Health effects of sunlight exposure

File:XrayRicketsLegssmall.jpg, usually caused by insufficient vitamin D]]

{{Main|Vitamin D}} UVB radiation with a wavelength of 290–315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D₃, which in turn becomes vitamin D₃.{{cite journal | vauthors = Hayes CE, Nashold FE, Spach KM, Pedersen LB | title = The immunological functions of the vitamin D endocrine system | journal = Cellular and Molecular Biology | volume = 49 | issue = 2 | pages = 277–300 | date = March 2003 | pmid = 12887108 }}{{cite journal | vauthors = Holick MF | title = McCollum Award Lecture, 1994: vitamin D--new horizons for the 21st century | journal = The American Journal of Clinical Nutrition | volume = 60 | issue = 4 | pages = 619–630 | date = October 1994 | pmid = 8092101 | doi = 10.1093/ajcn/60.4.619 | url = http://www.ajcn.org/cgi/pmidlookup?view=long&pmid=8092101 | access-date = January 13, 2010 | url-status = live | archive-url = https://web.archive.org/web/20200315174831/https://academic.oup.com/ajcn | archive-date = March 15, 2020 }}{{cite journal | vauthors = Holick MF |date=February 2002 |title=Vitamin D: the underappreciated D-lightful hormone that is important for skeletal and cellular health |journal=Current Opinion in Endocrinology, Diabetes and Obesity |volume=9 |issue=1 |pages=87–98 |doi=10.1097/00060793-200202000-00011|s2cid=87725403 }} UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D.{{cite book |author=Holick MF |chapter=Photobiology of vitamin D |editor1=Feldman, David Henry |editor2=Glorieux, Francis H. |title=Vitamin D |publisher=Elsevier Academic Press |location=Amsterdam |year=2005 |isbn=978-0-12-252687-9}} Time of day, time of year, geographic latitude, ground altitude, cloud cover, smog, skin melanin content, and sunscreen are among the factors that greatly affect UV intensity and vitamin D synthesis, making it difficult to provide general guidelines. It has been suggested by some researchers, for example, that adequate amounts of vitamin D can be produced with moderate sun exposure to the face, arms and legs, averaging 5–30 minutes twice per week without sunscreen. (The darker the complexion, or the weaker the sunlight, the more minutes of exposure are needed, approximating 25% of the time for minimal sunburn. Vitamin D overdose is impossible from UV exposure; the skin reaches an equilibrium where the vitamin degrades as fast as it is created.){{cite journal | vauthors = Holick MF | title = Sunlight and vitamin D: both good for cardiovascular health | journal = Journal of General Internal Medicine | volume = 17 | issue = 9 | pages = 733–735 | date = September 2002 | pmid = 12220371 | pmc = 1495109 | doi = 10.1046/j.1525-1497.2002.20731.x }}{{cite journal | vauthors = Holick MF | title = Vitamin D deficiency | journal = The New England Journal of Medicine | volume = 357 | issue = 3 | pages = 266–281 | date = July 2007 | pmid = 17634462 | doi = 10.1056/NEJMra070553 | s2cid = 18566028 }} Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.

The only way to quantify adequate levels of vitamin D is with a serum 25(OH)D₃ (calcifediol) test.{{cite web |url=https://www.nlm.nih.gov/medlineplus/ency/article/003569.htm |title=25-hydroxy vitamin D test |website=MedlinePlus | vauthors = Topiwala S |publisher=US National Institutes of Health |date=July 19, 2012 |access-date=March 25, 2015 |archive-date=July 5, 2016 |archive-url= https://web.archive.org/web/20160705125938/https://www.nlm.nih.gov/medlineplus/ency/article/003569.htm |url-status=live }} In the United States, serum 25(OH)D₃ was below the recommended level for more than a third of white men in a 2005 study, with serum levels even lower in women and in most minorities. This indicates that vitamin D deficiency may be a common problem in the US.{{cite journal | vauthors = Zadshir A, Tareen N, Pan D, Norris K, Martins D | title = The prevalence of hypovitaminosis D among US adults: data from the NHANES III | journal = Ethnicity & Disease | volume = 15 | issue = 4 Suppl 5 | pages = S5–97–S5–101 | year = 2005 | pmid = 16315387 }} Australia and New Zealand have had similar findings, which indicate insufficient protection against rickets for children and osteoporosis for adults.{{cite journal | vauthors = Nowson CA, Margerison C | title = Vitamin D intake and vitamin D status of Australians | journal = The Medical Journal of Australia | volume = 177 | issue = 3 | pages = 149–152 | date = August 2002 | pmid = 12149085 | doi = 10.5694/j.1326-5377.2002.tb04702.x | url = http://www.mja.com.au/public/issues/177_03_050802/now10763_fm.html | access-date = December 23, 2014 | url-status = live | s2cid = 20278782 | archive-url = https://web.archive.org/web/20120301154857/http://www.mja.com.au/public/issues/177_03_050802/now10763_fm.html | archive-date = March 1, 2012 }}

