Editing Ultraviolet (section)

==Human health-related effects==
{{Further|Health effects of sunlight exposure}}
The impact of ultraviolet radiation on [[human health]] has implications for the risks and benefits of sun exposure and is also implicated in issues such as [[fluorescent lamps and health]]. Getting too much sun exposure can be harmful, but in moderation, sun exposure is beneficial.<ref name="sivamani">{{cite journal |last1=Sivamani |first1=R.K. |last2=Crane |first2=L.A. |last3=Dellavalle |first3=R.P. |title=The benefits and risks of ultraviolet tanning and its alternatives: The role of prudent sun exposure |journal=Dermatologic Clinics |date=April 2009 |volume=27 |issue=2 |pages=149–154 |pmid=19254658 |doi=10.1016/j.det.2008.11.008 |pmc=2692214}}</ref>

===Beneficial effects===

UV (specifically, UVB) causes the body to produce [[vitamin D]],<ref>{{Cite journal |last1=Wacker |first1=Matthias |last2=Holick |first2=Michael F. |date=2013-01-01 |title=Sunlight and Vitamin D |journal=Dermato-endocrinology |volume=5 |issue=1 |pages=51–108 |doi=10.4161/derm.24494 |issn=1938-1972 |pmc=3897598 |pmid=24494042}}</ref> which is essential for life. Humans need some UV radiation to maintain adequate vitamin&nbsp;D levels. According to the World Health Organization:<ref name="who.int"/>
<blockquote>There is no doubt that a little sunlight is good for you! But 5–15&nbsp;minutes of casual sun exposure of hands, face and arms two to three times a week during the summer months is sufficient to keep your vitamin D levels high.</blockquote>

Vitamin D can also be obtained from food and supplementation.<ref>{{cite journal |last1=Lamberg-Allardt |first1=Christel |title=Vitamin&nbsp;D in foods and as supplements |journal=Progress in Biophysics and Molecular Biology |date=1 September 2006 |volume=92 |issue=1 |pages=33–38 |doi=10.1016/j.pbiomolbio.2006.02.017 |pmid=16618499 |language=en |issn=0079-6107 |doi-access=free }}</ref> Excess sun exposure produces harmful effects, however.<ref name="who.int">{{cite report |url=https://www.who.int/uv/faq/uvhealtfac/en/index1.html |title=The known health effects of UV: Ultraviolet radiation and the INTERSUN Programme |archive-url=https://web.archive.org/web/20161016090300/http://www.who.int/uv/faq/uvhealtfac/en/index1.html |archive-date=16 October 2016 |publisher=World Health Organization}}</ref>

Vitamin D promotes the creation of [[serotonin]]. The production of serotonin is in direct proportion to the degree of bright sunlight the body receives.<ref>{{cite magazine |author=Korb, Alex |date=17 November 2011 |title=Boosting your serotonin activity |magazine=Psychology Today |url=https://www.psychologytoday.com/blog/prefrontal-nudity/201111/boosting-your-serotonin-activity |archive-url=https://archive.today/20170801135657/https://www.psychologytoday.com/blog/prefrontal-nudity/201111/boosting-your-serotonin-activity |archive-date=1 August 2017}}</ref> Serotonin is thought to provide sensations of happiness, well-being and serenity to human beings.<ref>{{cite journal |last = Young |first = S.N. |year = 2007 |title = How to increase serotonin in the human brain without drugs |journal = Journal of Psychiatry and Neuroscience |volume = 32 |issue = 6 |pages = 394–399 |pmid = 18043762 |pmc = 2077351}}</ref>

