A sunburn is a burn to living tissue such as skin produced by overexposure to ultraviolet (UV) radiation, commonly from the sun's rays. Exposure of the skin to lesser amounts of UV will often produce a suntan. Usual mild symptoms in humans and animals are red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV-radiation can be life-threatening.

Excessive UV-radiation is the leading cause of skin cancer. While sunburn is prevented through the use of sunscreen the effectiveness of sunscreen to prevent malignant melanoma is debated and many scientists believe it is counterproductive in this respect. Therefore clothing (and hats) is considered the better method to protect skin. Moderate sun tanning without burning can also prevent sunburn as it increases the amount of melanin in the skin, which is the skin's natural defense against overexposure. Importantly, the sunburn and the increase in melanin production are both 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. Malignant melanoma may occur as a result of the indirect DNA damage if the damage is not properly repaired. Proper repair occurs in the majority of DNA damage, and as a result not every exposure to UVA results in cancer. The only cure for sunburn is slow healing, although some skin creams can help with the symptoms.

Cause

Sunburn is caused by the UV-radiation stemming from the sun, from tanning lamps, or from welding arcs. It is a reaction of the body to the direct DNA damage which can result from the excitation of DNA by UV-B light. This damage is mainly the formation of a thymine-thymine dimer. The damage is recognized by the body and it triggers several defense mechanisms. These include DNA repair to revert the damage and increased melanin production to prevent future damage. Melanin transforms UV-photons quickly into harmless amounts of heat without generating free radicals and therefore it is an excellent photoprotectant against direct and indirect DNA damages.

On an evolutionary level the sunburn may have developed as a warning signal that deters humans from sun seeking behaviour which induces infertility. Importantly it has been shown that protecting against sunburn with chemical sunscreens does not imply protection against other damaging effects of UV-radiation.

Sunburn and skin cancer

Ultraviolet B (UVB) radiation causes sunburns and increases the risk of certain types of skin cancer (basal-cell carcinoma and squamous cell carcinoma).

Controversy over sunscreen

The statement that "sunburn causes skin cancer" is adequate when it refers to basal-cell carcinoma and squamous cell carcinoma. But it is false when it comes to malignant melanoma (see picture: UVR sunburn melanoma). The statistical correlation between sunburn and melanoma is due to a common cause — the UV-radiation. However, they are generated via two different mechanisms (direct DNA damage versus indirect DNA damage).

Topically applied sunscreens block the UV rays as long as they do not penetrate into the skin. This prevents sunburn, suntanning and skin cancer. If however the sunscreen filter is absorbed into the skin it only prevents the sunburn but it increases the amount of free radicals which in turn increases the risk for malignant melanoma. The harmful effect of photoexcited sunscreen filters on living tissue has been shown in many photobiological studies. Whether sunscreen prevents or promotes the development of melanoma depends on the relative importance of the protective effect from the topical sunscreen and the harmful effects of the absorbed sunscreen. The controversy of this subject is presented on the sunscreen and on the sunscreen controversy pages.

The use of sunscreen is known to prevent the direct DNA damages that cause sunburn and the two most common forms of skin cancer, basal-cell carcinoma and squamous cell carcinoma. However, if sunscreen penetrates into the skin it promotes the indirect DNA damages which cause the most lethal form of skin cancer, malignant melanoma. This form of skin cancer is rare, but it is responsible for 75% of all skin cancer-related deaths.

The increased rate of malignant melanoma of the sunscreen user that had been found in the majority of the epidemiological studies is ascribed by many medical doctors to a change in behaviour of the sunscreen user due to a false sense of security afforded by the sunscreen. (Other researchers blame insufficient correction for confounding factors: light skinned individuals use more sunscreen and they are more likely to develop melanoma. But this is an unlikely explanation, because Queensland had an increase in melanoma after sunscreens had been heavily endorsed

Other risk factors

Location

Due to the higher intensity of the UV-radiation the risk of sunburn increases with proximity to the tropic latitudes which are located between 23.5° north and south latitude. Everything 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 the same amount of UV radiation. It is in the temperate zones between 23.5° and 66.5° where UV radiation varies by latitude. The higher the latitude, the lower the UV-intensity. During each of the annual four seasons, the earth is tilted at 23.5 degrees, which equals about 2600 km (1600 miles). Thus, on the first day of summer (assuming no other variables), any given location will receive the same amount of UV radiation as the location 2600 km in the direction of the equator did on the first day of spring. (Temperate zones may receive slightly more UV radiation due to the longer periods of daylight.) There is no place on earth that doesn't have a least some sunburn risk during the late spring and early summer.

