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. Sunburn can be life-threatening and is a leading cause of skin cancer.[1][2] Sunburn can easily be prevented through the use of sunscreen, clothing (and hats), and by limiting solar exposure, especially during the middle of the day. The only cure for skin burn is slow healing, although skin creams can help.

The condition occurs when incident UV radiation exceeds the protective capacity of melanin in the skin. Concentrations of this pigment vary greatly among individuals, but in general, darker-skinned people have more melanin than those with lighter skin. Correspondingly, the incidence of sunburn among dark-skinned individuals is lower.

The sun is not the only origin — a similar burn can be produced by overexposure to other sources of UV such as from tanning lamps, or occupationally, such as from welding arcs.

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, 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. Nevertheless, the inflicted harm is often not immediately obvious.

After the exposure, skin may turn red in 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.

UV radiation is divided into the UVA (380–315 nm), UVB (315–280 nm) and UVC (280-180 nm) sub-bands. Ozone in the Earth's atmosphere filters out a portion of this before it reaches the planet's surface. UVC is almost entirely eliminated by the atmosphere, but enough UVA and UVB penetrates it in large enough quantities that sunburn occurs.

With respect to the spectral components of sunlight, the severity of sunburn has been found to peak in the low-frequency UVB range near the 320 nm transition to UVA. This is based on two factors:

erythemal activity - the specific effect of different wavelengths of radiation on the skin
solar irradiance - how much of any solar radiation wavelength can be expected to be incident on the surface of the earth
The resulting erythemal irradiance metric is calculated by weighting measurements of solar irradiance with experimental measurements of erythemal activity. When this product is graphed, there is a peak at 308 nm.
At the cellular level, UVB light causes DNA damage which may be passed onto subsequent generations of a cell's progeny, leading to increased risk of skin cancer. Damaged cells die and release toxins which are responsible for nausea and fever. If many die, peeling may result.

The risk of sunburn increases with proximity to the earth's equator.[3] It can also be increased by pharmaceutical products that sensitise some users to UV radiation. Certain antibiotics, contraceptives, and tranquillizers have this effect.[4] People with red hair and/or freckles generally have a greater risk of sunburn than others because of their lighter skin tone.[3]

Suntans, which naturally develop in some individuals as a protective mechanism against the sun, are viewed by many in the Western world as desirable.[5] This has led to increases in sunburn incidences and in solarium popularity as individuals attempt to tan.

A 2003 study found that 36% of US adults have a sunburn at least once a year; one study found 50% of Canadians to have been burnt in the two months before the study.[citation needed]

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 [6]. 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.[7]

No one is immune from sun-induced skin cancer, but there are several factors that dramatically increase the risk. Fair-skinned individuals are the most prone to sun damage, as are people taking medications that contraindicate sun exposure.

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).