====Skin conditions====
UV rays also treat certain skin conditions. Modern phototherapy has been used to successfully treat [[psoriasis]], [[eczema]], [[jaundice]], [[vitiligo]], [[atopic dermatitis]], and localized [[scleroderma]].<ref>{{cite journal |last1=Juzeniene |first1=Asta |last2=Moan |first2=Johan |title=Beneficial effects of UV radiation other than via vitamin&nbsp;D production |journal=Dermato-Endocrinology|date=27 October 2014 |volume=4 |issue=2 |pages=109–117 |doi=10.4161/derm.20013 |pmid=22928066 |pmc=3427189}}</ref><ref>[http://healthycanadians.gc.ca/healthy-living-vie-saine/environment-environnement/sun-soleil/effects-uv-effets-eng.php "Health effects of ultraviolet radiation"] {{webarchive|url=https://web.archive.org/web/20161008013441/http://healthycanadians.gc.ca/healthy-living-vie-saine/environment-environnement/sun-soleil/effects-uv-effets-eng.php |date=8 October 2016 }}. Government of Canada.</ref> In addition, UV radiation, in particular UVB radiation, has been shown to induce [[cell cycle]] arrest in [[keratinocytes]], the most common type of skin cell.<ref>{{Cite journal |last1=Herzinger |first1=T. |last2=Funk |first2=J.O. |last3=Hillmer |first3=K. |last4=Eick |first4=D. |last5=Wolf |first5=D.A. |last6=Kind |first6=P. |year=1995 |title=Ultraviolet&nbsp;B irradiation-induced G2 cell cycle arrest in human keratinocytes by inhibitory phosphorylation of the cdc2 cell cycle kinase |journal=Oncogene |volume=11 |issue=10 |pages=2151–2156 |pmid=7478536}}</ref> As such, sunlight therapy can be a candidate for treatment of conditions such as psoriasis and [[exfoliative cheilitis]], conditions in which skin cells divide more rapidly than usual or necessary.<ref>{{cite journal |last1=Bhatia |first1=Bhavnit K. |last2=Bahr |first2=Brooks A. |last3=Murase |first3=Jenny E. |year=2015 |title=Excimer laser therapy and narrowband ultraviolet&nbsp;B therapy for exfoliative cheilitis |journal=International Journal of Women's Dermatology |volume=1 |issue=2 |pages=95–98 |doi=10.1016/j.ijwd.2015.01.006 |pmid=28491966 |pmc=5418752}}</ref>

=== Harmful effects ===
[[File:Erythemal action spectrum.svg|thumb|right|Sunburn effect (as measured by the [[UV index]]) is the product of the sunlight spectrum (radiation intensity) and the erythemal action spectrum (skin sensitivity) across the range of UV wavelengths. Sunburn production per milliwatt of radiation intensity is increased by nearly a factor of 100 between the near UVB wavelengths of 315–295&nbsp;nm.]]

In humans, excessive exposure to UV radiation can result in acute and chronic harmful effects on the eye's dioptric system and [[retina]]. The risk is elevated at high [[altitude]]s and people living in high [[latitude]] areas where snow covers the ground right into early summer and sun positions even at [[zenith]] are low, are particularly at risk.<ref name="Meyer-Rochow 2000">{{cite journal |last=Meyer-Rochow |first=Victor Benno |year=2000 |title= Risks, especially for the eye, emanating from the rise of solar UV-radiation in the Arctic and Antarctic regions |journal=International Journal of Circumpolar Health |volume=59 |issue=1 |pages=38–51 |pmid=10850006}}</ref> Skin, the [[circadian]] system, and the [[immune system]] can also be affected.<ref>{{cite web |title=Health effects of UV radiation |url=https://www.who.int/uv/health/uv_health2/en/ |publisher=World Health Organization |url-status=dead |archive-url=https://web.archive.org/web/20150317041542/http://www.who.int/uv/health/en/ |archive-date=17 March 2015 }}</ref>

