Health effects of sunlight exposure
On the positive side, UV exposure enables the synthesis of vitamin D3, which is essential for bone health[1] and potentially plays a role in inhibiting certain cancers.
[5][6][7][8][9] Additionally, exposure to visible light supports melatonin synthesis, maintains circadian rhythms, and reduces the risk of seasonal affective disorder.
They recommend avoiding sunburn at all costs and advocate for moderation in sun exposure to minimize the risks associated with UV radiation while still reaping its health benefits.
[17][18][19] UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D.[20] 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,[19] 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.
[26] 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.
[27] Effects of UV-radiation at high latitudes, where snow stays on the ground into early summer and the sun then remains at a low position even at its zenith, have been reviewed by Meyer-Rochow.
[citation needed] If an adult's arms and legs are exposed to a half minimal erythemal UV radiation, it is the same as taking 3,000 IU of vitamin D3 through an oral supplement.
Individuals whose metabolism makes taking oral vitamin D ineffective are able, through exposure to an ultraviolet lamp that emits UV-B radiation, to achieve a 25 (OH) D blood level.
This vitamin helps to regulate calcium metabolism (vital for the nervous system and bone health), immunity, cell proliferation, insulin secretion, and blood pressure.
Most people in the world depend on the sun to get vitamin D,[31] and elderly populations in low UVB countries experience higher rates of cancer.
[43] The major acute effects of UV irradiation on normal human skin comprise sunburn inflammation erythema, tanning, and local or systemic immunosuppression.
[45][failed verification] UVC is the highest-energy, most-dangerous type of ultraviolet radiation, and causes adverse effects that can variously be mutagenic or carcinogenic.
[50] Lifetime cumulative UV exposure to skin is also responsible for significant age-associated dryness, wrinkling, elastin and collagen damage, freckling, IGH, age spots and other cosmetic changes.
[58] Blood levels of folate, a nutrient vital for fetal development, can be degraded by UV radiation,[59] raising concerns about sun exposure for pregnant women.
[60] 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.
[63] The process begins when UV exposure triggers an increase in blood levels of urocanic acid (UCA), which possesses the unique ability to cross the blood-brain barrier.
Upon entering the brain, UCA initiates a cascade of biochemical reactions that promote glutamate biosynthesis, particularly in critical regions such as the motor cortex and hippocampus.
[66] 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.
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 to the right illustrates the geographic distribution of skin color for native populations prior to 1940, based on von Luschan's chromatic scale.
