Sunburn is a form of radiation burn that affects living tissue, such as skin, that results from an overexposure to ultraviolet (UV) radiation, usually from the Sun. Common symptoms in humans and other animals include red or reddish skin that is hot to the touch or painful, general fatigue, and mild dizziness. Other symptoms include blistering, peeling skin, swelling, itching, and nausea. Excessive UV radiation is the leading cause of (primarily) non-malignant skin tumors, which in extreme cases can be life-threatening. Sunburn is an inflammatory response in the tissue triggered by the nucleic acid damage caused by UV radiation. Sun protective measures like sunscreen and sun protective clothing are widely accepted to prevent sunburn and some types of skin cancer. Special populations, including children, are especially susceptible to sunburn and protective measures should be used to prevent damage.
Signs and symptoms
Sunburn will show as varying degrees of skin redness (erythema) and swelling, depending on the extent and severity of sun exposure. Severe sunburn can cause blistering, itching, peeling skin, rash, and even nausea, fever, chills, and fainting, and so is sometimes informally termed sun poisoning. Capillaries close to the sunburned skin surface produce heat, making the affected area warm to the touch. Sunburns may be classified as superficial or partial-thickness burns. Blistering is a sign of second-degree sunburn.
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 in response to sun exposure. However, the first signs of damage may not appear for a few hours. Sunburn can occur in seconds when exposed to non-shielded welding arcs or other sources of intense ultraviolet light. Sunburn symptoms are typically at their worst 24 to 36 hours after sun exposure and usually begin to improve after 3 to 5 days. After a few days, the skin may start to flake and peel, with full healing typically occurring within 7 days. Small amounts of skin may continue to peel for days or even weeks after.
Skin cancer Ultraviolet radiation causes sunburns and increases the risk of three types of skin cancer: melanoma, basal-cell carcinoma and squamous-cell carcinoma. Of greatest concern is that the melanoma risk increases dose-dependently proportional to the number of a person's lifetime cumulative episodes of sunburn. An estimated 1/3 of melanomas in the United States and Australia could be prevented with regular sunscreen use.
Mechanism Sunburn is caused by prolonged exposure to ultraviolet radiation from the Sun or artificial sources such as tanning lamps, welding arcs, or ultraviolet germicidal irradiation. Two types of ultraviolet radiation from the Sun reach the Earth’s surface, UVA and UVB. UVA is 1,000–2,000 times less effective at causing sunburn than UVB, as it has less energy. UVB damages mRNA. This triggers a fast pathway that leads to inflammation of the skin, and sunburn. mRNA damage initially triggers a response in ribosomes through a protein known as ZAK-alpha in a stress response. This response acts as a cell surveillance system. Following this, detection of RNA damage leads to inflammatory signaling and recruitment of immune cells. RNA damage, but not DNA damage, results in UVB induced skin inflammation and acute sunburn. The stress also 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. By preventing UV photons from disrupting chemical bonds, melanin inhibits both the direct alteration of DNA, as well as the generation of free radicals, to prevent them from indirectly damaging DNA. 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. Sunburn causes an inflammation process that includes the 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). The pain may be caused by the overproduction of a protein called CXCL5, which activates nerve fibers.
Risk factors The most significant risk factor for sunburn is the duration and intensity of UV exposure to the skin. The face, neck and trunk are two to four times more sensitive than the extremities. Additionally, individuals with lighter skin tones or those on specific medications are more susceptible to experiencing sunburn.
UV intensity The UV Index indicates the risk of sunburn at a given time and location. Contributing factors include:
The time of day. In most locations, the sun's rays are strongest between approximately 10 am and 4 pm daylight saving time. Cloud cover. Clouds partially block UV, 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 sunburns. The season of the year. The Sun's position in late spring and early summer can cause a more-severe sunburn. 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 a year. For example, the southern United States gets fifty percent more sunlight than the northern United States.
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