The differential effects of various wavelengths of light on the human cornea and skin are sometimes called the "erythemal action spectrum".<ref>{{cite report
 |title=Ultraviolet Radiation Guide
 |date=April 1992
 |publisher=U.S.Navy
 |department=Environmental Health Center
 |place=Norfolk, Virginia
 |url=https://www.med.navy.mil/sites/nmcphc/Documents/policy-and-instruction/ih-ultraviolet-radiation-technical-guide.pdf
 |access-date=21 December 2019
 |archive-date=21 December 2019
 |archive-url=https://web.archive.org/web/20191221165034/https://www.med.navy.mil/sites/nmcphc/Documents/policy-and-instruction/ih-ultraviolet-radiation-technical-guide.pdf
 |url-status=dead
 }}</ref> The action spectrum shows that UVA does not cause immediate reaction, but rather UV begins to cause [[photokeratitis]] and skin redness (with lighter skinned individuals being more sensitive) at wavelengths starting near the beginning of the UVB band at 315&nbsp;nm, and rapidly increasing to 300&nbsp;nm. The skin and eyes are most sensitive to damage by UV at 265–275&nbsp;nm, which is in the lower UVC band. At still shorter wavelengths of UV, damage continues to happen, but the overt effects are not as great with so little penetrating the atmosphere. The [[WHO]]-standard [[ultraviolet index]] is a widely publicized measurement of total strength of UV wavelengths that cause sunburn on human skin, by weighting UV exposure for action spectrum effects at a given time and location. This standard shows that most sunburn happens due to UV at wavelengths near the boundary of the UVA and UVB bands.{{cn|date=May 2024}}

==== Skin damage ====
[[File:DNA UV mutation.svg|thumb|right|Ultraviolet photons harm the [[DNA]] molecules of living organisms in different ways. In one common damage event, adjacent [[thymine]] bases bond with each other, instead of across the "ladder". This "[[thymine dimer]]" makes a bulge, and the distorted DNA molecule does not function properly.]]

Overexposure to UVB radiation not only can cause [[sunburn]] but also some forms of [[skin cancer]]. However, the degree of redness and eye irritation (which are largely not caused by UVA) do not predict the long-term effects of UV, although they do mirror the direct damage of DNA by ultraviolet.<ref>{{cite web |title=What is ultraviolet (UV) radiation? |website=cancer.org |url=https://www.cancer.org/cancer/skin-cancer/prevention-and-early-detection/what-is-uv-radiation.html |access-date=2017-06-11 |url-status=live |archive-url=https://web.archive.org/web/20170403181332/https://www.cancer.org/cancer/skin-cancer/prevention-and-early-detection/what-is-uv-radiation.html |archive-date=3 April 2017}}</ref>

All bands of UV radiation damage [[collagen]] fibers and accelerate aging of the skin. Both UVA and UVB destroy vitamin&nbsp;A in skin, which may cause further damage.<ref>
{{cite journal
 |last1 = Torma      |first1 = H.
 |last2 = Berne      |first2 = B.
 |last3 = Vahlquist  |first3 = A.
 |year=1988
 |title = UV irradiation and topical vitamin A modulate retinol esterification in hairless mouse epidermis
 |journal = Acta Derm. Venereol.
 |volume = 68 |issue = 4 |pages = 291–299
 |pmid = 2459873
}}
</ref>

UVB radiation can cause direct DNA damage.<ref name="Bernstein-2002">{{cite journal |vauthors=Bernstein C, Bernstein H, Payne CM, Garewal H |date=June 2002 |title=DNA repair / pro-apoptotic dual-role proteins in five major DNA repair pathways: Fail-safe protection against carcinogenesis |journal=Mutat. Res. |volume=511 |issue=2 |pages=145–78 |pmid=12052432 |doi= 10.1016/S1383-5742(02)00009-1|bibcode=2002MRRMR.511..145B }}</ref> This cancer connection is one reason for concern about [[ozone depletion]] and the [[ozone hole]].

