The skin may respond to sunlight with acute (eg, photosensitivity, sunburn) or chronic (eg, dermatoheliosis [photoaging], actinic keratosis) changes.
Ultraviolet (UV) radiation
The sun emits a wide range of electromagnetic radiation. Most of the dermatologic effects of sunlight are caused by UV radiation, which is divided into 3 bands: UVA (320 to 400 nm), UVB (280 to 320 nm), and UVC (100 to 280 nm). Because the atmosphere filters the radiation, only UVA and UVB reach the earth’s surface.
The character and amount of sunburn-producing rays (primarily wavelengths < 320 nm) reaching the earth's surface vary greatly with the following environmental factors:
Atmospheric and surface conditions
Latitude
Season
Time of day
Altitude
Ozone layer
Air pollution
Cloud cover
Surface reflectivity (eg, snow, sand)
The degree of exposure of skin to sunlight also depends on multiple lifestyle factors (eg, clothing, occupation, recreational activities).
Sunburn-producing rays are filtered out by glass and to a great extent by heavy clouds, smoke, and smog; however, they may still pass through light clouds, fog, or 30 cm of clear water, potentially causing severe burns. Snow, sand, and water enhance exposure by reflecting the rays. Exposure is increased at low latitudes (nearer the equator), in the summer, and during midday (10 AM to 4 PM) because sunlight passes through the atmosphere more directly (ie, at less of an angle) in these settings. Exposure is also increased at high altitudes primarily because of a thinner atmosphere. Stratospheric ozone, which filters out UV radiation, especially shorter wavelengths, is depleted by man-made chlorofluorocarbons (eg, refrigerants, aerosols). A depleted ozone layer increases the amount of UVA and UVB reaching the earth's surface.
Medical light treatments (narrowband UVB [NB-UVB] phototherapy) deliver a targeted wavelength of light, approximately 311 to 313 nm. These treatments are effective in inflammatory skin disease and can minimize sunburn, compared with shorter, more erythemogenic UVB wavelengths. Tanning beds emit a broad spectrum (primarily UVA [320 to 400 nm] with some UVB [280 to 320 nm]) for cosmetic tanning. Many of the same long-term deleterious effects can occur with UVA as with UVB exposure, including photoaging and skin cancer. UV radiation emitted from tanning beds has been classified as a human carcinogen, and indoor tanning, as well as the amount and cumulative exposure of UV exposure, has been shown to increase the risk of melanoma (1). Quite simply, there is no "safe" suntan with UV exposure.
General reference
1. Colantonio S, Bracken MB, Beecker J. The association of indoor tanning and melanoma in adults: systematic review and meta-analysis. J Am Acad Dermatol. 2014 May;70(5):847-57.e1-18. doi: 10.1016/j.jaad.2013.11.050.
Pathophysiology of the Cutaneous Effects of Sun Exposure
The adverse effects of UV exposure include acute sunburn and several chronic changes. Ultraviolet radiation damages the skin through a combination of direct DNA injury, oxidative damage, and inflammation, leading to cutaneous changes and increasing melanoma risk by promoting mutations in key genes (1, 2). UV exposure also leads to inactivation and loss of epidermal Langerhans cells, which are an important part of the skin's immune system.
Chronic changes include skin thickening, wrinkling, and lesions such as actinic keratosis and cancer. As a protective response after exposure to sunlight, the epidermis thickens, and melanocytes produce the pigment melanin at an increased rate, causing what is commonly referred to as a "tan." Tanning provides some natural protection against UV radiation but otherwise has no health benefits.
People differ greatly in their sensitivity and response to sunlight, based mainly on the amount of melanin in the skin. Lightly pigmented skin is more sensitive to UV radiation compared with darkly pigmented skin, which is more protected from UV radiation. However, people with dark skin are not immune to the effects of the sun, and darkly pigmented skin can develop sun damage with strong or prolonged exposure. Long-term effects of UV exposure in people with dark skin are the same as those in people with light skin, but are often delayed and less severe because the melanin in their skin provides inherent UV protection.