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. Regardless of one's latitude (assuming no other variables), equal shadow lengths mean equal amounts of UV radiation.

Pharmaceutical products

Sunburn can also be caused by pharmaceutical products that sensitise some users to UV radiation. Certain antibiotics, oral contraceptives, and tranquillizers have this effect. People with red hair and/or freckles generally have a greater risk of sunburn than others because of their lighter skin tone.

Ozone depletion

In recent years, 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 has led to dangerously high levels of UV radiation. Incidence of skin cancer in Queensland, Australia has risen to 75 percent among those over 64 years of age by about 1990, presumably due to thinning of the ozone layer. However it was pointed out by Garland et al. that the melanoma rate in Queensland had 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 and sunscreen use was encouraged in the rest of Australia only with a certain delay. An effect that would stem from the ozone depletion can not obey the borderline of different areas of Australia, but sunscreen endorsement programs can. Another study from Norway points out that there had been no change of the ozone layer during the period 1957 to 1984; and yet the yearly incidence of melanoma in Norway had increased by 350% for men and by 440% for women. They conclude that in Norway the "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 many in the Western world as desirable. This has led to an increased exposure to UV-radiation from the natural sun and from solariums.

Symptoms

Typically there is initial redness (erythema), followed by varying degrees of pain, both proportional in severity to the duration and intensity of exposure.

Other symptoms are edema, itching, red and/or peeling skin, rash, nausea and fever. 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.

One should immediately speak to a dermatologist if a skin lesion appears suddenly, with asymmetrical appearance, darker edges than center, that changes color, or becomes larger than 1/4 inch (6 mm).

Variations

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.

Duration

Sunburn can occur in less than 15 minutes. 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.

Protection

Skin

It is advisable to consult a UV index to determine what level of protection is necessary. Potential forms of protection include wearing long-sleeved garments and wide-brimmed hats, and using an umbrella when in the sun. Minimization of sun exposure between the hours of 10 a.m. to 4 p.m. is also recommended. It is important to keep in mind that locations that use daylight saving time can have the most intense rays significantly later than 12 pm. Usually it will be around 1 pm, but in places like western Europe (where standard/winter time is already about an hour ahead of the sun, excluding the UK) DST/Summer Time can make it be later than 2 pm. Commercial preparations are available that block UV light, known as sunscreens or sunblocks. They have a Sunburn 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.

A sunscreen rated SPF10 blocks 90% UVB (but only as long as it did not penetrate into the skin); an SPF20 rated sunscreen blocks 95%. It is best to use a broad spectrum sunscreen to protect against both UVA and UVB radiation. It is prudent to use waterproof formulations if one plans to engage in water-based activities. Modern sunscreens contain filters for UVA radiation as well as UVB. Note that the stated protection factors are only correct 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.

Contrary to the common advice that sunscreen should be reapplied every 2–3 hours, 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 the sun exposure begins. Further reapplication is only necessary 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.

When one is exposed to any artificial source of occupational UV, special protective clothing (for example, welding helmets/shields) should be worn.

There is also evidence that common foods may have some protective ability against sunburn if taken for a period before the exposure. Beta-Carotene and lycopene, chemicals found in tomatoes and other fruit, have been found to increase the skin's ability to resist the effects of UV light. In a 2007 study, after about 10-12 weeks of eating tomato-derived products, a decrease in sensitivity toward UV was observed in volunteers. Ketchup and tomato puree are both high in lycopene. Dark chocolate rich in flavinoids has also been found to have a similar effect if eaten for long periods before exposure.

Eyes

Eyes should not be neglected, and wrap-around sunglasses which block UV light should also be worn. UV light has been implicated in pterygium and cataract development.