Over the past several years, levels of ultraviolet radiation have been tracked at over 30 sites across North America as part of the United States Department of Agriculture's UVB Monitoring and Research Program at Colorado State University. The first map at right shows levels of UVB radiation in June 2008, expressed in Vitamin D Equivalents.{{cite web |url=http://uvb.nrel.colostate.edu/UVB/index.jsf |title=UV-B Monitoring and Research Program |publisher=Colorado State University |access-date=May 13, 2010 |archive-date=January 29, 2019 |archive-url=https://web.archive.org/web/20190129170605/http://uvb.nrel.colostate.edu/UVB/index.jsf |url-status=dead }}

File:All20080601-1206872731 con.png

Using satellite data, measurements from the European Space Agency produce similar maps expressed in units of the widely followed UV Index, for locations around the world.{{cite web |url=http://temis.nl/uvradiation/GOME/uvclim.php?fb=uviec&Clim=17 |title=UV index & UV dose based on GOME |publisher=KNMI/TEMIS |access-date=March 17, 2015 |archive-date=April 2, 2015 |archive-url=https://web.archive.org/web/20150402120126/http://temis.nl/uvradiation/GOME/uvclim.php?fb=uviec&Clim=17 |url-status=live }} Effects of UV-radiation at high latitudes, where snow stays on the ground into early summer and the sun then remains at a low position even at its zenith, have been reviewed by Meyer-Rochow.{{cite journal | vauthors = Meyer-Rochow VB | 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 | date = January 2000 | pmid = 10850006 }}

File:GOME.uviecclimyear lr.gif)]]

Exposure to ultraviolet radiation from the sun is a source of vitamin D. One minimal erythemal dose of sunlight UV radiation provides the equivalent of about 20,000 IU of vitamin D2, taken as an oral supplement.{{citation needed|date=January 2018}} If an adult's arms and legs are exposed to a half minimal erythemal UV radiation, it is the same as taking 3,000 IU of vitamin D₃ through an oral supplement. This exposure of 10–15 minutes, on a frequency of two to three times per week, will cause the adult's skin to produce enough vitamin D. It is not necessary to expose the face to the UV, as facial skin provides little vitamin D₃. Individuals whose metabolism makes taking oral vitamin D ineffective are able, through exposure to an ultraviolet lamp that emits UV-B radiation, to achieve a 25 (OH) D blood level.{{cite journal | vauthors = Hossein-nezhad A, Holick MF | title = Vitamin D for health: a global perspective | journal = Mayo Clinic Proceedings | volume = 88 | issue = 7 | pages = 720–755 | date = July 2013 | pmid = 23790560 | pmc = 3761874 | doi = 10.1016/j.mayocp.2013.05.011 }}

Three benefits of UV exposure are production of vitamin D, improvement in mood, and increased energy.{{cite journal | vauthors = Sivamani RK, Crane LA, Dellavalle RP | title = The benefits and risks of ultraviolet tanning and its alternatives: the role of prudent sun exposure | journal = Dermatologic Clinics | volume = 27 | issue = 2 | pages = 149–54, vi | date = April 2009 | pmid = 19254658 | pmc = 2692214 | doi = 10.1016/j.det.2008.11.008 }}

UVB induces production of vitamin D in the skin at rates of up to 1,000 IUs per minute. This vitamin helps to regulate calcium metabolism (vital for the nervous system and bone health), immunity, cell proliferation, insulin secretion, and blood pressure.{{cite web |url=http://lpi.oregonstate.edu/infocenter/vitamins/vitaminD/ |title=Vitamin D |publisher=Oregon State University |access-date=November 8, 2011 |url-status=live |archive-url=https://web.archive.org/web/20111026135239/http://lpi.oregonstate.edu/infocenter/vitamins/vitaminD/ |archive-date=October 26, 2011 }} In low and middle income countries, foods fortified with vitamin D are "practically nonexistent." Most people in the world depend on the sun to get vitamin D,{{cite journal | vauthors = Shah D, Gupta P | title = Vitamin D Deficiency: Is The Pandemic for Real? | journal = Indian Journal of Community Medicine | volume = 40 | issue = 4 | pages = 215–217 | date = 2015 | pmid = 26435592 | pmc = 4581139 | doi = 10.4103/0970-0218.164378 | doi-access = free }} and elderly populations in low UVB countries experience higher rates of cancer.{{cite journal | vauthors = Purushothaman VL, Cuomo RE, Garland CF, Mackey TK | title = Could age increase the strength of inverse association between ultraviolet B exposure and colorectal cancer? | journal = BMC Public Health | volume = 21 | issue = 1 | pages = 1238 | date = July 2021 | pmid = 34218809 | pmc = 8256562 | doi = 10.1186/s12889-021-11089-w | doi-access = free }}

