||This article needs attention from an expert in Medicine. (January 2010)|
The sunburn should always be avoided.
In the United States, serum levels of 25(OH) D3 are below the recommended levels for more than a third of white men, with serum levels lower in women and in most minorities. This indicates that Vitamin D deficiency is a common problem in the US.
According to the U.S. National Institutes of Health Office of Dietary Supplements, most people in the United States can meet their vitamin D needs through exposure to sunlight, even though a large portion have serum 25(OH)D3 levels below recommendations.
Synthesis of vitamin D3
Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 
Complete cloud cover reduces UV energy by 50%; shade (including that produced by severe pollution) reduces it by 60%.
The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines. It has been suggested by some vitamin D researchers, for example, that approximately 5–30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial  Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.
Despite the importance of the sun to vitamin D synthesis, it is prudent to limit exposure of skin to sunlight Lifetime cumulative UV damage to skin is also largely responsible for some age-associated dryness and other cosmetic changes.
It is not known whether a desirable level of regular sun exposure exists that imposes no (or minimal) risk of skin cancer over time. The American Academy of Dermatology advises that photoprotective measures be taken, including the use of sunscreen, whenever one is exposed to the sun.
Prolonged optical exposure to sunlight, especially intense snow blindness.
Safe level of sun exposure
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 at this time.
There is no consensus on which component of 
On average over a day, 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA. UVC is almost completely absorbed by the ozone layer and does not penetrate the atmosphere in any appreciable quantities.
Sunlight is therefore the only listed carcinogen that is known to have health benefits, in the form of helping the human body to make Vitamin D. This makes sunlight unique on the list of known carcinogens.
With new evidence of Vitamin D receptors in all body tissues, experts advise having a balance between Vitamin D from sun exposure and Vitamin D from supplements. The only way to quantify adequate levels of Vitamin D is with a serum 25(OH) D3 test.
Lifetime sun exposure
There are currently no recommendations on the total safe level of lifetime sun exposure.
If one is fair skinned, 10 minutes of exposure to sunshine at high noon (in summer) will produce 10,000 IU of Vitamin D; darker skin requires longer exposure.
Note that summer peak daily UVB radiation can be one thousand times higher than winter peak daily UVB radiation in temperate regions. The reason is that UVB radiation is strongly absorbed by the atmosphere, and, when the sun is closer to the horizon, its ultraviolet light is attenuated due to having to pass through greater thickness of atmosphere. This effect is far stronger for the ultraviolet light than for the visible light. For example, in Boston, the summer solstice sun peak altitude is 71 degrees and the corresponding UVB radiation is 73% of max (90 degree sun altitude); the winter solstice sun peak altitude is 24 degrees and the corresponding UVB radiation is 0.03% of max (90 degree sun altitude).
The current recommendations for Vitamin D supplementation (between 200 IU and 400 IU)
It has recently been discovered that vitamin D receptors are present in most if not all cells in the body. Additionally, experiments using cultured cells have demonstrated that vitamin D has potent effects on the growth and differentiation of many types of cells. These findings suggest that vitamin D has physiologic effects much broader than a role in mineral homeostasis and bone function. This is an active area of research and a much better understanding of this area will likely be available in the near future.
Sun exposure and survival from malignant melanoma
A study in the February 2, 2005 issue of the Journal of the National Cancer Institute looked at markers of sun exposure in more than 500 people who had recently been diagnosed with malignant melanoma. The researchers found that solar elastosis, or sun damage to the skin, was independently associated with a surprising increased survival from melanoma.
Sunscreen use within the last 10 years or during childhood was not associated with worse survival from melanoma. And all measures of sun exposure (i.e., history of severe sunburn, high levels of intermittent sun exposure, solar elastosis) were associated with improved survival from melanoma. Furthermore, participants who reported high skin awareness, but not those who reported skin examinations, had better survival.
If confirmed, the results of this study suggest that whereas excessive sun exposure leads to the development of melanoma, sun exposure may protect against the progression of melanoma into more fatal disease.
There is ample evidence that the death rate in elderly patients increases in winter months. In a recent study of seasonal mortality in terminal cancer patients in the United States, it was found that compared to those patients who died during June, July, and August, the number of deaths of patients increased an average of 20% in January, February, and March. This near-sinusoidal pattern was remarkably consistent over a five-year period.
Mortality in the general population is lowest in late summer or early fall. The fewest deaths in Japan occurred in July, in Sweden and North America the fewest deaths occurred in August and in Mediterranean countries, the lowest average daily mortality was observed in September. In the southern hemisphere, the lowest mortality in New Zealand occurred in February and in Australia the lowest mortality occurred in March.
Deaths due to variceal bleeding in 
The preceding details evidence that does not address direct sun exposure, but only seasonal variations in death rate. Factors involved in the observed increase in mortality events during winter months, such as lower temperatures in winter, must be kept in mind, as the corresponding increase in 
UV-B monitoring in the United States
Over the past several years, levels of ultraviolet radiation have been tracked at over 30 sites across North America as part of the UV-B Monitoring and Research Program (UVMRP) at 
UVB Levels in Vitamin D Equivalents:
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