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 and a mutagen. A dietary supplement can supply vitamin D without this mutagenic effect, 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 and possibly inhibiting the growth of some cancers. UV exposure also has positive effects for endorphin levels, and possibly for protection against multiple sclerosis. Visible sunlight to the eyes gives health benefits through its association with the timing of melatonin synthesis, maintenance of normal and robust circadian rhythms, and reduced risk of seasonal affective disorder.
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. Short-term over-exposure is the cause of sunburn, snow blindness, and solar retinopathy.
UV rays, and therefore sunlight and sunlamps, are the only listed carcinogens that are known to have health benefits, 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. There is a general consensus that sunburn should always be avoided.
Synthesis of vitamin D3
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. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D. 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.) 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. 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. Australia and New Zealand have had similar findings, which indicate insufficient protection against rickets for children and osteoporosis for adults.
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.
Lower prevalence of multiple sclerosis
Multiple sclerosis (MS) is least prevalent in the sunniest regions. Exposure to the ultraviolet-B radiation of sunlight appears to be most important and this may operate via vitamin D synthesis.
Risks to skin
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. The major acute effects of UV irradiation on normal human skin comprise sunburn inflammation erythema, tanning, and local or systemic immunosuppression. The most deadly form, malignant melanoma, is mostly caused by indirect DNA damage from UVA radiation. This can be seen from the absence of a direct UV signature mutation in 92% of all melanoma. UVC is the highest-energy, most-dangerous type of ultraviolet radiation, and causes adverse effects that can variously be mutagenic or carcinogenic.
Despite the importance of the sun to vitamin D synthesis, it is prudent to limit the exposure of skin to UV radiation from sunlight and from tanning beds. 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. Lifetime cumulative UV exposure to skin is also responsible for significant age-associated dryness, wrinkling, elastin and collagen damage, freckling, 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. Short-term over-exposure causes the pain and itching of sunburn, which in extreme cases can produce more-severe effects like blistering.
Benefits of optic exposure
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. Bright morning light has been shown to be effective against insomnia, premenstrual syndrome and seasonal affective disorder (SAD).
Effects on eyes
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.
Blood levels of folate, a nutrient vital for fetal development, can be degraded by UV radiation, raising concerns about sun exposure for pregnant women. 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.
Safe level of sun exposure
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. In addition, there is not yet conclusive evidence on which components of ultraviolet radiation (UVA, UVB, UVC) are actually carcinogenic. 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. UVC is almost completely absorbed by the atmosphere and does not reach the surface in any appreciable quantity. 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.
Lifetime sun exposure
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, and it is inappropriate to recommend total avoidance of sunlight.
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 below 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.
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