Health effects of sun exposure

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The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D3 and a mutagen.[1] A dietary supplement can supply vitamin D without this mutagenic effect,[2] 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[3] and possibly inhibiting the growth of some cancers.[4][5] Sun exposure has also been associated with the timing of melatonin synthesis, maintenance of normal circadian rhythms, and reduced risk of seasonal affective disorder.[6]

Long-term sunlight exposure is known to be associated with the development of skin cancer, skin aging, immune suppression, and eye diseases such as cataracts and macular degeneration.[7] Short-term overexposure is the cause of sunburn and snow blindness.

UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits,[8] 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.[9] There is a general consensus that sunburn should always be avoided.

Synthesis of vitamin D3[edit]

Radiograph of a child with rickets, usually caused by insufficient vitamin D
Main article: 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.[10][11][12] UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D.[13] 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,[12] 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, the more minutes of exposure, 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.)[12][14][15] 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.[16] 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.[17] Australia and New Zealand have had similar findings.[18]


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 (UVMRP) at Colorado State University. The image below shows levels of UVB radiation in June 2008, expressed in Vitamin D Equivalents:[19]

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Organizations like the United States Environmental Protection Agency produce similar maps expressed in the units of the widely followed UV Index:[20]

July map new.png

Risks to skin and eyes[edit]

Melanoma on human skin
Sunburn peeling

Despite the importance of the sun to vitamin D synthesis, it is prudent to limit the exposure of skin to UV radiation from sunlight[21] and from tanning beds.[22] According to the National Toxicology Program Report on Carcinogens from the 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.[21][23] Lifetime cumulative UV damage to skin is also responsible for significant age-associated dryness, wrinkling 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.[24] Short-term overexposure causes the pain and itching of sunburn, which in extreme cases can produce more-severe effects like blistering.

Prolonged optical exposure to sunlight, especially intense ultraviolet light, may be linked to cataracts, and high levels of visible light may be linked to macular degeneration. Short-term overexposure can cause snow blindness, which is analogous to sunburn of the cornea.

Safe level of sun exposure[edit]

According to a 2007 study submitted by the University of Ottawa to the Department of Health and Human Services in Washington, D.C., there is not enough information to determine a safe level of sun exposure that imposes minimal risk of skin cancer.[25] In addition, there is not yet conclusive evidence on which components of ultraviolet radiation (UVA, UVB, UVC) are actually carcinogenic.[8] The composition reaching the ground changes through the day and year: approximately 95% UVA and 5% UVB at high noon, shifting toward 99% UVA and 1% UVB when the sun is at a low angle in the sky.[26] UVC is almost completely absorbed by the atmosphere and does not reach the surface in any appreciable quantity.[27] As a result, only the broad-spectrum combination (UVA, UVB, and 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.[8]

Lifetime sun exposure[edit]

There are currently no recommendations on a safe level of total lifetime sun exposure.[25] According to epidemiologist Robyn Lucas at Australian National University, analysis of worldwide lifespan versus disease shows that far more lives are lost to diseases caused by lack of sunlight than to those caused by too much,[28] and it is inappropriate to recommend total avoidance of sunlight.[29]

See also[edit]

References[edit]