The most deadly form of [[skin cancer]], malignant [[melanoma]], is mostly caused by DNA damage independent from UVA radiation. This can be seen from the absence of a direct UV signature mutation in 92% of all melanoma.<ref name=Davies>{{cite journal |author1=Davies, H. |author2=Bignell, G.R. |author3=Cox, C. |date=June 2002 |title=Mutations of the ''BRAF'' gene in human cancer |journal=Nature |volume=417 |issue=6892 |pages=949–954 |doi=10.1038/nature00766 |pmid=12068308 |url=http://eprints.gla.ac.uk/121/1/Davis%2CH_2002_pdf.pdf |bibcode=2002Natur.417..949D |s2cid=3071547 |s2cid-access=free |access-date=30 November 2019 |archive-date=5 August 2020 |archive-url=https://web.archive.org/web/20200805053025/http://eprints.gla.ac.uk/121/1/Davis%2CH_2002_pdf.pdf |url-status=live }}</ref> Occasional overexposure and sunburn are probably greater risk factors for melanoma than long-term moderate exposure.<ref name=NS>{{cite magazine |first=Richard |last=Weller |date=10 June 2015 |title=Shunning the sun may be killing you in more ways than you think |magazine=[[New Scientist]] |url=https://www.newscientist.com/article/mg22630250-500-shunning-the-sun-may-be-killing-you-in-more-ways-than-you-think |url-access=subscription |url-status=live |archive-url=https://web.archive.org/web/20170609062643/https://www.newscientist.com/article/mg22630250-500-shunning-the-sun-may-be-killing-you-in-more-ways-than-you-think/ |archive-date=9 June 2017}}</ref> UVC is the highest-energy, most-dangerous type of ultraviolet radiation, and causes adverse effects that can variously be mutagenic or carcinogenic.<ref>{{cite book |author=Hogan, C. Michael |orig-date=November 12, 2010 |date=May 25, 2012 |article=Sunlight |editor1=Saundry, P. |editor2=Cleveland, C. |title=Encyclopedia of Earth |article-url=http://www.eoearth.org/view/article/160592/ |archive-url=https://web.archive.org/web/20131019060416/http://www.eoearth.org/view/article/160592/ |archive-date=19 October 2013}}</ref>

In the past, UVA was considered not harmful or less harmful than UVB, but today it is known to contribute to skin cancer via [[indirect DNA damage]] (free radicals such as reactive oxygen species).<ref>{{Cite journal |last1=D'Orazio |first1=John |last2=Jarrett |first2=Stuart |last3=Amaro-Ortiz |first3=Alexandra |last4=Scott |first4=Timothy |date=2013-06-07 |title=UV Radiation and the Skin |journal=International Journal of Molecular Sciences |language=en |volume=14 |issue=6 |pages=12222–12248 |doi=10.3390/ijms140612222 |doi-access=free |issn=1422-0067 |pmc=3709783 |pmid=23749111}}</ref> UVA can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. The DNA damage caused indirectly to skin by UVA consists mostly of single-strand breaks in DNA, while the damage caused by UVB includes direct formation of [[thymine dimer]]s or [[cytosine dimer]]s and double-strand DNA breakage.<ref>{{cite journal | pmid = 22271212 | doi=10.1007/s00403-012-1212-x | title=DNA damage after acute exposure of mice skin to physiological doses of UVB and UVA light |date=January 2012 | journal=Arch. Dermatol. Res. |vauthors=Svobodová AR, Galandáková A, Sianská J |display-authors=etal | volume=304 | issue=5 | pages=407–412| s2cid=20554266 }}</ref> UVA is immunosuppressive for the entire body (accounting for a large part of the immunosuppressive effects of sunlight exposure), and is mutagenic for basal cell keratinocytes in skin.<ref>{{cite journal | pmid = 22123419 | doi=10.1016/j.sder.2011.08.002 | volume=30 | issue=4 | title=Ultraviolet&nbsp;A radiation: Its role in immunosuppression and carcinogenesis |date=December 2011 | journal=Semin. Cutan. Med. Surg. | pages=214–21 |vauthors=Halliday GM, Byrne SN, Damian DL | doi-broken-date=1 November 2024 }}</ref>

