Background
Sunburn is an acute cutaneous inflammatory reaction that follows excessive exposure of the skin to ultraviolet (UV) radiation (UVR). UVR exposure can come from a variety of sources, including sun, tanning beds, phototherapy lamps, and arc lamps. [1] Long-term adverse health effects of repeated exposure to UVR are well described but are beyond the scope of this article. Most sunburns are classified as superficial or first-degree burns. (See the image below.)
Pathophysiology
Exposure to solar radiation has the beneficial effects of stimulating the cutaneous synthesis of vitamin D and providing radiant warmth. Unfortunately, when the skin is subjected to excessive radiation in the UV range, deleterious effects may occur. The most conspicuous is acute sunburn or solar erythema. [2]
The principal injury responsible for sunburn is direct damage to DNA by UVR, resulting in inflammation and apoptosis of skin cells. [3] Sunburn inflammation causes vasodilation of cutaneous blood vessels, resulting in the characteristic erythema.
Within 1 hour after UVR exposure, mast cells release preformed mediators, including histamine, serotonin, and tumor necrosis factor (TNF), leading to prostaglandin and leukotriene synthesis. [2, 4] Cytokine release additionally contributes to the inflammatory reaction, leading to an infiltrate of neutrophils and T lymphocytes. [5] Within 2 hours after UV exposure, damage to epidermal skin cells is seen. Both epidermal keratinocytes ("sunburn cells") and Langerhans cells undergo apoptotic changes as a consequence of UVR-induced DNA damage (see the image below). [6, 7] Erythema usually occurs 3-4 hours after exposure, with peak levels at 24 hours. [8]
Note apoptotic sunburn cells in the epidermis. Image from David Shum, MD, Division of Dermatology, University of Western Ontario.
UVR exposure that is less intense or of shorter duration exposure results in an increase in skin pigmentation, known as tanning, which provides some protection against further UVR-induced damage. [9] The increased skin pigmentation occurs in the following two phases:
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Immediate pigment darkening - This occurs during exposure to UVR and results from alteration of existing melanin (oxidation, redistribution); it may fade rapidly or persist for several days
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Delayed tanning - This results from increased synthesis of epidermal melanin and requires a longer period of time to become visible (24-72 h)
With repeated exposure to UVR, the skin thickens, primarily through epidermal hyperplasia with thickening of the stratum corneum. UVR exposure also suppresses cutaneous cell-mediated immunity, which might contribute to nonmelanoma skin cancer and certain infections. [2]
Etiology
Sunburn is caused by excessive exposure of the skin to UVR. The UV spectrum can be divided into the following segments [1] :
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Ultraviolet A (UVA)-I, 340-400 nm
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UVA-II, 320-340 nm
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Ultraviolet B (UVB), 290-320 nm
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Ultraviolet C (UVC), 200-290 nm
Solar UVR of wavelengths shorter than 290 nm is filtered out or absorbed in the outer atmosphere and is not encountered at sea level. [1] Shorter-wavelength UVB rays are much more effective at inducing erythema than UVA rays are and thus are the principal cause of sunburn. [1, 10] However, UVA constitutes the majority of the UVR reaching the surface of the earth (~95-98% at midday) and therefore accounts for a significant percentage of the immediate and long-term cutaneous effects of UVR. [1]
The minimal single dose of UVR (energy per unit area) required to produce erythema after 24 hours at an exposed site is known as the minimal erythema dose (MED). This dose varies according to skin type. [2]
Multiple factors influence UVR-induced erythema, including the following [2, 11, 12, 13] :
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Wavelength - UVB is more erythemogenic than UVA
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Skin type/pigmentation - Compared with patients who have type I-II skin, patients who have type IV-V skin require three to five times more UVR exposure to cause erythema
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Hydration - UVR causes erythema more effectively in moist skin than in dry skin
