Health effects of sunlight exposure

The only way to quantify adequate levels of vitamin D is with a serum 25(OH)D₃ (calcifediol) test.^[23] 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.^[24] Australia and New Zealand have had similar findings, which indicate insufficient protection against rickets for children and osteoporosis for adults.^[25]

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.^[26]

Using satellite data, measurements from the

Three benefits of UV exposure are production of vitamin D, improvement in mood, and increased energy.[29]

UVB induces production of vitamin D in the skin at rates of up to 1,000 IUs per minute. This vitamin helps to regulate calcium metabolism (vital for the nervous system and bone health), immunity, cell proliferation, insulin secretion, and blood pressure.^[30] In low and middle income countries, foods fortified with vitamin D are "practically nonexistent." 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.^[32]

There are not many foods that naturally have vitamin D.^[33] Examples are cod liver oil and oily fish. If people cannot get sunlight, then they will need 1,000 IU of vitamin D per day to stay healthy.^[34] A person would have to eat oily fish three or four times per week in order to get enough vitamin D from that food source alone.

People with higher levels of vitamin D tend to have lower rates of diabetes, heart disease, and stroke and tend to have lower blood pressure. However, it has been found that vitamin D supplementation does not improve cardiovascular health or metabolism, so the link with vitamin D must be in part indirect.^{[citation needed]} People who get more sun are generally healthier, and also have higher vitamin D levels. It has been found that ultraviolet radiation (even UVA) produces nitric oxide (NO) in the skin, and nitric oxide can lower blood pressure. High blood pressure increases the risk of stroke and heart disease. Although long-term exposure to ultraviolet contributes to non-melanoma skin cancers that are rarely fatal, it has been found in a Danish study that those who get these cancers were less likely to die during the study, and were much less likely to have a heart attack, than those who did not have these cancers.^[35]

People in certain situations, such as people with intellectual disabilities and neurodevelopmental disorders who stay inside most of the time have low vitamin D levels. Getting enough vitamin D can help stave off "autoimmune diseases, cardiovascular disease, many types of cancer, dementia, types 1 and 2 diabetes mellitus, and respiratory tract infections."^[36]

Fetuses and children who do not get enough vitamin D can result in "growth retardation and skeletal deformities."^[33]

Multiple sclerosis risk

Multiple sclerosis (MS) is least prevalent in the sunniest regions.^[37][38][39] Exposure to the ultraviolet-B radiation of sunlight appears to be most important and this may operate via vitamin D synthesis.^[40]

Effects on 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. Skin surface lipids, including unsaturated lipids such as squalene, sebaleic aicd, linoleic acid, and cholesterol can be a subject of oxidation by singlet oxygen and ozone as well as free radicals. Ultraviolet radiation activates lipoxygenase and cyclooxygenase, inducing specific enzymatic oxidation of lipids. Free radical mediated lipid peroxidation gives multiple oxidation products which may induce various skin diseases^[41]

The major acute effects of UV irradiation on normal human skin comprise sunburn inflammation erythema, tanning, and local or systemic immunosuppression.^[42] 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.^[43] UVC is the highest-energy, most-dangerous type of ultraviolet radiation, and causes adverse effects that can variously be mutagenic or carcinogenic.^[44]

Despite the importance of the sun to vitamin D synthesis, it is prudent to limit the exposure of skin to UV radiation from sunlight

However, significant daily exposure to bright light may be necessary for children to avoid myopia (nearsightedness).^[51]

Short-term over-exposure can cause

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).^[10]

Folate degradation

Blood levels of folate, a nutrient vital for fetal development, can be degraded by UV radiation,^[55] raising concerns about sun exposure for pregnant women.^[56] 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.^[57]

Blood pressure

A seasonal variation in blood pressure has been noted for decades. Research indicates that skin exposure to sunlight results in a modest reduction in systolic blood pressure. The effect is independent of vitamin D status, instead being mediated by nitric oxide release from skin upon exposure to UV light. The effect is greater in fair-skinned individuals.^[58]

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.^[59] In addition, there is not yet conclusive evidence on which components of ultraviolet radiation (UVA, UVB, UVC) are actually carcinogenic.^[12] UVC is almost completely absorbed by the atmosphere and does not reach the surface in any appreciable quantity.^[60] 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.^[12]

Lifetime sun exposure

There are currently no recommendations on a safe level of total lifetime sun exposure.^[59] 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,^[62] and it is inappropriate to recommend total avoidance of sunlight.^[63]

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 to the right 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.^[64][65]

See also

• Sunburn
• Fluorescent lamps and health
• Heat stroke
• Photosensitivity in humans
• UV Index
• High-energy visible light

References