Editing Sunlight (section)

==Effects on human health==
{{Main|Health effects of sunlight exposure}}
The [[ultraviolet radiation]] in sunlight has both positive and negative health effects, as it is both a principal source of [[vitamin D3|vitamin D<sub>3</sub>]] and a [[mutagen]].<ref>{{cite journal | pmid = 12174089 | volume=147 | issue=2 | title=Vitamin D and systemic cancer: is this relevant to malignant melanoma? |date=August 2002 | journal=Br. J. Dermatol. | pages=197–213 | author=Osborne JE | author2=Hutchinson PE | doi = 10.1046/j.1365-2133.2002.04960.x| s2cid=34388656 }}</ref> A dietary supplement can supply [[vitamin D]] without this mutagenic effect,<ref name="ODS-vitamin-D-sheet">{{cite web|url=http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp|title=Dietary Supplement Fact Sheet: Vitamin D|publisher=Office of Dietary Supplements, National Institutes of Health|url-status=live|archive-url=https://web.archive.org/web/20070716065832/http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp|archive-date=2007-07-16}}</ref> but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Vitamin D has a wide range of positive health effects, which include strengthening bones<ref>{{cite journal | display-authors = 4| author = Cranney A| author2 = Horsley T| author3 = O'Donnell S| author4 = Weiler H| author5 = Puil L| author6 =  Ooi D| author7 = Atkinson S| author8 = Ward L| author9 = Moher D| author10 = Hanley D| author11 = Fang M| author12 = Yazdi F| author13 = Garritty C| author14 = Sampson M| author15 = Barrowman N| author16 = Tsertsvadze A| author17 = Mamaladze V | title = Effectiveness and safety of vitamin D in relation to bone health | journal = Evidence Report/Technology Assessment | issue = 158 | pages = 1–235 | date = August 2007 | pmid = 18088161 | pmc=4781354}}</ref> and possibly inhibiting the growth of some cancers.<ref>{{cite journal | display-authors = 4| author = John E| author2 = Schwartz G| author3 = Koo J| author4 = Van Den Berg D| author5 = Ingles S | title = Sun Exposure, Vitamin D Receptor Gene Polymorphisms, and Risk of Advanced Prostate Cancer | journal = Cancer Research | volume = 65 | issue = 12 | pages = 5470–5479 | date = June 15, 2005 | doi=10.1158/0008-5472.can-04-3134 | pmid=15958597| doi-access = free }}</ref><ref>{{cite journal | author = Egan K | author2 = Sosman J | author3 = Blot W | title = Sunlight and Reduced Risk of Cancer: Is The Real Story Vitamin D? | journal = J Natl Cancer Inst | volume = 97 | issue = 3 | pages = 161–163 | date = February 2, 2005 | doi = 10.1093/jnci/dji047 | pmid = 15687354 | doi-access = free }}</ref> Sun exposure has also been associated with the timing of [[melatonin]] synthesis, maintenance of normal [[circadian rhythm]]s, and reduced risk of [[seasonal affective disorder]].<ref>{{cite journal |author=Mead MN |title=Benefits of sunlight: a bright spot for human health |journal=Environmental Health Perspectives |volume=116 |issue=4 |pages=A160–A167 |date=April 2008 |pmid=18414615 |pmc=2290997 |doi=10.1289/ehp.116-a160}}</ref>

Long-term sunlight exposure is known to be associated with the development of [[skin cancer]], [[photoaging|skin aging]], [[immune suppression]], and eye diseases such as [[cataracts]] and [[macular degeneration]].<ref>{{cite journal |author=Lucas RM|author2=Repacholi MH|author3=McMichael AJ |title=Is the current public health message on UV exposure correct? |journal=Bulletin of the World Health Organization |volume=84 |issue=6 |pages=485–491 |date=June 2006 |pmid=16799733 |pmc=2627377 |doi=10.2471/BLT.05.026559}}</ref> Short-term overexposure is the cause of [[sunburn]], [[snow blindness]], and [[solar retinopathy]].

UV rays, and therefore sunlight and sunlamps, are the only listed [[carcinogen]]s that are known to have health benefits,<ref name="13th Report on Carcinogens">{{cite web |url=http://ntp.niehs.nih.gov/ntp/roc/content/profiles/ultravioletradiationrelatedexposures.pdf |title=13th Report on Carcinogens: Ultraviolet-Radiation-Related Exposures |publisher=National Toxicology Program |date=October 2014 |access-date=2014-12-22 |url-status=dead |archive-url=https://web.archive.org/web/20141222172504/http://ntp.niehs.nih.gov/ntp/roc/content/profiles/ultravioletradiationrelatedexposures.pdf |archive-date=2014-12-22 }}</ref> and a number of public health organizations state that there needs to be a balance between the risks of having too much sunlight or too little.<ref>{{cite web |url=http://www.cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf |title=Risks and Benefits |access-date=2010-05-13 |url-status=live |archive-url=https://web.archive.org/web/20101120224439/http://cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf |archive-date=2010-11-20 }}</ref> There is a general consensus that sunburn should always be avoided.

Epidemiological data shows that people who have more exposure to sunlight have less high blood pressure and cardiovascular-related mortality. While sunlight (and its UV rays) are a risk factor for skin cancer, "sun avoidance may carry more of a cost than benefit for over-all good health".<ref name="wellersunlight">{{cite journal|last1=Weller|first1=RB|title=Sunlight Has Cardiovascular Benefits Independently of Vitamin D.|journal=Blood Purification|date=2016|volume=41|issue=1–3|pages=130–4|doi=10.1159/000441266|pmid=26766556|hdl=20.500.11820/8f7d93d4-db22-418d-a1cc-3dbf9ddad8c3|s2cid=19348056|hdl-access=free}}</ref> A study found that there is no evidence that UV reduces lifespan in contrast to other risk factors like smoking, alcohol and high blood pressure.<ref name="wellersunlight" />