UVB photons can cause direct DNA damage. UVB radiation [[excites]] DNA molecules in skin cells, causing aberrant [[covalent bond]]s to form between adjacent [[pyrimidine]] bases, producing a [[pyrimidine dimers|dimer]]. Most UV-induced pyrimidine dimers in DNA are removed by the process known as [[nucleotide excision repair]] that employs about 30&nbsp;different proteins.<ref name="Bernstein-2002"/> Those pyrimidine dimers that escape this repair process can induce a form of programmed cell death ([[apoptosis]]) or can cause DNA replication errors leading to [[mutation]].{{cn|date=May 2024}}

UVB damages [[Messenger RNA|mRNA]]<ref>{{Cite journal |last1=Wurtmann |first1=Elisabeth J. |last2=Wolin |first2=Sandra L. |date=2009-02-01 |title=RNA under attack: Cellular handling of RNA damage |journal=Critical Reviews in Biochemistry and Molecular Biology |volume=44 |issue=1 |pages=34–49 |doi=10.1080/10409230802594043 |issn=1040-9238 |pmc=2656420 |pmid=19089684}}</ref> This triggers a fast pathway that leads to inflamination of the skin and sunburn. mRNA damage initially triggers a response in [[Ribosome|ribosomes]] though a protein known as [[ZAK|ZAK-alpha]] in a ribotoxic stress response. This response acts as a cell surveillance system. Following this detection of RNA damage leads to inflammatory signaling and recruitment of immune cells. This, not DNA damage (which is slower to detect) results in UVB skin inflammation and acute sunburn.<ref>{{Cite journal |last1=Vind |first1=Anna Constance |last2=Wu |first2=Zhenzhen |last3=Firdaus |first3=Muhammad Jasrie |last4=Snieckute |first4=Goda |last5=Toh |first5=Gee Ann |last6=Jessen |first6=Malin |last7=Martínez |first7=José Francisco |last8=Haahr |first8=Peter |last9=Andersen |first9=Thomas Levin |last10=Blasius |first10=Melanie |last11=Koh |first11=Li Fang |last12=Maartensson |first12=Nina Loeth |last13=Common |first13=John E.A. |last14=Gyrd-Hansen |first14=Mads |last15=Zhong |first15=Franklin L. |date=2024 |title=The ribotoxic stress response drives acute inflammation, cell death, and epidermal thickening in UV-irradiated skin in vivo |journal=Molecular Cell |language=en |volume=84 |issue=24 |pages=4774–4789.e9 |doi=10.1016/j.molcel.2024.10.044 |pmc=11671030 |pmid=39591967}}</ref>

As a defense against UV radiation, the amount of the brown pigment [[melanin]] in the skin increases when exposed to moderate (depending on [[human skin color|skin type]]) levels of radiation; this is commonly known as a [[sun tan]]. The purpose of melanin is to absorb UV radiation and dissipate the energy as harmless heat, protecting the skin against both [[direct DNA damage|direct]] and [[indirect DNA damage]] from the UV. UVA gives a quick tan that lasts for days by oxidizing melanin that was already present and triggers the release of the [[melanin]] from [[melanocyte]]s. UVB yields a tan that takes roughly 2&nbsp;days to develop because it stimulates the body to produce more melanin.{{cn|date=May 2024}}

====Sunscreen safety debate====
{{Main|Sunscreen}}

[[File:UV and Vis Sunscreen.jpg|thumb|left|Demonstration of the effect of sunscreen. The left image is a regular photograph of his face; the right image is of reflected UV light. The man's face has sunscreen on his right side only. It appears darker because the sunscreen absorbs the UV light.]]

