Sunburn is a form of radiation burn that affects living tissue, such as skin.

Sunburn is a result of an overexposure to ultraviolet (UV) radiation, usually from the sun. 

Common findings include: red or reddish skin that is hot to the touch or painful, general fatigue, mild dizziness,blistering, peeling skin, swelling, itching, and nausea. 

Excessive UV radiation is the leading cause of non-malignant skin tumors.

In extreme exposure it can be life-threatening. 

Sunburn is an inflammatory response in the tissue triggered by direct DNA damage by UV radiation. 

A sunburned cell is a keratinocyte with a pyknotic nucleus and eosinophilic cytoplasm that appears after exposure to UVC or UVB radiation or UVA in the presence of psoralens. 

Cells show premature and abnormal keratinization, and has been described as an example of apoptosis.

When cells’ DNA is overly damaged by UV radiation, cell-death is triggered and the tissue is replaced.

Risk factors:

Working outdoors, skin unprotected by clothes or sunscreen, skin type, age

Prevention by use of sunscreen, sun protective clothing.

Sunburn typically begins as redness, followed by varying degrees of pain, proportional in severity to both the duration and intensity of exposure.

Other symptoms include:  blistering, edema, pruritus, peeling of skin, rash, nausea, fever, chills, and fainting.

A small amount of heat is given off from the burn, caused by the concentration of blood in the healing process.

Sunburns are classified as superficial, or partial thickness burns. 

Blistering is a sign of second degree sunburn.

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. 

After sun the exposure, skin may turn red in as little as 30 minutes,  but most often takes 2 to 6 hours. 

Pain is usually strongest 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.

Ultraviolet radiation caused  sunburn increases the risk of three types of skin cancer: melanoma, basal-cell carcinoma and squamous-cell carcinoma.

Melanoma risk increases in a dose-dependent manner with the number of a person’s lifetime cumulative episodes of sunburn.

It has been estimated that over 1/3 of melanomas in the United States and Australia could be prevented with regular sunscreen use.

The cause of sunburn is the direct damage that a UVB photon can induce in cellular DNA.

Sunburn is the body‘s reaction to direct DNA damage from UVB light.

Sunburn is caused by UV radiation from the sun, but may be a result of artificial sources, such as tanning lamps, welding arcs, or ultraviolet germicidal irradiation. 

The damage from sunburn is mainly the formation of a thymine dimer. 

DNA damage by sunburn leads to defense mechanisms: DNA repair to revert the damage, apoptosis and peeling that removes irreparably damaged skin cells, and increased melanin production to prevent future sun damage.

Melanin’s role in skin : absorbs UV wavelength light, and acting as a photoprotectant. 

Melanin prevents UV photons from disrupting chemical bonds, and inhibits both the direct alteration of DNA and the generation of free radicals, thus indirect DNA damage. 

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 with production of prostanoids and bradykinin, which increase sensitivity to heat by reducing the threshold of heat receptor (TRPV1) activation.

Sunburn pain may be caused by overproduction of CXCL5 protein which activates nerve fibers.

Skin type is a determinant of  the ease of sunburn. 

Individuals with a lighter skin tone have limited capacity to develop a tan after UV radiation exposure and have a greater risk of sunburn. 

The Fitzpatrick’s Skin phototypes classification describes the normal variations of skin responses to UV radiation. 

Patients with type I skin have the greatest capacity to sunburn.

Individuals with type VI have the least capacity to burn. 

All skin types can develop sunburn.

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 affects how skin reacts to sun: Children younger than six and adults older than sixty are more sensitive to sunlight.

Genetic conditions (xeroderma pigmentosum) can  increase a person’s susceptibility to sunburn and subsequent skin cancers by manifesting defects in DNA repair mechanisms which in turn decreases the ability to repair DNA that has been damaged by UV radiation.

Sunburn can be increased by pharmaceutical products that sensitize users to UV radiation: Certain antibiotics, oral contraceptives, antidepressants, acne medications, and tranquillizers.

The UV Index indicates the risk of getting a sunburn at a given time and location. 

The sun’s  rays are strongest between approximately 10 am and 4 pm daylight saving time.

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 can reflect the sun’s rays and can cause sunburns.

The position of the sun in late spring and early summer can cause a more-severe sunburn.

With 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.

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: the southern United States gets fifty percent more sunlight than the northern United States.