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Environmental reflection - Radiation is 80% reflected by snow and ice but only 15% reflected by sand
-
Ozone coverage - Increased levels of ozone filter out more UVR
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Altitude - Thinner atmosphere at higher altitudes absorbs less UVR
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Latitude - Exposure is greater nearer the equator
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Time of day - UVR exposure is greatest from 10:00 AM to 4:00 PM, when the sun is highest in the sky
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Season - In locations outside the tropics, UVR is much greater in summer than in winter
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Cloud cover - Light clouds attenuate UVR by 10%, which may not be enough to protect from sunburn
Epidemiology
US and international statistics
Previous reports have stated that about one third of US adults have a sunburn each year, [14, 15] and about two thirds of US children have a sunburn each summer. [16] The US Centers for Disease Control and Prevention (CDC) reported in 2012 that just over 50% of all adults reported at least one sunburn in the past 12 months and that just over 65% of whites aged 18-29 years reported at least one sunburn in the past 12 months. [17] Rural US residents may be at higher risk than urban residents. [18]
Risk of sunburn is increased in regions that are closer to the equator and that are higher in altitude. [13]
Age-, sex-, and race-related demographics
Sunburn is more common in children than in adults. [16] Easy sunburning during infancy may indicate a serious underlying disease, such as porphyria or xeroderma pigmentosum. Referral for further evaluation is prudent.
Surveys of US adults show that men have a slightly higher prevalence of sunburn than women. [14]
Lighter-skinned individuals are affected more frequently and severely. Skin types are traditionally classified into one of six Fitzpatrick categories, based on an individual's tendency to tan, to burn, or both (see Table 1 below). [2, 11, 19]
Table 1. Fitzpatrick Skin Types and Recommended Sunscreen Sun Protection Factor (SPF) Levels (Open Table in a new window)
Skin Type |
Description |
Skin Color |
Routine SPF |
SPF for Outdoor Activity |
I |
Always burns, never tans |
White |
15 |
25-30 |
II |
Always burns, tans minimally |
White |
12-15 |
25-30 |
III |
Burns minimally, tans slowly |
White |
8-10 |
15 |
IV |
Burns minimally, tans well |
Olive |
6-8 |
15 |
V |
Rarely burns, tans profusely/darkly |
Brown |
6-8 |
15 |
VI |
Rarely burns, always tans |
Black |
6-8 |
15 |
Prognosis
Uncomplicated sunburn is associated with minimal short-term morbidity. Most cases resolve spontaneously with no significant sequelae. In rare cases, sunburn may be so severe and diffuse that it results in second-degree burns, dehydration, or secondary infection. [8]
Morbidity and mortality associated with long-term sun exposure are related primarily to the development of cutaneous neoplasms, including basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and malignant melanoma. [2] For more information on skin cancers, see the Skin Cancer Resource Center.
Patient Education
Patients should be educated regarding the short- and long-term complications of sunburn (see Complications). They should also be instructed in means of preventing sunburn (see Prevention). [10] There is clearly a need for better education in this area. A narrative review of 24 studies addressing skin cancer–related knowledge, attitudes, beliefs, and prevention practices among beachgoers and sunbathers found that 21-22.8% did not use sun protection and that 62% had experienced a blistering sunburn at some point. [20]
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Note apoptotic sunburn cells in the epidermis. Image from David Shum, MD, Division of Dermatology, University of Western Ontario.
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Acute sunburn on face of 15 year-old female after soccer match.
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Subacute sunburn on shoulder of 21-year-old male, with peeling.
Tables
Skin Type |
Description |
Skin Color |
Routine SPF |
SPF for Outdoor Activity |
I |
Always burns, never tans |
White |
15 |
25-30 |
II |
Always burns, tans minimally |
White |
12-15 |
25-30 |
III |
Burns minimally, tans slowly |
White |
8-10 |
15 |
IV |
Burns minimally, tans well |
Olive |
6-8 |
15 |
V |
Rarely burns, tans profusely/darkly |
Brown |
6-8 |
15 |
VI |
Rarely burns, always tans |
Black |
6-8 |
15 |
