019 |archive-url=https://web.archive.org/web/20191012144703/https://www.consumerreports.org/sun-protection/get-the-best-sun-protection/ |url-status=live }}</ref>

Mathematically, the SPF (or the UPF) is calculated from measured data as:{{citation needed|date=July 2019}}

<math display="block">\mathrm{SPF} = \frac{\int A(\lambda) E(\lambda)d\lambda}{\int A(\lambda) E(\lambda)/\mathrm{MPF}(\lambda) \, d\lambda},</math>

where <math>E(\lambda)</math> is the [[solar irradiance]] spectrum, <math>A(\lambda)</math> the erythemal action spectrum, and <math>\mathrm{MPF}(\lambda)</math> the monochromatic protection factor, all functions of the wavelength <math>\lambda</math>. The MPF is roughly the inverse of the transmittance at a given wavelength.{{citation needed|date=July 2019}}

The combined SPF of two layers of sunscreen may be lower than the square of the single-layer SPF.<ref name=squared/>