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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. A dietary supplement can supply vitamin D without this mutagenic effect,〔(【引用サイトリンク】title= Dietary Supplement Fact Sheet: Vitamin D )〕 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 == (詳細は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.〔(【引用サイトリンク】title=UV-B Monitoring and Research Program )〕 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:〔(【引用サイトリンク】title=UV index & UV dose based on GOME )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Health effects of sunlight exposure」の詳細全文を読む スポンサード リンク
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