A sunburnt back that was partially protected by a bathing suit top.
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A sunburn is a form of radiation burn that affects living tissue, such as skin, that results from an overexposure to ultraviolet (UV) radiation, commonly from the sun. Normal symptoms in humans and other animals include red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV radiation can be life-threatening in extreme cases. Exposure of the skin to lesser amounts of UV radiation will often produce a suntan.
Excessive UV radiation is the leading cause of primarily non-malignant skin tumors. Sunscreen is widely agreed to prevent sunburn and some types of skin cancer. Clothing, including hats, is considered the preferred skin protection method. Moderate sun tanning without burning can also prevent subsequent sunburn, as it increases the amount of melanin, a photoprotectant pigment that is the skin's natural defense against overexposure. Importantly, both sunburn and the increase in melanin production are triggered by direct DNA damage. When the skin cells' DNA is damaged by UV radiation, type I cell-death is triggered and the skin is replaced.
Signs and symptoms
Other symptoms can include edema, itching, peeling skin, rash, nausea, fever, chills, and syncope. Also, a small amount of heat is given off from the burn, caused by the concentration of blood in the healing process, giving a warm feeling to the affected area. Sunburns may be first- or second-degree burns.
Minor sunburns typically cause nothing more than slight redness and tenderness to the affected areas. In more serious cases, blistering can occur. Extreme sunburns can be painful to the point of debilitation and may require hospital care.
Sunburn can occur in less than 15 minutes, and in seconds when exposed to non-shielded welding arcs or other sources of intense ultraviolet light. Nevertheless, the inflicted harm is often not immediately obvious.
After the exposure, skin may turn red in as little as 30 minutes but most often takes 2 to 6 hours. Pain is usually most extreme 6 to 48 hours after exposure. The burn continues to develop for 24 to 72 hours, occasionally followed by peeling skin in 3 to 8 days. Some peeling and itching may continue for several weeks.
Long-term low-intensity exposure to sunlight is known to cause significant aging of the skin; other health effects are not accurately known. A particular example with very noticeable aging is that of a 69-year-old truck driver of Chicago, IL in the United States, who drove in the city for 28 years. A photograph of his face shows a great deal of ageing on the left side, where he was exposed to sunlight all day, while the right side has the "taut, unblemished face of an apparently much younger man". Window glass does not absorb UVA, which can penetrate the epidermis and upper layers of dermis. Chronic UVA exposure can cause photoaging: thickening of the epidermis and stratum corneum and destruction of elastic fibers; it can cause DNA mutations and toxicity which can lead to cancer, although less carcinogenic than UVB.
Ultraviolet radiation causes sunburns and increases the risk of three types of skin cancer: melanoma, basal-cell carcinoma and squamous cell carcinoma. Of greatest concern is that the melanoma risk increases in a dose-dependent manner with the number of a person's lifetime cumulative episodes of sunburn.
Sunburn is caused by UV radiation, either from the sun or from artificial sources, such as tanning lamps, welding arcs, and ultraviolet germicidal irradiation. It is a reaction of the body to the direct DNA damage, which can result from the excitation of DNA by UVB light. This damage is mainly the formation of a thymine dimer. The damage is recognized by the body, which then triggers several defense mechanisms, including DNA repair to revert the damage and increased melanin production to prevent future damage. Melanin readily absorbs UV wavelength light, acting as a photoprotectant. By preventing the disruption of bonds that higher-energy photons can produce, it inhibits both direct alteration of DNA and generation of free radicals, thus indirect DNA damage.
Experiments with mice found that protection against sunburn by chemical sunscreens does not necessarily provide protection against other damaging effects of UV radiation such as enhanced melanoma growth. These were pre-2000 sunscreens that offered only limited protection against UVA radiation.