The variations in the intensity of UV radiation passing through the atmosphere cause the risk of sunburn to  increase with proximity to the tropic latitudes, located between 23.5° north and south latitude. 

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. 

The amount of UV radiation is dependent on the angle of the sun. 

The greatest risk is at solar noon, when shadows are at their minimum and the sun’s radiation passes most directly through the atmosphere. 

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.

This  lower UVC band that is almost never encountered except from artificial sources like welding arcs. 

Most sunburn is caused by longer wavelengths,

The  incidence and severity of sunburn has increased worldwide, partly because of chemical damage to the atmosphere’s ozone layer. 

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.

Suntans as a form of sun protection is not recommended.

Sunburns associated with indoor tanning can be severe.

Avoiding artificial UV sources such as tanning beds is recommended.

Differential diagnosis: skin pathology induced by UV radiation including photoallergic reactions, phototoxic reactions to topical or systemic medications, and other dermatologic disorders that are aggravated by exposure to sunlight. 

Diagnostic considerations include: duration and intensity of UV exposure, use of topical or systemic medications, history of dermatologic disease, and nutritional status.

Phototoxic reactions are non-immunological response to sunlight interacting with certain drugs and chemicals in the skin which resembles an exaggerated sunburn. 

Phototoxic reaction to drugs include:amiodarone, dacarbazine, fluoroquinolones, 5-fluorouracil, furosemide, nalidixic acid, phenothiazines, psoralens, retinoids, sulfonamides, sulfonylureas, tetracyclines, thiazides, and vinblastine.

Photoallergic reactions: Uncommon immunological response to sunlight interacting with certain drugs and chemicals in the skin. 

When in excited state by UVR, these drugs and chemicals form free radicals that react to form functional antigens and induce a Type IV hypersensitivity reaction. 

These drugs include 6-methylcoumarin, aminobenzoic acid and esters, chlorpromazine, promethazine, diclofenac, sulfonamides, and sulfonylureas. 

Unlike phototoxic reactions which resemble exaggerated sunburns, photoallergic reactions can cause intense itching and can lead to thickening of the skin.

Phytophotodermatitis occurs when UV radiation induces inflammation of the skin after contact with certain plants: limes, celery, and meadow grass.

It causes pain, redness, and blistering of the skin in the distribution of plant exposure.

Polymorphic light eruption is recurrent reaction to UVR, that can preset with pink-to-red bumps, blisters, plaques and urticaria.

Solar urticaria: UVR-induced wheals that occurs within minutes of exposure and fades within hours.

Several dermatologic conditions can be exacerbated in severity with exposure to UVR: systemic lupus erythematosus, dermatomyositis, acne, atopic dermatitis, and rosacea.

Sunburn’s  effects are the product of the sunlight spectrum at the earth’s surface and the skin sensitivity.

Long-wavelength UV is more prevalent, but each milliwatt at 295 nm produces almost 100 times more sunburn than at 315 nm.

The most effective way to prevent sunburn is to reduce the amount of UV radiation reaching the skin. 

The following measures to prevent excessive UV exposure and skin cancer are recommended:

Limiting sun exposure between the hours of 10 am and 4 pm, when UV rays are the strongest.

Seeking shade when UV rays are most intense

Wearing sun-protective clothing including a wide brim hat, sunglasses, and tightly-woven, loose-fitting clothing

Using sunscreen

Avoiding tanning beds and artificial UV exposure

The strength of sunlight is published as a UV Index. 

Sunlight is generally strongest when the sun is close to the highest point in the sky. 

Seeking shade including using umbrellas and canopies can reduce the amount of UV exposure, but does not block all UV rays. 

Short shadow, seek shade to prevent sunburn.

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 sun protectant factor (SPF) is correct only if 2 mg 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.

Sunscreens function as chemicals such as oxybenzone and dioxybenzone (organic sunscreens) or opaque materials such as zinc oxide or titanium oxide (inorganic sunscreens) that both mainly absorb UV radiation. 

Chemical and mineral sunscreens vary in the wavelengths of UV radiation blocked. 

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.

Sunscreen is effective and thus recommended for preventing melanoma and squamous cell carcinoma.

There is little evidence that sunscreen is effective in preventing basal cell carcinoma.

Typical use of sunscreen does not usually result in vitamin D deficiency.

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