People with blond or red hair are especially susceptible to the acute and chronic effects of UV radiation. Uneven melanocyte activation occurs in many of these fair-haired people and results in freckling. Polymorphisms in the melanocortin 1 receptor (MC1R) gene (3) can predispose to this phenotype by impairing the ability of melanocytes to produce protective eumelanin and instead favors the synthesis of pheomelanin (4). Pheomelanin is a pigment with poor UV shielding and pro-oxidant properties. Some evidence also suggests that people with MC1R variants may be at increased risk of melanoma independent of UV radiation exposure (5).
There is no skin pigmentation in people with albinism because of a defect in melanin metabolism. Patchy areas of depigmentation are present in patients with vitiligo because of immunologic destruction of melanocytes. These and any other group of people who are unable to produce melanin at a rapid and complete rate are especially susceptible to sun damage.
Pathophysiology references
1. Halliday GM. Inflammation, gene mutation and photoimmunosuppression in response to UVR-induced oxidative damage contributes to photocarcinogenesis. Mutat Res. 2005 Apr 1;571(1-2):107-20. doi: 10.1016/j.mrfmmm.2004.09.013
2. Long GV, Swetter SM, Menzies AM, et al. Cutaneous melanoma. Lancet. 2023 Aug 5;402(10400):485-502. doi: 10.1016/S0140-6736(23)00821-8. Epub 2023 Jul 24. Erratum in: Lancet. 2023 Aug 5;402(10400):450. doi: 10.1016/S0140-6736(23)01581-7
3. Medline Plus. MC1R gene: genemelanocortin 1 receptor. Accessed November 19, 2025.
4. Cerdido S, Sánchez-Beltrán J, Lambertos A, et al. A Side-by-Side Comparison of Wildtype and Variant Melanocortin 1 Receptor Signaling with Emphasis on Protection against Oxidative Damage to DNA. Int J Mol Sci. 2023;24(18):14381. Published 2023 Sep 21. doi:10.3390/ijms241814381
5. Wendt J, Rauscher S, Burgstaller-Muehlbacher S, et al. Human Determinants and the Role of Melanocortin-1 Receptor Variants in Melanoma Risk Independent of UV Radiation Exposure. JAMA Dermatol. 2016 Jul 1;152(7):776-82. doi: 10.1001/jamadermatol.2016.0050
Prevention of Effects of Sun Exposure
Simple precautions help prevent acute changes (ie, sunburn) and the chronic effects of sunlight. These precautions are recommended for people of all skin types, and especially in those who have light skin and burn easily. Avoiding the sun, wearing protective clothing, and applying sunscreen help minimize UV exposure. Two oral supplements—Polypodium leucotomos and nicotinamide—can also provide some protection from sun damage (eg, skin thickening, wrinkling).
Avoiding the sun
Exposure to bright midday sun and other high-UV environments (see Ultraviolet [UV] radiation) should be minimized (30 minutes or less), even for people with dark skin. In temperate zones, UV ray intensity is less before 10 AM and after 4 PM because more sunburn-producing wavelengths are filtered out. Fog and clouds do not reduce the risk significantly. The risk is also increased at high altitudes and low latitudes (eg, at or near the equator).
Although sun exposure helps generate vitamin D, many experts recommend maintaining adequate vitamin D levels by consuming supplements if needed rather than through excessive sunlight exposure., many experts recommend maintaining adequate vitamin D levels by consuming supplements if needed rather than through excessive sunlight exposure.