There are not many foods that naturally have vitamin D.{{cite journal | vauthors = Holick MF | title = Vitamin D deficiency | journal = The New England Journal of Medicine | volume = 357 | issue = 3 | pages = 266–281 | date = July 2007 | pmid = 17634462 | doi = 10.1056/NEJMra070553 | s2cid = 18566028 }} Examples are cod liver oil and oily fish. If people cannot get sunlight, then they will need 1,000 IU of vitamin D per day to stay healthy.{{cite journal | vauthors = Holick MF | title = Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis | journal = The American Journal of Clinical Nutrition | volume = 79 | issue = 3 | pages = 362–371 | date = March 2004 | pmid = 14985208 | doi = 10.1093/ajcn/79.3.362 | doi-access = free }} A person would have to eat oily fish three or four times per week in order to get enough vitamin D from that food source alone.

People with higher levels of vitamin D tend to have lower rates of diabetes, heart disease, and stroke and tend to have lower blood pressure. However, it has been found that vitamin D supplementation does not improve cardiovascular health or metabolism, so the link with vitamin D must be in part indirect.{{citation needed|date=January 2018}} People who get more sun are generally healthier, and also have higher vitamin D levels. It has been found that ultraviolet radiation (even UVA) produces nitric oxide (NO) in the skin, and nitric oxide can lower blood pressure. High blood pressure increases the risk of stroke and heart disease. Although long-term exposure to ultraviolet contributes to non-melanoma skin cancers that are rarely fatal, it has been found in a Danish study that those who get these cancers were less likely to die during the study, and were much less likely to have a heart attack, than those who did not have these cancers.{{cite journal|author1=Richard Weller|title=Shunning the sun may be killing you in more ways than you think|journal=New Scientist|date=June 10, 2015|url=https://www.newscientist.com/article/mg22630250-500-shunning-the-sun-may-be-killing-you-in-more-ways-than-you-think|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=June 9, 2017}}

People in certain situations, such as people with intellectual disabilities and neurodevelopmental disorders who stay inside most of the time have low vitamin D levels. Getting enough vitamin D can help stave off "autoimmune diseases, cardiovascular disease, many types of cancer, dementia, types 1 and 2 diabetes mellitus, and respiratory tract infections."{{cite journal | vauthors = Grant WB, Wimalawansa SJ, Holick MF, Cannell JJ, Pludowski P, Lappe JM, Pittaway M, May P | display-authors = 6 | title = Emphasizing the health benefits of vitamin D for those with neurodevelopmental disorders and intellectual disabilities | journal = Nutrients | volume = 7 | issue = 3 | pages = 1538–1564 | date = February 2015 | pmid = 25734565 | pmc = 4377865 | doi = 10.3390/nu7031538 | doi-access = free }}

Fetuses and children who do not get enough vitamin D can result in "growth retardation and skeletal deformities."

Multiple sclerosis (MS) is least prevalent in the sunniest regions.{{cite journal | vauthors = Ascherio A, Munger KL | title = Environmental risk factors for multiple sclerosis. Part II: Noninfectious factors | journal = Annals of Neurology | volume = 61 | issue = 6 | pages = 504–513 | date = June 2007 | pmid = 17492755 | doi = 10.1002/ana.21141 | s2cid = 36999504 | doi-access = free }}{{cite journal | vauthors = Milo R, Kahana E | title = Multiple sclerosis: geoepidemiology, genetics and the environment | journal = Autoimmunity Reviews | volume = 9 | issue = 5 | pages = A387–A394 | date = March 2010 | pmid = 19932200 | doi = 10.1016/j.autrev.2009.11.010 }}{{cite journal | vauthors = Ascherio A, Munger KL, Simon KC | title = Vitamin D and multiple sclerosis | journal = The Lancet. Neurology | volume = 9 | issue = 6 | pages = 599–612 | date = June 2010 | pmid = 20494325 | doi = 10.1016/S1474-4422(10)70086-7 | s2cid = 12802790 }} Exposure to the ultraviolet-B radiation of sunlight appears to be most important and this may operate via vitamin D synthesis.{{cite journal | vauthors = Koch MW, Metz LM, Agrawal SM, Yong VW | title = Environmental factors and their regulation of immunity in multiple sclerosis | journal = Journal of the Neurological Sciences | volume = 324 | issue = 1–2 | pages = 10–16 | date = January 2013 | pmid = 23154080 | pmc = 7127277 | doi = 10.1016/j.jns.2012.10.021 }}