Medical organizations recommend that patients protect themselves from UV radiation by using [[sunscreen]]. Five sunscreen ingredients have been shown to protect mice against skin tumors. However, [[Potential health risks of sunscreen|some sunscreen chemicals]] produce potentially harmful substances if they are illuminated while in contact with living cells.<ref name=Parsons>{{cite journal |author1=Xu, C. |author2=Green, Adele |author3=Parisi, Alfio |author4=Parsons, Peter G |year= 2001 |title= Photosensitization of the sunscreen octyl p‑dimethylaminobenzoate&nbsp;b UV‑A in human melanocytes but not in keratinocytes |journal= Photochemistry and Photobiology |volume= 73 |issue= 6 |pages=600–604 |doi=10.1562/0031-8655(2001)073<0600:POTSOP>2.0.CO;2 |pmid=11421064|s2cid=38706861 }}</ref><ref name=Knowland1993>{{cite journal |author1=Knowland, John |author2=McKenzie, Edward A. |author3=McHugh, Peter J. |author4=Cridland, Nigel A. |title= Sunlight-induced mutagenicity of a common sunscreen ingredient | journal= FEBS Letters |volume= 324 |pages=309–313 |year=1993 |pmid=8405372 |doi= 10.1016/0014-5793(93)80141-G | issue=3|bibcode=1993FEBSL.324..309K |s2cid=23853321 }}</ref> The amount of sunscreen that penetrates into the lower layers of the skin may be large enough to cause damage.<ref>{{cite journal | last1 = Chatelaine | first1 = E. | last2 = Gabard | first2 = B. | last3 = Surber | first3 = C. | year = 2003 | title = Skin penetration and sun protection factor of five UV filters: Effect of the vehicle | url = http://www.karger.com/Article/FullText/68291 | journal = Skin Pharmacol. Appl. Skin Physiol | volume = 16 | issue = 1 | pages = 28–35 | doi = 10.1159/000068291 | pmid = 12566826 | s2cid = 13458955 | access-date = 26 December 2013 | archive-date = 27 December 2013 | archive-url = https://web.archive.org/web/20131227063745/http://www.karger.com/Article/FullText/68291 | url-status = live | url-access = subscription }}</ref>

Sunscreen reduces the direct DNA damage that causes sunburn, by blocking UVB, and the usual [[Sun Protection Factor|SPF rating]] indicates how effectively this radiation is blocked. SPF is, therefore, also called UVB-PF, for "UVB protection factor".<ref>{{cite journal |pmid=21283919 |volume=10 |issue=2 |title=The impact of natural sunlight exposure on the UV‑B – sun protection factor (UVB-SPF) and UVA protection factor (UVA-PF) of a UV‑A / UV‑B SPF&nbsp;50 sunscreen |date=February 2011 |journal=J. Drugs Dermatol. |pages=150–155 |vauthors=Stephens TJ, Herndon JH, Colón LE, Gottschalk RW }}</ref> This rating, however, offers no data about important protection against UVA,<ref>{{cite journal |pmid=21669263 |doi=10.1016/j.ijpharm.2011.05.071 |volume=415 |issue=1–2 |title=Sunscreen products: what do they protect us from? |date=August 2011 |journal=Int. J. Pharm. |pages=181–184 |vauthors=Couteau C, Couteau O, Alami-El Boury S, Coiffard LJ }}</ref> which does not primarily cause sunburn but is still harmful, since it causes indirect DNA damage and is also considered carcinogenic. Several studies suggest that the absence of UVA filters may be the cause of the higher incidence of melanoma found in sunscreen users compared to non-users.<ref name=Garland>{{cite journal |vauthors=Garland C, Garland F, Gorham E |title=Could sunscreens increase melanoma risk? |journal=Am. J. Public Health |volume=82 |issue=4 |pages=614–615 |year=1992 |pmid=1546792 |doi=10.2105/AJPH.82.4.614 |pmc=1694089}}</ref><ref name=Westerdahl2000>{{cite journal |vauthors=Westerdahl J, Ingvar C, Masback A, Olsson H |title= Sunscreen use and malignant melanoma | journal= International Journal of Cancer |volume=87 |issue=1 |pages=145–150 |year=2000 |pmid=10861466 |doi=10.1002/1097-0215(20000701)87:1<145::AID-IJC22>3.0.CO;2-3 |doi-access= }}</ref><ref name=Autier>{{cite journal |vauthors=Autier P, Dore JF, Schifflers E |title=Melanoma and use of sunscreens: An EORTC case control study in Germany, Belgium and France |journal=Int. J. Cancer |volume=61 |issue= 6|pages=749–755 |year=1995| doi = 10.1002/ijc.2910610602 |pmid=7790106|s2cid=34941555 |display-authors=etal}}</ref><ref name="Weinstock">{{cite journal |author=Weinstock |first=M. A. |year=1999 |title=Do sunscreens increase or decrease melanoma risk: An epidemiologic evaluation |url=https://www.jidsponline.org/action/showPdf?pii=S1087-0024%2815%2930243-4 |url-status=live |journal=Journal of Investigative Dermatology Symposium Proceedings |volume=4 |issue=1 |pages=97–100 |doi=<!-- Deny Citation Bot--> |pmid=10537017 |archive-url=https://web.archive.org/web/20221205021844/https://www.jidsponline.org/action/showPdf?pii=S1087-0024(15)30243-4 |archive-date=5 December 2022 |access-date=5 December 2022}}</ref><ref name=Vainio>{{cite journal |author1=Vainio, H. |author2=Bianchini, F. |title=Commentary: Cancer-preventive effects of sunscreens are uncertain |journal= Scandinavian Journal of Work, Environment & Health |volume=26 |issue=6 |pages=529–531 |year=2000 |doi=10.5271/sjweh.578 |doi-access=free}}</ref> Some sunscreen lotions contain [[titanium dioxide]], [[zinc oxide]], and [[avobenzone]], which help protect against UVA rays.

