Treatment for sun-damage, age spots, liver spots, broken capillaries, hyperkeratosis, and sebacous hyperplasia.

The body produces vitamin D from sunlight (specifically from the UVB band of ultraviolet light), and excessive seclusion from the sun can lead to deficiency unless adequate amounts are obtained through diet.
Sunburn can have mild to severe inflammation effects on skin; this can be avoided by using a proper sunscreen cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called skin photodamage), which produces a difficult to treat cosmetic effect. Some people are concerned that ozone depletion is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer.[19]
A lack of sunlight, on the other hand, is considered one of the primary causes of seasonal affective disorder (SAD), a serious form of the "winter blues". SAD occurrence is more prevalent in locations further from the tropics, and most of the treatments (other than prescription drugs) involve light therapy, replicating sunlight via lamps tuned to specific (visible, not ultra-violet) wavelengths of light or full-spectrum bulbs.
A recent study indicates that more exposure to sunshine early in a person’s life relates to less risk from multiple sclerosis (MS) later in life.[20

Lifetime sun exposure
There are currently no recommendations on the total safe level of lifetime sun exposure.[12] According to epidemiologist Robyn Lucas at Australian National University,[28] analysis of lifespan versus disease shows that far more lives are lost to diseases caused by lack of sunlight than by those caused by too much.[29] Some scientists suggest that it is inappropriate to recommend total avoidance of sunlight.[30]
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.[29]
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).[31][32] Conversely, the intensity of ultraviolet radiation is increased at higher altitudes, by 4-5% for every 1000 feet of elevation.[33]
The current recommendations for Vitamin D supplementation (between 200 IU and 400 IU)[34] are not based on sun exposure levels of Vitamin D production, but on fears of toxicity as each person's Vitamin D status depends on dozens of environmental and nutritional factors.[35] Because of this balance between internal production and external supplementation of Vitamin D, it is up to each individual to be aware of how they feel, and to consider sun exposure and Vitamin D status as part of their overall health.[36]
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.[37]

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.[12]
There is no consensus on which component of ultraviolet radiation (UVA or UVB or UVC) actually is carcinogenic,[22] and the composition of ultraviolet radiation changes throughout the day: At high noon, ultraviolet radiation reaching ground level is 95% UVA and 5% UVB, while before 10am and after 2pm this percentage changes over time to 99% UVA and 1% UVB.[23] This is caused by the reflection of UVB rays back into space due to sun angle as the earth slowly rotates on its axis. The rate of change is faster the farther the position is away from the equator (i.e. more north or south).[24]
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.[25] As a result, only the 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, also known as "sunlight" is also listed as a carcinogen because it contains ultraviolet radiation. This means also that the UV Index is a measure of total ultraviolet radiation, and not just Vitamin D-producing UVB.[26]
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.[22]
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.[27]

Risks
Despite the importance of the sun to vitamin D synthesis, it is prudent to limit exposure of skin to sunlight[16] and UV radiation from tanning beds.[19] According to the National Toxicology Program Report on Carcinogens from the Department of Health and Human Services, broad-spectrum UV radiation is a carcinogen 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.[16][20] 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.[21]
Prolonged optical exposure to sunlight, especially intense ultraviolet light, may be linked to cataracts and high levels of high energy visible light may be linked to age related macular degeneration. See also snow blindness.