  1. ^ Osborne JE, Hutchinson PE (August 2002). "Vitamin D and systemic cancer: is this relevant to malignant melanoma?". Br. J. Dermatol. 147 (2): 197–213. doi:10.1046/j.1365-2133.2002.04960.x. PMID 12174089. 
  2. ^ "Dietary Supplement Fact Sheet: Vitamin D". Office of Dietary Supplements, National Institutes of Health. 
  3. ^ 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 (August 2007). "Effectiveness and safety of vitamin D in relation to bone health". Evidence report/technology assessment (158): 1–235. PMID 18088161. 
  4. ^ John E, Schwartz G, Koo J, Van Den Berg D, Ingles S (June 15, 2005). "Sun Exposure, Vitamin D Receptor Gene Polymorphisms, and Risk of Advanced Prostate Cancer". Cancer Research 65 (12): 5470–5479. 
  5. ^ Egan K, Sosman J, Blot W (February 2, 2005). "Sunlight and Reduced Risk of Cancer: Is The Real Story Vitamin D?". J Natl Cancer Inst 97 (3): 161–163. doi:10.1093/jnci/dji047. 
  6. ^ Mead MN (April 2008). "Benefits of sunlight: a bright spot for human health". Environmental Health Perspectives 116 (4): A160–A167. doi:10.1289/ehp.116-a160. PMC 2290997. PMID 18414615. 
  7. ^ Lucas RM, Repacholi MH, McMichael AJ (June 2006). "Is the current public health message on UV exposure correct?". Bulletin of the World Health Organization 84 (6): 485–491. doi:10.2471/BLT.05.026559. PMC 2627377. PMID 16799733. 
  8. ^ a b c "13th Report on Carcinogens: Ultraviolet-Radiation-Related Exposures". National Toxicology Program. October 2014. Retrieved 2014-12-22. 
  9. ^ "Risks and Benefits" (PDF). Retrieved 2010-05-13. 
  10. ^ Hayes CE, Nashold FE, Spach KM, Pedersen LB (March 2003). "The immunological functions of the vitamin D endocrine system". Cellular and Molecular Biology 49 (2): 277–300. PMID 12887108. 
  11. ^ Holick MF (October 1994). "McCollum Award Lecture, 1994: vitamin D--new horizons for the 21st century". The American Journal of Clinical Nutrition 60 (4): 619–630. PMID 8092101. 
  12. ^ a b c Holick, Michael F. (February 2002). "Vitamin D: the underappreciated D-lightful hormone that is important for skeletal and cellular health". Current Opinion in Endocrinology, Diabetes and Obesity 9 (1): 87–98. doi:10.1097/00060793-200202000-00011. 
  13. ^ Holick MF (2005). "Photobiology of vitamin D". In Feldman, David Henry; Glorieux, Francis H. Vitamin D. Amsterdam: Elsevier Academic Press. ISBN 0-12-252687-2. 
  14. ^ Holick MF (September 2002). "Sunlight and Vitamin D". Journal of General Internal Medicine 17 (9): 733–735. doi:10.1046/j.1525-1497.2002.20731.x. PMC 1495109. 
  15. ^ Holick MF (July 2007). "Vitamin D deficiency". The New England Journal of Medicine 357 (3): 266–281. doi:10.1056/NEJMra070553. PMID 17634462. 
  16. ^ "25-hydroxy vitamin D test: MedlinePlus Medical Encyclopedia". Nlm.nih.gov. Retrieved 2010-05-13. 
  17. ^ Zadshir A, Tareen N, Pan D, Norris K, Martins D (2005). "The prevalence of hypovitaminosis D among US adults: data from the NHANES III". Ethnicity & Disease 15 (4 Suppl 5): S5–97–101. PMID 16315387. 
  18. ^ Nowson C, Margerison C (2002). "Vitamin D intake and vitamin D status of Australians". Med J Aust 177 (3): 149–52. PMID 12149085. 
  19. ^ "UV-B Monitoring and Research Program at Colorado State University". Uvb.nrel.colostate.edu. Retrieved 2010-05-13. 
  20. ^ "Monthly Average UV Index". EPA Sunwise. Retrieved 2014-12-22. 
  21. ^ a b Wolpowitz D, Gilchrest BA (February 2006). "The vitamin D questions: how much do you need and how should you get it?". Journal of the American Academy of Dermatology 54 (2): 301–317. doi:10.1016/j.jaad.2005.11.1057. PMID 16443061. 
  22. ^ International Agency for Research on Cancer Working Group on artificial ultraviolet (UV) light and skin cancer (March 2007). "The association of use of sunbeds with cutaneous malignant melanoma and other skin cancers: A systematic review". International Journal of Cancer 120 (5): 1116–1122. doi:10.1002/ijc.22453. PMID 17131335. 
  23. ^ "Ultraviolet (UV) Radiation, Broad Spectrum and UVA, UVB, and UVC". National Toxicology Program. 2009-01-05. Retrieved 2010-05-13. 
  24. ^ American Academy of Dermatology. Position statement on vitamin D. November 1, 2008.
  25. ^ a b Cranney A, Horsley T, O'Donnell S, et al. (August 2007). "Effectiveness and safety of vitamin D in relation to bone health". Evidence Report/technology Assessment (158): 1–235. PMID 18088161. 
  26. ^ "Ultraviolet (UV) Radiation". Food and Drug Administration. Retrieved 2010-05-13. 
  27. ^ "UVC Radiation". Dermatology.about.com. Retrieved 2010-05-13. 
  28. ^ "Time in the Sun: How Much Is Needed for Vitamin D?". US News and World Report. 2008-06-23. Retrieved 2010-05-13. 
  29. ^ Lucas RM, Ponsonby AL (December 2002). "Ultraviolet radiation and health: friend and foe". Medical Journal of Australia 177 (11): 594–598.