File:Melanoma.jpg

File:Sun burn.JPG

Ultraviolet (UV) irradiation present in sunlight is an environmental human carcinogen. The toxic effects of UV from natural sunlight and therapeutic artificial lamps are a major concern for human health. Skin surface lipids, including unsaturated lipids such as squalene, sebaleic acid, linoleic acid, and cholesterol can be a subject of oxidation by singlet oxygen and ozone as well as free radicals. Ultraviolet radiation activates lipoxygenase and cyclooxygenase, inducing specific enzymatic oxidation of lipids. Free radical mediated lipid peroxidation gives multiple oxidation products which may induce various skin diseases{{cite journal | vauthors = Niki E | title = Lipid oxidation in the skin | journal = Free Radical Research | volume = 49 | issue = 7 | pages = 827–834 | date = August 2014 | pmid = 25312699| doi = 10.3109/10715762.2014.976213 | s2cid = 19975554 }}

UVB damages mRNA{{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}} This triggers a fast pathway that leads to inflamination of the skin and sunburn. mRNA damage initially triggers a response in ribosomes though a protein known as 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.{{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}}

The major acute effects of UV irradiation on normal human skin comprise sunburn inflammation erythema, tanning, and local or systemic immunosuppression.{{cite journal | vauthors = Matsumura Y, Ananthaswamy HN | title = Toxic effects of ultraviolet radiation on the skin | journal = Toxicology and Applied Pharmacology | volume = 195 | issue = 3 | pages = 298–308 | date = March 2004 | pmid = 15020192 | doi = 10.1016/j.taap.2003.08.019 | bibcode = 2004ToxAP.195..298M }} The most deadly form, malignant melanoma, is mostly caused by indirect DNA damage from UVA radiation.{{Dubious|date=August 2024|reason=The source doesn't seem to specify UVA, and most other resources I can find say that UVB is most associated with melanoma}} This can be seen from the absence of a direct UV signature mutation in 92% of all melanoma.{{cite journal | vauthors = Davies H, Bignell GR, Cox C, Stephens P, Edkins S, Clegg S, Teague J, Woffendin H, Garnett MJ, Bottomley W, Davis N, Dicks E, Ewing R, Floyd Y, Gray K, Hall S, Hawes R, Hughes J, Kosmidou V, Menzies A, Mould C, Parker A, Stevens C, Watt S, Hooper S, Wilson R, Jayatilake H, Gusterson BA, Cooper C, Shipley J, Hargrave D, Pritchard-Jones K, Maitland N, Chenevix-Trench G, Riggins GJ, Bigner DD, Palmieri G, Cossu A, Flanagan A, Nicholson A, Ho JW, Leung SY, Yuen ST, Weber BL, Seigler HF, Darrow TL, Paterson H, Marais R, Marshall CJ, Wooster R, Stratton MR, Futreal PA | display-authors = 6 | title = Mutations of the BRAF gene in human cancer | journal = Nature | volume = 417 | issue = 6892 | pages = 949–954 | date = June 2002 | pmid = 12068308 | doi = 10.1038/nature00766 | url = http://eprints.gla.ac.uk/121/1/Davis%2CH_2002_pdf.pdf | access-date = September 28, 2019 | url-status = live | s2cid = 3071547 | bibcode = 2002Natur.417..949D | archive-url = https://web.archive.org/web/20200805053025/http://eprints.gla.ac.uk/121/1/Davis%2CH_2002_pdf.pdf | archive-date = August 5, 2020 }}{{Not in citation|date=August 2024}} UVC is the highest-energy, most-dangerous type of ultraviolet radiation, and causes adverse effects that can variously be mutagenic or carcinogenic.{{cite encyclopedia | vauthors = Hogan CM | date = 2011 | url = http://www.eoearth.org/view/article/160592/ | title = Sunlight | veditors = Saundry P, Cleveland C | encyclopedia = Encyclopedia of Earth | archive-url = https://web.archive.org/web/20131019060416/http://www.eoearth.org/view/article/160592/ | archive-date = October 19, 2013 }}