Skin type determines the ease of sunburn. In general, people with lighter skin tone have a greater risk of sunburn. Age also affects how skin reacts to sun: the skin of children younger than 6 and adults older than 60 is more sensitive to sunlight. Factors of sunburn include:
- The time of day. Between 10am and 4pm daylight saving time, the sun's rays are the strongest. UV is partially blocked by clouds; but even on an overcast day, a significant percentage of the sun's damaging UV radiation can pass through clouds.
- Proximity to reflective surfaces, such as water, white sand, concrete, snow, and ice. All of these reflect the sun's rays and can cause sunburns.
- The season of the year. The position of the sun on late spring and summer days can cause a more severe sunburn.
- Altitude. At a higher altitude it is easier to become burnt because there is less of the earth's atmosphere to block the sunlight. UV exposure increases about 4% for every 1000 ft (305 m) gain in elevation.
- Proximity to the equator (latitude). The closer to the equator, the more direct sunlight passes through the atmosphere. For example, the southern United States gets fifty percent more sunlight than the northern United States.
- The UV Index, which indicates the risk of getting a sunburn at a given time and location.
Because of variations in the intensity of UV radiation passing through the atmosphere, the risk of sunburn increases with proximity to the tropic latitudes, located between 23.5° north and south latitude. All else being equal (e.g., cloud cover, ozone layer, terrain, etc.), over the course of a full year, each location within the tropic or polar regions receives approximately the same amount of UV radiation. In the temperate zones between 23.5° and 66.5°, UV radiation varies by latitude and season. The higher the latitude, the lower the intensity of the UV rays. Intensity in the northern hemisphere is greatest during the months of May, June and July — and in the southern hemisphere, November, December and January. On a minute-by-minute basis, the amount of UV radiation is dependent on the angle of the sun. This is easily determined by the height ratio of any object to the size of its shadow. The greatest risk is at solar noon, when shadows are at their minimum and the sun's radiation passes more directly through the atmosphere. Regardless of one's latitude (assuming no other variables), equal shadow lengths mean equal amounts of UV radiation.
In recent decades, the incidence and severity of sunburn has increased worldwide, especially in the southern hemisphere, because of damage to the ozone layer. Ozone depletion and the seasonal ozone hole have led to dangerously high levels of UV radiation in some locations. Incidence of skin cancer in Queensland, Australia had risen to 75 percent among those over 64 years of age by about 1990, due, it is presumed, to thinning of the ozone layer. It was pointed out by Garland et al. that the melanoma rate in Queensland had taken a steep rise before the rest of Australia experienced the same increase of melanoma numbers. They blamed the vigorous promotion of sunscreen, which was first done in Queensland, while sunscreen use was encouraged in the rest of Australia some time later. An effect that would stem from the ozone depletion could not differ from territory to territory within Australia, but sunscreen endorsement programs could. Another study from Norway points out that there had been no change in the ozone layer during the period 1957 to 1984, yet the yearly incidence of melanoma in Norway had increased by 350% for men and by 440% for women. They concluded that in Norway "ozone depletion is not the cause of the increase in skin cancers".
Popularity of tanning
Suntans, which naturally develop in some individuals as a protective mechanism against the sun, are viewed by most in the Western world as desirable. This has led to increased exposure to UV radiation from both the natural sun and solaria.
One of the most effective ways to prevent sunburn is to reduce the amount of UV radiation reaching the skin. The strength of sunlight is published in many locations as a UV Index. The World Health Organization recommends to limit time in midday summer sun (between 10 a.m. and 4 p.m.), to watch the UV Index, to seek shade, to wear protective clothing and a wide-brim hat, and to use sunscreen. Sunlight is generally strongest when the sun is close to the highest point in the sky. Due to time zones and daylight saving time, this is not necessarily at 12 p.m., but often one to two hours later.
The skin and eyes are most sensitive to damage by UV at 265–275 nm, which is in the lower UVC band that is almost never encountered except from artificial sources like welding arcs. Most sunburn is caused by longer wavelengths simply because those are more prevalent in sunlight at ground level.