The photochemical properties of melanin make it an excellent [[photoprotection|photoprotectant]]. However, sunscreen chemicals cannot dissipate the energy of the excited state as efficiently as melanin and therefore, if sunscreen ingredients penetrate into the lower layers of the skin, the amount of [[reactive oxygen species]] may be increased.<ref name="Hanson">{{cite journal |author1=Hanson, Kerry M. |author2=Gratton, Enrico |author3=Bardeen, Christopher J. |title=Sunscreen enhancement of UV-induced reactive oxygen species in the skin |doi=10.1016/j.freeradbiomed.2006.06.011 |journal=Free Radical Biology and Medicine |volume=41 |issue=8 |pages=1205–1212 |year=2006 |pmid=17015167 |s2cid=13999532 |url=https://escholarship.org/content/qt9f14s2dd/qt9f14s2dd.pdf?t=oe9hj9 |access-date=6 September 2018 |archive-date=14 March 2020 |archive-url=https://web.archive.org/web/20200314065450/https://escholarship.org/content/qt9f14s2dd/qt9f14s2dd.pdf?t=oe9hj9 |url-status=live }}</ref><ref name=Parsons/><ref name=Knowland1993 /><ref name=Damiani1999>{{cite journal |author1=Damiani, E. |author2=Greci, L. |author3=Parsons, R. |author4=Knowland, J. |title=Nitroxide radicals protect DNA from damage when illuminated in vitro in the presence of dibenzoylmethane and a common sunscreen ingredient |journal= Free Radic. Biol. Med. |volume=26 |issue=7–8 |pages=809–816 |year=1999 |doi=10.1016/S0891-5849(98)00292-5 |pmid=10232823}}</ref> The amount of sunscreen that penetrates through the [[stratum corneum]] may or may not be large enough to cause damage.

In an experiment by Hanson ''et al''. that was published in 2006, the amount of harmful [[reactive oxygen species]] (ROS) was measured in untreated and in sunscreen treated skin. In the first 20&nbsp;minutes, the film of sunscreen had a protective effect and the number of ROS species was smaller. After 60&nbsp;minutes, however, the amount of absorbed sunscreen was so high that the amount of ROS was higher in the sunscreen-treated skin than in the untreated skin.<ref name="Hanson"/> The study indicates that sunscreen must be reapplied within 2&nbsp;hours in order to prevent UV light from penetrating to sunscreen-infused live skin cells.<ref name="Hanson"/>

