Sunburn

# Overview
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. 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 sunburn is slow healing, although skin creams can help.

# Cause
The condition occurs when incident UV radiation exceeds the existing 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.

# 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, giving a warm feeling to the affected area. Sunburns may be first- or second-degree 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. 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.

# UVA and UVB
UV radiation is divided into the UVA (380315 nm), UVB (315280 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 cells die, peeling may result.

# Increased risk
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 and 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 lower the latitude, the greater the risk.  In the late spring and early summer, higher latitudes have many hours of daylight, which partially compensates for the less direct sunlight in this region if exposed all day. People can still receive severe sunburns at these higher latitudes.
On a minute by minute basis though, 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 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.
Sunburn can also be caused by pharmaceutical products that sensitise some users to UV radiation. Certain antibiotics, 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.
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 increases in sunburn incidences and in solarium popularity as individuals attempt to tan.
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.
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).

# 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 3 p.m. is also recommended.
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 reduce the UVB radiation at the skin: The higher the SPF rating, the greater the protection. A sunscreen rated SPF15 blocks 93.3% UVB; an SPF30 rated sunscreen blocks 96.7%. 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. The best sunscreens attenuate UVA radiation as well as UVB. Note that the stated protection factors are only correct if 2l 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 23 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.

## 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
There is no immediate cure for sunburns, but the pain can be relieved by hydrating the skin. This is done by applying products containing aloe, vitamin E, or both. Vinegar is a remedy for the stinging sensation on a burn and any products with lidocaine can prevent healing and damage skin. Drinking fluids can aid in hydration, and eating high protein foods will assist tissue repair. Analgesics such as acetaminophen (e.g. Tylenol) or ibuprofen (e.g. Advil) can also reduce pain. One method to treatment involves applying a clean washcloth soaked with cool milk like a cold compress, since the lactic acid will help reduce inflammation and the cool towel will soothe the pain.
Blistered skin, with or without open sores, should heal on its own. As with any other open skin wound, it is best to avoid lotions or other directly applied ointments. However, antibacterial solutions and gauze can prevent skin infections.
The best treatment for most sunburns is time. Given a few weeks, they will heal. Overall, the most important aspect of sunburn care is to avoid the sun while healing, and to take precautions to prevent future burns.

# Skin cancer
The more critical and long-term danger posed by sunburn is an increased risk of future skin cancer, which is believed to be highly correlated. One incident of blistering sunburn doubles the risk of malignant melanoma .  But while sunburn severity gives an indication of short-term radiation over-exposure, there is also deeper penetration by UVA that occurs in the absence of perceptible symptoms.  UVB was thought to be the sole causative agent in skin cancer, but there is a growing body of evidence to support the theory that both UVA and UVB are implicated.

# 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 animalseven fish, given the right conditionscan 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 humans and other 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.

# Related Chapters
- Hyperthermia (heat stroke)
- Windburn