Protective clothing
Skin exposure to UV radiation can be minimized through the use of protective coverings such as hats, long-sleeved shirts and pants, and sunglasses. Fabrics with a tight weave block the sun better than fabrics with a loose weave. Special clothing that provides high sun protection is commercially available. This type of clothing is labeled with ultraviolet protection factor (UPF) followed by a number that indicates the level of protection (similar to sunscreen labeling). Broad-brimmed hats help protect the face, ears, and neck, but these areas still need supplemental protection with a topical sunscreen. Regular use of UV-protective, wrap-around sunglasses helps shield the eyes and eyelids.
Sunscreens
Sunscreens are topical agents that help protect the skin from sunburn and chronic sun damage by absorbing or reflecting the sun's UV rays (1). Most sunscreens now available effectively filter both UVA and UVB light and are labeled "broad spectrum." In the United States, the Food and Drug Administration (FDA) rates sunscreens by sun protection factor (SPF): the higher the number, the greater the protection. The SPF only quantifies the protection against UVB exposure; there is no FDA-approved scale in the United States for UVA protection; however, protection grade of UVA (PA rating) may be listed on some sunscreens manufactured in several countries around the world. People should typically use a broad-spectrum sunscreen with an SPF rating of 30 or higher. In addition, water resistance is an important feature of many sunscreens.
Sunscreens are available in a wide variety of formulations, including creams, gels, foams, sprays, powders, and sticks. Sunscreen ingredients function by absorbing and/or reflecting light. Self-tanning products do not provide significant protection from UV exposure.
Chemical sunscreens include ingredients that absorb UV radiation. Cinnamates, salicylates, and para-aminobenzoic acid (PABA) derivatives provide UVB protection. Benzophenones are commonly used to provide UVB and shortwave UVA protection. Avobenzone and ecamsule filter in the UVA range and may be added to provide further UVA protection.
Physical blocking (mineral sunscreens) reflect or scatter light and contain the ingredients zinc oxide and titanium dioxide, which physically reflect both UVB and UVA rays. Although formulations of these products were previously very white and pasty when applied, micronization and nanotechnology have allowed them to form a more transparent layer while providing broad-spectrum protection. Physical blocking (mineral sunscreens) reflect or scatter light and contain the ingredients zinc oxide and titanium dioxide, which physically reflect both UVB and UVA rays. Although formulations of these products were previously very white and pasty when applied, micronization and nanotechnology have allowed them to form a more transparent layer while providing broad-spectrum protection.
All chemical sunscreen ingredients are thought to be systemically absorbed to some degree. Although most ingredients have minimal adverse effects, some have potential risk, and others are currently being investigated. For people concerned about systemic absorption, mineral sunscreens that have not been micronized may be preferred, because their molecules are too large to be absorbed through the skin.
Sunscreen failure is common and usually results from delaying application (sunscreens should optimally be applied 30 minutes before exposure), failure to reapply after swimming or sweating, failure to apply every 2 to 3 hours during sun exposure, or insufficient application of the product. One ounce (approximately 30 mL) should be used to cover the entire body surface of an average-sized person; most people apply less than half the recommended amount.
Allergic or photoallergic reactions can occur with sunscreens that contain chemical filters and must be distinguished from other photosensitive skin eruptions. Patch or photopatch testing with sunscreen components may be necessary to make the diagnosis. This testing is usually performed by dermatologists or allergists with expertise in allergic contact dermatitis.
Protective supplements
Polypodium leucotomos (a natural tropical fern extract) (2) and nicotinamide are oral supplements that provide some protection against damaging effects of sunlight but should not be considered a replacement for other methods of sun protection. Caution should be used with higher doses of nicotinamide because they may cause liver damage and elevate blood glucose levels.
Prevention references
1. Glaser KS, Tomecki KJ. Sunscreens in the United States: Current Status and Future Outlook. Adv Exp Med Biol. 2020;1268:355-379. doi: 10.1007/978-3-030-46227-7_18
2. Nestor MS, Berman B, Swenson N. Safety and efficacy of oral Polypodium leucotomos extract in healthy adult subjects. J Clin Aesthet Dermatol 8(2):19–23, 2015