Despite the importance of the sun to vitamin D synthesis, it is prudent to limit the exposure of skin to UV radiation from sunlight{{cite journal | vauthors = Wolpowitz D, Gilchrest BA | title = The vitamin D questions: how much do you need and how should you get it? | journal = Journal of the American Academy of Dermatology | volume = 54 | issue = 2 | pages = 301–317 | date = February 2006 | pmid = 16443061 | doi = 10.1016/j.jaad.2005.11.1057 }} and from tanning beds.{{cite journal | vauthors = | title = The association of use of sunbeds with cutaneous malignant melanoma and other skin cancers: A systematic review | journal = International Journal of Cancer | volume = 120 | issue = 5 | pages = 1116–1122 | date = March 2007 | pmid = 17131335 | doi = 10.1002/ijc.22453 | doi-access = free }} According to the National Toxicology Program Report on Carcinogens from the US Department of Health and Human Services, broad-spectrum UV radiation is a carcinogen whose DNA damage is thought to contribute to most of the estimated 1.5 million skin cancers and the 8,000 deaths due to metastatic melanoma that occur annually in the United States.{{cite web|url=https://ntp.niehs.nih.gov/pubhealth/roc/listings/index.html?substance=Ultraviolet+Radiation|archive-url=https://web.archive.org/web/20100528035215/http://ntp.niehs.nih.gov/index.cfm?objectid=BD4CD88D-F1F6-975E-792094AC1CE4B062 |archive-date=May 28, 2010 |title=Ultraviolet (UV) Radiation, Broad Spectrum and UVA, UVB, and UVC |publisher=National Toxicology Program |date=January 5, 2009 |access-date=May 13, 2010}} The use of sunbeds is reported by the World Health Organization to be responsible for over 450,000 cases of non-melanoma skin cancer and over 10,000 cases of melanoma every year in the U.S., Europe, as well as Australia.{{Cite web|url=https://www.who.int/uv/en/|title=More can be done to restrict sunbeds to prevent increasing rates of skin cancer|website=World Health Organization|access-date=May 5, 2018|archive-date=April 30, 2018|archive-url=https://web.archive.org/web/20180430044436/http://www.who.int/uv/en/|url-status=live}} Lifetime cumulative UV exposure to skin is also responsible for significant age-associated dryness, wrinkling, elastin and collagen damage, freckling, IGH, age spots and other cosmetic changes. The American Academy of Dermatology advises that photoprotective measures be taken, including the use of sunscreen, whenever one is exposed to the sun.{{cite web | work = American Academy of Dermatology. | url = http://www.aad.org/Forms/Policies/Uploads/PS/PS-Vitamin%20D.pdf | title = Position statement on vitamin D. | archive-url = https://web.archive.org/web/20180114161930/https://www.aad.org/forms/policies/uploads/ps/ps-vitamin%20d.pdf | archive-date = January 14, 2018 | date = November 1, 2008 }} Short-term over-exposure causes the pain and itching of sunburn, which in extreme cases can produce more-severe effects like blistering.

Several countries (such as Australia) provide public forecasts of UV irradiation in the form of the UV Index. The index can be used as a guide to the public of dangers from over-exposure to sunlight, especially around noon, when direct sunlight is at its most intense.

Prolonged optical exposure to sunlight, especially intense ultraviolet light, may be linked to cortical cataracts,{{cite journal | vauthors = West SK, Duncan DD, Muñoz B, Rubin GS, Fried LP, Bandeen-Roche K, Schein OD | title = Sunlight exposure and risk of lens opacities in a population-based study: the Salisbury Eye Evaluation project | journal = JAMA | volume = 280 | issue = 8 | pages = 714–718 | date = August 1998 | pmid = 9728643 | doi = 10.1001/jama.280.8.714 | s2cid = 24926534 | doi-access = }} and high levels of visible light is maybe linked to macular degeneration.