==== Aggravation of certain skin conditions ====
Ultraviolet radiation can aggravate several skin conditions and diseases, including<ref name=euroderm>{{cite report |title=§2 Photoaggravated disorders |series=European guidelines for photodermatoses |website=European Dermatology Forum |url=http://www.euroderm.org/images/stories/guidelines/guideline_Photoaggravated_dermatoses.pdf |access-date=1 January 2016 }}{{Dead link|date=October 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> [[systemic lupus erythematosus]], [[Sjögren's syndrome]], [[Sinear Usher syndrome]], [[rosacea]], [[dermatomyositis]], [[Darier's disease]], [[Kindler–Weary syndrome]] and [[Porokeratosis]].<ref name=Medscape>[https://emedicine.medscape.com/article/1059123-overview#a4 Medscape: Porokeratosis] {{Webarchive|url=https://web.archive.org/web/20210624201419/https://emedicine.medscape.com/article/1059123-overview#a4 |date=24 June 2021 }}.</ref>

====Eye damage====
[[File:UV Warning.jpg|thumb|Signs are often used to warn of the hazard of strong UV sources.]]
The eye is most sensitive to damage by UV in the lower UVC band at 265–275&nbsp;nm. Radiation of this wavelength is almost absent from sunlight at the surface of the Earth but is emitted by artificial sources such as the [[electrical arcs]] employed in [[arc welding]]. Unprotected exposure to these sources can cause "welder's flash" or "arc eye" ([[photokeratitis]]) and can lead to [[cataract]]s, [[pterygium]] and [[pinguecula]] formation. To a lesser extent, UVB in sunlight from 310 to 280&nbsp;nm also causes photokeratitis ("snow blindness"), and the [[cornea]], the [[Lens (anatomy)|lens]], and the [[retina]] can be damaged.<ref>{{cite report |url=https://www.who.int/uv/faq/uvhealtfac/en/index3.html |title=The known health effects of UV |publisher=World Health Organization |url-status=live |archive-url=https://web.archive.org/web/20161024170159/http://www.who.int/uv/faq/uvhealtfac/en/index3.html |archive-date=24 October 2016}}</ref>

[[Protective eyewear]] is beneficial to those exposed to ultraviolet radiation. Since light can reach the eyes from the sides, full-coverage eye protection is usually warranted if there is an increased risk of exposure, as in high-altitude mountaineering. Mountaineers are exposed to higher-than-ordinary levels of UV radiation, both because there is less atmospheric filtering and because of reflection from snow and ice.<ref>{{cite web |url=https://www.who.int/uv/faq/whatisuv/en/index3.html |title=UV radiation |publisher=World Health Organization |url-status=live |archive-url=https://web.archive.org/web/20161025234856/http://www.who.int/uv/faq/whatisuv/en/index3.html |archive-date=25 October 2016}}</ref><ref>{{cite report |title=What is UV radiation and how much does it increase with altitude? |publisher=U.S. [[National Oceanographic and Atmospheric Administration]] |url=http://www.wrh.noaa.gov/fgz/science/uv.php?wfo=fgz |url-status=live |archive-url=https://web.archive.org/web/20170103102305/http://www.wrh.noaa.gov/fgz/science/uv.php?wfo=fgz |archive-date=3 January 2017}}</ref>
Ordinary, untreated [[eyeglasses]] give some protection. Most plastic lenses give more protection than glass lenses, because, as noted above, glass is transparent to UVA and the common acrylic plastic used for lenses is less so. Some plastic lens materials, such as [[polycarbonate]], inherently block most UV.<ref>{{cite web |url=https://www.opticianonline.net/features/optical-properties-of-lens-materials-2 |title=Optical properties of lens materials |website=Optician Online |date=6 June 2005 |url-access=subscription |url-status=live |archive-url=https://web.archive.org/web/20161026000953/https://www.opticianonline.net/features/optical-properties-of-lens-materials-2 |archive-date=26 October 2016}}</ref>