Commercial preparations are available that block UV light, known as sunscreens or sunblocks. They have a Sun Protection Factor (SPF) rating, based on the sunblock's ability to suppress sunburn: The higher the SPF rating, the lower the amount of direct DNA damage.
Modern sunscreens contain filters for UVA radiation as well as UVB. The stated protection factors are correct only if 2 μL of sunscreen is applied per square cm of exposed skin. This translates into about 28 mL (1 oz) to cover the whole body of an adult male, which is much more than many people use in practice. Although UVA radiation does not cause sunburn, it does contribute to skin aging and an increased risk of skin cancer. Many sunscreens provide broad-spectrum protection, meaning that they protect against both UVA and UVB radiation.
Research has shown that the best protection is achieved by application 15 to 30 minutes before exposure, followed by one reapplication 15 to 30 minutes after exposure begins. Further reapplication is necessary only after activities such as swimming, sweating, and rubbing. This varies based on the indications and protection shown on the label — from as little as 80 minutes in water to a few hours, depending on the product selected.
Sunscreen is effective and thus recommended for preventing melanoma and squamous cell carcinoma. There is little evidence that it is effective in preventing basal cell carcinoma. Other advice to reduce rates of skin cancer includes: avoiding sunburning, wearing protective clothing, sunglasses and hats, and attempting to avoid sun exposure or periods of peak exposure. The U.S. Preventive Services Task Force recommends that people aged between 9 and 25 years of age be advised to avoid ultraviolet light. Typical use of sunscreen does not usually result in vitamin D deficiency, but extensive usage may.
When one is exposed to any artificial source of occupational UV, special protective clothing (for example, welding helmets/shields) should be worn. Such sources can produce UVC, an extremely carcinogenic wavelength of UV which ordinarily is not present in normal sunlight, having been filtered out by the atmosphere.
The eyes are also sensitive to sun exposure at about the same wavelengths in the UV as skin. Wrap-around sunglasses or the use by spectacle-wearers of glasses that block UV light reduce harmful UV radiation. UV light has been implicated in the development of age-related macular degeneration, pterygium and cataract. Concentrated clusters of melanin, commonly known as freckles, are often found within the iris.
Dietary factors influence susceptibility to sunburn, recovery from sunburn, and risk of secondary complications from sunburn. Several dietary antioxidants, including essential vitamins, have been shown to have some effectiveness for protecting against sunburn and skin damage associated with ultraviolet radiation, both in human and animal studies. Supplementation with Vitamin C and Vitamin E was shown in one study to reduce the amount of sunburn after a controlled amount of UV exposure. A review of scientific literature through 2007 found that beta carotene (Vitamin A) supplementation had a protective effect against sunburn, but that the effects were only evident in the long-term, with studies of supplementation for periods less than 10 weeks in duration failing to show any effects. There is also evidence that common foods may have some protective ability against sunburn if taken for a period before the exposure.
The most important aspects of sunburn care are to avoid exposure to the sun while healing and to take precautions to prevent future burns. The best treatment for most sunburns is time. Most sunburns heal completely within a few weeks. Home treatments that help manage the discomfort or facilitate the healing process include using cool and wet cloths on the sunburned areas, taking frequent cold showers or baths, and applying soothing lotions that contain aloe vera to the sunburn areas. Topical steroids (such as 1% hydrocortisone cream) may also help with sunburn pain and swelling. The peeling that comes after some sunburn is inevitable. However, there are lotions such as calamine that may relieve the itching. Paracetamol (acetaminophen in the US), non-steroidal anti-inflammatory drugs (such as ibuprofen or naproxen), and aspirin have all been shown to reduce the pain of sunburns.
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The sun's damaging UV light can pass through clouds.
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Up to 80% of solar UV radiation can penetrate light cloud cover.
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Clear skies allow virtually 100% of UV to pass through, scattered clouds transmit 89%, broken clouds transmit 73%, and overcast skies transmit 31%.
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