However, significant daily exposure to bright light may be necessary for children to avoid myopia (nearsightedness).{{cite journal | vauthors = Dolgin E | title = The myopia boom | journal = Nature | volume = 519 | issue = 7543 | pages = 276–278 | date = March 2015 | pmid = 25788077 | doi = 10.1038/519276a | doi-access = free | bibcode = 2015Natur.519..276D }}

Short-term over-exposure can cause snow blindness, which is analogous to sunburn of the cornea, or can cause solar retinopathy, which is long-lasting retinal damage and vision impairment from sungazing.{{cite journal | vauthors = Chen JC, Lee LR | title = Solar retinopathy and associated optical coherence tomography findings | journal = Clinical & Experimental Optometry | volume = 87 | issue = 6 | pages = 390–393 | date = November 2004 | pmid = 15575813 | doi = 10.1111/j.1444-0938.2004.tb03100.x | doi-access = free }}{{cite journal | vauthors = Källmark FP, Ygge J | title = Photo-induced foveal injury after viewing a solar eclipse | journal = Acta Ophthalmologica Scandinavica | volume = 83 | issue = 5 | pages = 586–589 | date = October 2005 | pmid = 16187997 | doi = 10.1111/j.1600-0420.2005.00511.x | doi-access = free }}

Frequent exposure to the sun can cause yellow non-cancerous bumps on the middle part of the sclera of the eye, called pingueculae. It is most common in younger people, mainly those who spend a lot of their time outdoors and do not protect their eyes from UV rays. To decrease the risk of developing pingueculae, it may be wise to wear sunglasses when outdoors, even on overcast days.{{cite web| vauthors = Lusby F, Zieve D, Ogilvie I |title=Pinguecula |url= https://medlineplus.gov/ency/article/001025.htm |website=MedicinePlus |publisher=U.S. National Library of Medicine|access-date=October 28, 2016}}

Light to the eyes, primarily blue-wavelength light, is important for the entrainment and maintenance of robust circadian rhythms. Exposure to sunlight in the morning is particularly effective; it leads to earlier melatonin onset in the evening and makes it easier to fall asleep. Natural daylight exposure, particularly at high intensities, has been shown to have numerous beneficial effects on sleep patterns. It can advance sleep timing (leading to earlier bedtimes), affect sleep duration, and improve overall sleep quality.{{Cite journal |last1=Blume |first1=Christine |last2=Garbazza |first2=Corrado |last3=Spitschan |first3=Manuel |date=20 August 2019 |title=Effects of light on human circadian rhythms, sleep and mood |journal=Somnologie|volume=23 |issue=3 |pages=147–156 |doi=10.1007/s11818-019-00215-x |pmid=31534436 |pmc=6751071 }} Bright morning light has been shown to be effective against insomnia, premenstrual syndrome and seasonal affective disorder (SAD).{{Cite journal |last=Mead |first=M Nathaniel |date=May 2008 |title=Benefits of Sunlight: A Bright Spot for Human Health |journal=Environmental Health Perspectives |volume=116 |issue=4 |pages=A160–A167 |doi=10.1289/ehp.116-a160 |pmid=18414615 |pmc=2290997 }}

Blood levels of folate, a nutrient vital for fetal development, can be degraded by UV radiation,{{cite journal | vauthors = Borradale D, Isenring E, Hacker E, Kimlin MG | title = Exposure to solar ultraviolet radiation is associated with a decreased folate status in women of childbearing age | journal = Journal of Photochemistry and Photobiology. B, Biology | volume = 131 | pages = 90–95 | date = February 2014 | pmid = 24509071 | doi = 10.1016/j.jphotobiol.2014.01.002 | bibcode = 2014JPPB..131...90B | hdl = 10072/432100 | url = http://eprints.qut.edu.au/67491/1/67491.pdf | access-date = September 19, 2019 | url-status = live | archive-url = https://web.archive.org/web/20181103234519/http://eprints.qut.edu.au/67491/1/67491.pdf | archive-date = November 3, 2018 }} raising concerns about sun exposure for pregnant women.{{cite web |url=http://www.americanpregnancy.org/pregnancyhealth/tanningmethods.html |title=Pregnancy and Tanning |publisher=American Pregnancy Association |date=January 2014 |access-date=January 11, 2015 |archive-date=July 3, 2014 |archive-url=https://web.archive.org/web/20140703081719/http://americanpregnancy.org/pregnancyhealth/tanningmethods.html |url-status=live }} Lifespan and fertility can be adversely affected for individuals born during peaks of the 11-year solar cycle, possibly because of UV-related folate deficiency during gestation.{{cite journal | vauthors = Skjærvø GR, Fossøy F, Røskaft E | title = Solar activity at birth predicted infant survival and women's fertility in historical Norway | journal = Proceedings. Biological Sciences | volume = 282 | issue = 1801 | pages = 20142032 | date = February 2015 | pmid = 25567646 | pmc = 4308994 | doi = 10.1098/rspb.2014.2032 }}