Treatment

The most important aspect of sunburn care is to avoid exposure to the sun while healing and to take precautions to prevent future burns. The best treatment for most sunburns is time. Given a few weeks, they will heal; however, there are a number of treatments that help manage the discomfort or facilitate the healing process. Blistered skin, with or without open sores, should heal on its own, but refer to "WebMD: When to see a doctor" for suggestions about whether or not you may need medical attention.

Topical applications

The pain and burning associated with a sunburn can be relieved with a number of different remedies applied to the burn site. The skin can be hydrated by applying topical products containing Aloe vera and/or vitamin E, which reduce inflammation. Hydrocortisone cream may also help reduce inflammation and itching. Avoid the use of petroleum jelly, benzocaine, lidocaine, and butter; these are false remedies which can prevent healing, damage skin, but relieve pain. A dilute solution of white cider vinegar (approx. 1 cup in a tub of water) applied in a similar fashion may also ease pain.

Oral medication

Sunburns can cause headaches or a mild fever in addition to the pain, so an analgesic may be indicated. Acetaminophen relieves pain, and NSAIDs like ibuprofen, naproxen, and aspirin can reduce both pain and inflammation.

Non-human sunburn

Many non-human animals can suffer from sunburn; however, many are protected by a layer of dense fur. Despite myths stating that only hippopotamuses and pigs can be affected by sunburn, almost all animals—even fish, given the right conditions—can suffer sunburn (though pigs and hippopotamuses are more prone due to their hairless skin producing less oil, a natural sun protector). The Tamworth Pig has adapted a special bristle density to minimize sunburn.

Variations in pigment, fur density, and genetic mutations such as albinism can make some individuals within the same species more or less prone to sunburn. Special care must be taken to protect individuals with variations that are more prone.

Sunburn is not limited to animals. Sunburn is a significant and common cause of damage to trees and plants. Plant related sunburn also involves damage to tissue, caused by light from the sun. "Sunscald" on trees is not the same as sunburn on trees. Sunscald is typically a winter or cool season injury to trees. Trunk and branch tissue can be damaged from exposure to sunlight. Damage typically occurs on the west side, to bark (tissue beneath) facing afternoon warm-season sunlight. Bark can fall off, leaving exposed dry wood - clearly seen here where a cavity developed after undamaged tissue continued to grow on either side of the sunburned area. Prevention includes protective trunk cover for newly planted trees, and avoiding excess foliage removal while pruning.

Some nectar producing foliage can suffer sun scorching as the nectar magnifies the sun's rays and can burn through the leaf in certain circumstances.

See also

• Sun unit
• Hyperthermia (heat stroke)
• Windburn

Notes

References

• Agar N, Halliday G, Barnetson R, Ananthaswamy H, Wheeler M, Jones A. The basal layer in human squamous tumors harbors more UVA than UVB fingerprint mutations: a role for UVA in human skin carcinogenesis. Proceedings of the National Academy of Sciences of the USA 6 April 2004;101(14):4954-9.

• Baron E, Fourtanier A, Compan D, Medaisko C, Cooper K, Stevens S. High ultraviolet A protection affords greater immune protection confirming that ultraviolet A contributes to photoimmunosuppression in humans. Journal of Investigative Dermatology October 2003;121(4):869-75.

• Hall H, Saraiya M, Thompson T, Hartman A, Glanz K, Rimer B. Correlates of Sunburn Experiences Among U.S. Adults: Results of the 2000 National Health Interview Survey. Public Health Reports 2003;118.

• Haywood R, Wardman P, Sanders R, Linge C. Sunscreens inadequately protect against ultraviolet-A-induced free radicals in skin: implications for skin aging and melanoma? Journal of Investigative Dermatology October 2003;121(4):862-8.
• NOAA UV-Index Summary with Data Graphs

......

External links

• WebMD: When to see a doctor
• Tips and facts to know before playing in the sun
• Information on Treating Sunburn from The Skin Cancer Foundation
• Sunburn Damage to Trees and Plants: and Prevention
• Avoid sunburn - online sun protection time calculator
• Sun Protection Tips
• Protecting Kids from the Sun
• Medline - Sunburn
• Medline - Sunburn First Aid