Editing Ultraviolet (section)

==== 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}}