A seasonal variation in blood pressure has been noted for decades. Research indicates that skin exposure to sunlight results in a modest reduction in systolic blood pressure. The effect is independent of vitamin D status, instead being mediated by nitric oxide release from skin upon exposure to UV light. The effect is greater in fair-skinned individuals.{{cite journal | vauthors = Weller RB, Wang Y, He J, Maddux FW, Usvyat L, Zhang H, Feelisch M, Kotanko P | display-authors = 6 | title = Does Incident Solar Ultraviolet Radiation Lower Blood Pressure? | journal = Journal of the American Heart Association | volume = 9 | issue = 5 | pages = e013837 | date = March 2020 | pmid = 32106744 | pmc = 7335547 | doi = 10.1161/JAHA.119.013837 }}

UV exposure plays a role in motor learning and object recognition memory. Moderate UV exposure enhances cognitive functioning through a specialized glutamate (GLU) biosynthetic pathway in the brain.{{Cite journal |last=Zhu |first=Hongying |date=June 14, 2018 |title=Moderate UV Exposure Enhances Learning and Memory by Promoting a Novel Glutamate Biosynthetic Pathway in the Brain |journal=Cell |volume=173 |issue=7 |pages=1716–1727.e17 |doi=10.1016/j.cell.2018.04.014 |pmid=29779945 |url=https://www.cell.com/cell/fulltext/S0092-8674(18)30507-5?sf193287443=1}} The process begins when UV exposure triggers an increase in blood levels of urocanic acid (UCA), which possesses the unique ability to cross the blood-brain barrier. Upon entering the brain, UCA initiates a cascade of biochemical reactions that promote glutamate biosynthesis, particularly in critical regions such as the motor cortex and hippocampus. This increased glutamate production leads to notable cognitive enhancements, including improved motor learning capabilities, enhanced object recognition memory, and strengthened synaptic plasticity.{{Cite journal |last=cao |first=Jianping |date=26 December 2018 |title=The glutamate biosynthetic pathway in brain: a novel mechanism of moderate UV-induced neurobehavioral changes |journal=Acta Biochimica et Biophysica Sinica |volume=51 |issue=2 |pages=227–228 |doi=10.1093/abbs/gmy166 |pmid=30590380 |url=https://academic.oup.com/abbs/article/51/2/227/5261072|doi-access=free }}

Ultraviolet (UV) light has been shown to influence various neurological functions, including mood, addiction, cognition, and memory.{{Cite journal |last=Slominski |first=Andrzej |date=May 2018 |title=How UV Light Touches the Brain and Endocrine System Through Skin, and Why |journal=Endocrinology|volume=159 |issue=5 |pages=1992–2007 |doi=10.1210/en.2017-03230 |pmid=29546369 |pmc=5905393 }} This impact likely occurs through UV-induced changes in brain chemistry, potentially altering neurotransmitter systems and neuroplasticity. UV exposure might, for example, affect serotonin levels, which play a significant role in mood regulation and cognitive functions. This hormonal effect can helps stabilize circadian rhythms. This stabilizing effect is particularly important for individuals with mental disorders, who often experience circadian desynchronization and sleep problems. As a result, light therapy has emerged as an increasingly popular treatment option for various mood and psychiatric disorders.{{Cite journal |last1=Blume |first1=Christine |last2=Garbazza |first2=Corrado |last3=Spitschan |first3=Manuel |date=20 August 2019 |title=Effects of light on human circadian rhythms, sleep and mood |journal=Somnologie|volume=23 |issue=3 |pages=147–156 |doi=10.1007/s11818-019-00215-x |pmid=31534436 |pmc=6751071 }}

