Sunburn

A sunburn is a burn to living tissue, such as skin, which is produced by overexposure to ultraviolet (UV) radiation, commonly from the sun's rays. Usual mild 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, although some scientists argue that it may not effectively protect against malignant melanoma, which either is caused by a different part of the ultraviolet spectrum or is not caused by sun exposure at all. 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 skin 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. Malignant melanoma may occur as a result of indirect DNA damage if the damage is not properly repaired. Proper repair occurs in the majority of DNA damage. The only cure for sunburn is slow healing, although some skin creams can help with the symptoms.

Cause
Sunburn is caused by UV radiation, either from the sun or from artificial sources, such as welding arcs, the lamps used in sunbeds, 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 UV-B 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 transforms UV-photons quickly into harmless amounts of heat without generating free radicals, and is therefore an excellent photoprotectant against direct and indirect DNA damage.

The pain may be caused by overproduction of a protein called CXCL5, which activates nerve fibres.

It has been shown that protection 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 dangerous sunburns and increases the risk of two types of skin cancer: basal-cell carcinoma and squamous cell carcinoma.

Controversy over sunscreen
The statement sunburn causes skin cancer is accurate when it refers to either basal-cell carcinoma, the mildest form of cancer, or squamous cell carcinoma. But the statement 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 &mdash; UV radiation. However, they are generated via two different mechanisms: Direct DNA damage 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 versus indirect DNA damage.

Topically applied sunscreen blocks UV rays as long as it does not penetrate into the skin. This prevents sunburn, suntanning, and skin cancer. If the sunscreen filter is absorbed into the skin, it prevents sunburn, but increases the amount of free radicals, which in turn increases the risk for malignant melanoma. The harmful effect of photo-excited sunscreen filters on living tissue has been shown in many photo-biological studies. Whether sunscreen prevents or promotes the development of melanoma depends on the relative importance of the protective effect from the topical sunscreen versus the harmful effects of the absorbed sunscreen.

The use of sunscreen is known to prevent the direct DNA damage that causes 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 indirect DNA damage, which causes the most lethal form of skin cancer, malignant melanoma. This form of skin cancer is rare, but it causes 75% of all skin cancer-related deaths. Increased risk of malignant melanoma in sunscreen users has been the subject of many epidemiological studies.

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. The higher the latitude, the lower the intensity of the UV rays. 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.

Pharmaceutical products
Sunburn can also be caused by pharmaceutical products that sensitize some users to UV radiation. Certain antibiotics, oral contraceptives, and tranquillizers have this effect. In general, people with fair hair and/or freckles 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 have led to dangerously high levels of UV radiation. 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 many in the Western world as desirable. This has led to increased exposure to UV radiation from both the natural sun and solaria.

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



Other symptoms are edema, itching, peeling skin, rash, nausea, fever, 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.

One should immediately speak to a dermatologist if one develops a skin lesion that has an asymmetrical form, has darker edges than center, changes color, or becomes larger than 1/4 inch (6 mm). (see Melanoma)

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, 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.

Skin


In order to prevent sunburn, the amount of UV radiation reaching the skin must be reduced. The strength of sunlight is published in many locations as a UV index. The World Health Organization recommends to limit time in the midday 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.



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 as SPF 10 blocks 90% of the sunburn-causing UVB radiation; an SPF20-rated sunscreen blocks 95%. 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 &mdash; 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 flavonoids has also been found to have a similar effect if eaten for long periods before exposure.

Eyes
The eyes are also sensitive to sun exposure, and wrap-around sunglasses or glasses that block UV light should also be worn. 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.

Treatment
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 unstoppable. However, lotion may relieve the itching. Acetaminophen (such as Tylenol), Nonsteroidal anti-inflammatory drugs (such as Ibuprofen or Naproxen), and Aspirin have all shown to reduce the pain of sunburns.