The cause of<unk>  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).<unk>  is caused by UV radiation from the sun, but<unk> " may result 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. However, human melanocytes contain over 2,000 genomic sites that are highly sensitive to UV, and such sites can be up to 170-fold more sensitive to UV induction of cyclobutane pyrimidine dimers than the average site These sensitive sites often occur at biologically significant locations near genes.<unk>  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 to . The pain may be caused by overproduction of a protein called CXCL5, which activates nerve fibres. Skin type determines the ease of<unk> . In general, people with lighter skin tone and limited capacity to develop a tan after UV radiation exposure have a greater risk of<unk> . The Fitzpatrick's Skin phototypes classification describes the normal variations of skin responses to UV radiation. Persons with type I skin have the greatest capacity to<unk>  and type VI have the least capacity to burn. However, all skin types can develop<unk> . Fitzpatrick's skin phototypes: * Type 0: Albino *Type I: Pale white skin, burns easily, does not tan * Type II: White skin, burns easily, tans with difficulty * Type III: White skin, may burn but tans easily * Type IV: Light brown/olive skin, hardly burns, tans easily * Type V: Brown skin, usually does not burn, tans easily * Type VI: Black skin, very unlikely to burn, becomes darker with UV radiation exposure Age also affects how skin reacts to sun. Children younger than six and adults older than sixty are more sensitive to sunlight. There are certain genetic conditions, for example xeroderma pigmentosum, that increase a person's susceptibility to<unk>  and subsequent skin cancers. These conditions involve defects in DNA repair mechanisms which in turn decreases the ability to repair DNA that has been damaged by UV radiation. Medications The risk of a<unk>  can be increased by pharmaceutical products that sensitize users to UV radiation. Certain antibiotics, oral contraceptives, antidepressants, acne medications, and tranquillizers have this effect. UV intensity The UV Index indicates the risk of getting a<unk>  at a given time and location. Contributing factors include: # The time of day. In most locations, the sun's rays are strongest between approximately 10am and 4pm daylight saving time. # 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. # Proximity to reflective surfaces, such as water, sand, concrete, snow, and ice. All of these reflect the sun's rays and can cause<unk> s. # The season of the year. The position of the sun in late spring and early summer can cause a more-severe<unk> . # 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. # 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<unk>  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 (if the height is measured vertical to the earth's gravitational field, the projected shadow is ideally measured on a flat, level surface; furthermore, for objects wider than skulls or poles, the height and length are best measured relative to the same occluding edge). 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<unk>  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<unk>  have 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. 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. 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.<unk> s associated with indoor tanning can be severe. The World Health Organization, American Academy of Dermatology, and the Skin Cancer Foundation recommend avoiding artificial UV sources such as tanning beds, and do not recommend suntans as a form of sun protection.
Sunburn
Windburn
Grocer's itch
Gangrene
Swimmer's itch
Photokeratitis
Pachyderma
Melasma