Sunburn

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. Common symptoms in humans and other animals include red or reddish skin that is hot to the touch, pain, 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.^[1][2] 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 photoprotective 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 overly damaged by UV radiation, type I cell-death is triggered and the skin is replaced.^[3]

Signs and symptoms

Blisters on a shoulder caused by sunburn.

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 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 classified as superficial, or partial thickness burns.

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 1 to 3 days, occasionally followed by peeling skin in 3 to 8 days. Some peeling and itching may continue for several weeks.

Skin cancer

Ultraviolet radiation causes sunburns and increases the risk of three types of skin cancer: melanoma, basal-cell carcinoma and squamous-cell carcinoma.^[1][2][4] 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.^[5]

Causes

Biological

The cause of sunburn is the direct damage that a UVB photon can induce in DNA (left). One of the possible reactions from the excited state is the formation of a thymine-thymine cyclobutane dimer (right).

Sunburn is caused by UV radiation, either from the sun or from artificial sources, such as tanning lamps, welding arcs, or ultraviolet germicidal irradiation. It is a reaction of the body to direct DNA damage from 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, apoptosis and peeling to remove irreparably damaged skin cells, and increased melanin production to prevent future damage. Melanin readily absorbs UV wavelength light, acting as a photoprotectant. By preventing UV photons from disrupting chemical bonds, melanin inhibits both the direct alteration of DNA and the generation of free radicals, thus indirect DNA damage.

Sunburn causes an inflammation process, including production of prostanoids and bradykinin. These chemical compounds increase sensitivity to heat by reducing the threshold of heat receptor (TRPV1) activation from 109 °F (43 °C) to 85 °F (29 °C).^[6] The pain may be caused by overproduction of a protein called CXCL5, which activates nerve fibres.^[7]

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. Children younger than six and adults older than sixty are more sensitive to sunlight.^[8]

Medications

The risk of a sunburn can be increased by pharmaceutical products that sensitize users to UV radiation. Certain antibiotics, oral contraceptives, and tranquillizers have this effect.^[9]

UV intensity

The UV Index indicates the risk of getting a sunburn at a given time and location. Contributing factors include:^[8]

1. The time of day. In most locations, the sun's rays are strongest between approximately 10am and 4pm daylight saving time.
2. Cloud cover. 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.^[10][11]
3. Proximity to reflective surfaces, such as water, sand, concrete, snow, and ice. All of these reflect the sun's rays and can cause sunburns.
4. The season of the year. The position of the sun in late spring and early summer can cause a more-severe sunburn.
5. 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.
6. Proximity to the equator (latitude). Between the polar and tropical regions, the closer to the equator, the more direct sunlight passes through the atmosphere over the course of a year. For example, the southern United States gets fifty percent more sunlight than the northern United States.

Erythemal dose rate at three Northern latitudes. (Divide by 25 to obtain the UV Index.) Source: NOAA.

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 substantially 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 most directly through the atmosphere. Regardless of one's latitude (assuming no other variables), equal shadow lengths mean equal amounts of UV radiation.

The skin and eyes are most sensitive to damage by UV at 265–275 nm wavelength, 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.

Ozone depletion

In recent decades, the incidence and severity of sunburn has increased worldwide, partly because of chemical damage to the atmosphere's ozone layer. Between the 1970s and the 2000s, average stratospheric ozone decreased by approximately 4%, contributing an approximate 4% increase to the average UV intensity at the earth's surface. Ozone depletion and the seasonal "ozone hole" have led to much larger changes in some locations, especially in the southern hemisphere.^[12]

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.^[13] This has led to an overall increase in exposure to UV radiation from both the natural sun and tanning lamps. Suntans can provide a modest sun protection factor (SPF) of 3, meaning that tanned skin would tolerate up to three times the UV exposure as pale skin.^[14]

Prevention

Skin

Sunburn effect (as measured by the UV Index) is the product of the sunlight spectrum at the earth's surface (radiation intensity) and the erythemal action spectrum (skin sensitivity). Long-wavelength UV is more prevalent, but each milliwatt at 295 nm produces almost 100 times more sunburn than at 315 nm.

Sunburn peeling. The destruction of lower layers of the epidermis causes rapid loss of the top layers.

Another sunburn peeling.

The most effective way 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 (including a wide-brim hat), and to use sunscreen.^[15] 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 noon, but often one to two hours later.

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. 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.^[16] Broad-spectrum sunscreens contain filters that protect against UVA radiation as well as UVB. Although UVA radiation does not primarily cause sunburn, it does contribute to skin aging and an increased risk of skin cancer.

Research has shown that the best sunscreen 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.^[17] 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^[18] and squamous cell carcinoma.^[19] There is little evidence that it is effective in preventing basal cell carcinoma.^[20] 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.^[21] The U.S. Preventive Services Task Force recommends that people between 9 and 25 years of age be advised to avoid ultraviolet light.^[22] Typical use of sunscreen does not usually result in vitamin D deficiency, but extensive usage may.^[23]

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.

Eyes

The eyes are also sensitive to sun exposure at about the same UV wavelengths as skin; snow blindness is essentially sunburn of the cornea. Wrap-around sunglasses or the use by spectacle-wearers of glasses that block UV light reduce the harmful radiation. UV light has been implicated in the development of age-related macular degeneration,^[24] pterygium^[25] and cataract.^[26] Concentrated clusters of melanin, commonly known as freckles, are often found within the iris.

Diet

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, in both 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.^[27] 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.^[28] There is also evidence that common foods may have some protective ability against sunburn if taken for a period before the exposure.^[29]

Treatment

The primary measure is avoiding further exposure to the sun. The best treatment for most sunburns is time. Most sunburns heal completely within a few weeks.

Non-steroidal anti-inflammatory drugs (such as ibuprofen or naproxen), and aspirin may decrease redness and pain.^[30][31] Local anesthetics such as benzocaine, however, are contraindicated.^[32] Schwellnus et al. states that topical steroids (such as hydrocortisone cream) do not help with sunburns,^[31] although the American Academy of Dermatology says they can be used on especially sore areas.^[32]

Home treatments that may help the discomfort include using cool and wet cloths on the sunburned areas.^[31] Applying soothing lotions that contain aloe vera to the sunburn areas was supported by one review.^[33] Others have found aloe vera to have no effect.^[31] Aloe vera has no ability to protect people from sunburns.^[34] Another treatment includes using a moisturizer that contains soy.^[32]

References

External links

• Sunburn at DMOZ