According to a 2007 study submitted by the University of Ottawa to the US Department of Health and Human Services, there is not enough information to determine a safe level of sun exposure that imposes minimal risk of skin cancer.{{cite journal | vauthors = Cranney A, Horsley T, O'Donnell S, Weiler H, Puil L, Ooi D, Atkinson S, Ward L, Moher D, Hanley D, Fang M, Yazdi F, Garritty C, Sampson M, Barrowman N, Tsertsvadze A, Mamaladze V | display-authors = 6 | 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 }} In addition, there is not yet conclusive evidence on which components of ultraviolet radiation (UVA, UVB, UVC) are actually carcinogenic.{{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=December 22, 2014 |archive-date=December 22, 2014 |archive-url=https://web.archive.org/web/20141222172504/http://ntp.niehs.nih.gov/ntp/roc/content/profiles/ultravioletradiationrelatedexposures.pdf |url-status=live }} UVC is almost completely absorbed by the atmosphere and does not reach the surface in any appreciable quantity.{{cite web |url=http://dermatology.about.com/od/glossaryu/g/uvc.htm |title=UVC Radiation | vauthors = Brannon H |publisher=About.com |date=January 1, 2014 |access-date=March 25, 2015 |archive-date=March 5, 2016 |archive-url=https://web.archive.org/web/20160305005600/http://dermatology.about.com/od/glossaryu/g/uvc.htm |url-status=dead }} As a result, only the broad-spectrum combination (UVA, UVB, UVC) known as "ultraviolet radiation" is listed as a carcinogen; the components are only "likely to become" known carcinogens. Solar radiation (sunlight) and sunlamps are listed as carcinogens because they contain ultraviolet radiation.

File:Imprecise-Biasutti-map.jpg for classifying skin color. It was reported that for areas with no data Biasutti simply filled in the map by extrapolation from findings obtained in other areas.{{cite journal |title=The Evolution of Human Skin Color | vauthors = Jablonski NG | journal = Annual Review of Anthropology | date = October 2004 | volume = 33 |issue = 1 | pages = 585-623 (600) | doi = 10.1146/annurev.anthro.33.070203.143955 |url=http://sites.psu.edu/ninajablonski/wp-content/uploads/sites/10224/2014/02/Jablonski.2004.ARA_.585ff.pdf |archive-url=https://web.archive.org/web/20180824034509/http://sites.psu.edu/ninajablonski/wp-content/uploads/sites/10224/2014/02/Jablonski.2004.ARA_.585ff.pdf |archive-date=August 24, 2018 |url-status=live }}]]

There are currently no recommendations on a safe level of total lifetime sun exposure. According to epidemiologist Robyn Lucas at Australian National University, analysis of lifespan versus disease shows that far more lives worldwide could be lost to diseases caused by lack of sunlight than to those caused by too much,{{cite journal | vauthors = Lucas RM, McMichael AJ, Armstrong BK, Smith WT | title = Estimating the global disease burden due to ultraviolet radiation exposure | journal = International Journal of Epidemiology | volume = 37 | issue = 3 | pages = 654–667 | date = June 2008 | pmid = 18276627 | doi = 10.1093/ije/dyn017 | doi-access = }} and it is inappropriate to recommend total avoidance of sunlight.{{cite journal | vauthors = Lucas RM, Ponsonby AL | title = Ultraviolet radiation and health: friend and foe | journal = The Medical Journal of Australia | volume = 177 | issue = 11–12 | pages = 594–598 | date = December 2002 | pmid = 12463975 | doi = 10.5694/j.1326-5377.2002.tb04979.x | url = https://www.mja.com.au/journal/2002/177/11/ultraviolet-radiation-and-health-friend-and-foe | access-date = March 24, 2015 | url-status = live | s2cid = 22389294 | archive-url = https://web.archive.org/web/20150402121603/https://www.mja.com.au/journal/2002/177/11/ultraviolet-radiation-and-health-friend-and-foe | archive-date = April 2, 2015 }}

Over thousands of years, in many climate zones, genetic selection has helped indigenous human populations adapt toward skin pigmentation levels that provide a healthy level of UV exposure. This largely explains the tendency toward darker-skinned populations in the sunniest tropical environments, and lighter skin tones in less-sunny regions and for those who most need vitamin D for rapid bone growth, specifically children and reproductive-age women. The map to the right illustrates the geographic distribution of skin color for native populations prior to 1940, based on von Luschan's chromatic scale. These long-term adaptations for optimal health can be confounded by patterns of food, clothing and shelter, especially at a time when large populations have migrated far from the climates for which their skin was genetically adapted.{{cite journal | vauthors = Webb AR | title = Who, what, where and when-influences on cutaneous vitamin D synthesis | journal = Progress in Biophysics and Molecular Biology | volume = 92 | issue = 1 | pages = 17–25 | date = September 2006 | pmid = 16766240 | doi = 10.1016/j.pbiomolbio.2006.02.004 | doi-access = }}{{cite book |last=Jablonski |first=Nina |title=Living Color |year=2012 |publisher=University of California Press |isbn=978-0-520-25153-3}}

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• Sunburn
• Fluorescent lamps and health
• Heat stroke
• Photosensitivity in humans
• UV Index
• High-energy visible light

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Sun

Category:Sun tanning

Category:Vitamin D