Sunburn is a radiation burn to the skin caused by too much exposure to the sun’s ultraviolet (UV) rays or artificial sources such as tanning beds. The biggest risk factors for sunburn is the amount of time the skin is exposed to UV rays, plus the intensity. Many factors such as time of day, medications, ozone depletion, high altitude, clear skies, and skin phototypes influence sunburns. An increase in the number of sunburns someone obtains is directly related to an increase in the risk of skin cancer. Therefore, by fully understanding the cause, treatment, and prevention of sunburns, patients can drastically decrease their risk of skin cancers. This will improve their quality of life. Additionally, by preventing sunburn, patients can reduce the solar effects of aging, which can lead to better cosmetic results.

Sunburn is caused by too much exposure to the ultraviolet radiation from the sun or similar artificial sources such as tanning beds. Many factors contribute to facility and severity of sunburn:

1. Medications: Sunburn risk is increased by tetracyclines (especially doxycycline), thiazide diuretics, sulfonamides, fluoroquinolones, nonsteroidal anti-inflammatory drugs, retinoids, and St. John's wort amongst other photosensitizing medication [1].
2. Increased UV index:
• The time between 10 am and 4 pm is when the sun's rays are at their strongest.
• Decreased cloud coverage corresponds with increased exposure to stronger UV rays.
• Higher altitudes are correlated with an increased risk of sunburn due to a smaller layer of the earth's atmospheric protection. 
• Proximity to the equator is correlated with more direct UV ray exposure. 
3. Ozone depletion: Certain areas of the world have decreased ozone or holes in the ozone layer. This equates to increased penetrance of the sun's UV rays.
4. Fitzpatrick skin phototype: Please see epidemiology for more information.  The lighter the skin color, the easier it is to sunburn.
5. Tanning: Tanning or spending increased amounts of time in the sun to acquire darker skin, has been a popular American pastime for decades. Tanning increases the risk of skin cancers and accelerated skin aging. Rapid tanning may cause sunburn. 

According to a cross-sectional study using a nationally representative sample of 31,162 US adults from the 2015 National Health Interview Survey, 34% of respondents reported having at least 1 sunburn in 2015. Prevalence was highest among Fitzpatrick skin types I to III, those in younger age groups (adults 18 to 29 years old), and non-Hispanic white individuals. Additionally, individuals who used tanning lotions, engaged in physical activity, binge drink, and are overweight were also more likely to get sunburned.[2]

Fitzpatrick's Skin Phototypes

• 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[3]

As mentioned above, individuals with Type I to III Fitzpatrick skin phototypes were at increased risk for sunburn. This is due to decreased melanin pigment in the skin that blocks UV radiation. People with a lower Fitzpatrick skin phototype have a lower MED, or minimal erythema dose, the amount of UV radiation measured in energy per unit area that leads to erythema of non-diseased skin.[4][5]

UVA and UVB rays both play a role in sunburn, though UVB rays are responsible for directly damaging DNA by inducing the formation of thymine-thymine cyclobutane dimers.[6] When these dimers are formed, the body generates a DNA repair response, which includes the induction of apoptosis of cells and the release of inflammatory markers such as prostaglandins, reactive oxygen species, and bradykinin.[7] This leads to vasodilation, edema, and pain which translates into the classically red, painful skin seen in a sunburn. Additionally, skin exposure to UVB causes an increase in chemokines such as CXCL5 and activates peripheral nociceptors, which results in over-activation of the pain receptors of the skin.[8][9]

There are major changes that occur at all levels of the skin that characterize the histology of sunburn, particularly in the epidermis and dermis. In the epidermis, loss of Langerhans cells and vacuolated keratinocytes were noted in sunburned skin cells. The dermis experienced vascular changes that could be seen within thirty minutes of exposure to radiation that included enlargement of endothelial cells and edema caused by mast cell degranulation. Histamine and prostaglandin E2 levels rose 4-fold after the exposure of UV radiation, providing evidence that histamine plays a role in a sunburn reaction of the skin. This was all reversed after 24 hours from the exposure of UV radiation.[10]