The skin may respond to sunlight with chronic (eg, dermatoheliosis [photoaging] Photoaging Chronic affects of sunlight include photoaging, actinic keratoses, and skin cancer. (See also Overview of Effects of Sunlight.) Chronic exposure to sunlight ages the skin (photoaging, dermatoheliosis... read more , actinic keratosis Actinic Keratoses Actinic keratoses are precancerous changes in skin cells (keratinocytes) that are a frequent consequence of many years of sun exposure. Diagnosis is clinical. Treatment typically includes lesion-directed... read more ) or acute (eg, photosensitivity Photosensitivity Photosensitivity is a cutaneous overreaction to sunlight. It can be related to photoallergy or phototoxicity and may be idiopathic or occur after exposure to certain toxic or allergenic drugs... read more , sunburn Sunburn Sunburn is characterized by erythema and sometimes pain and blisters caused by overexposure to solar ultraviolet radiation. Treatment is similar to that for thermal burns, including cool compresses... read more ) 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 factors:
Atmospheric and surface conditions
Time of day
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, in refrigerants and aerosols). A decreased ozone layer increases the amount of UVA and UVB reaching the earth's surface.
Sun-tanning lamps use artificial light that is more UVA than UVB. This UVA use is often advertised as a “safer” way to tan; however, many of the same long-term deleterious effects occur as with UVB exposure, including photoaging Photoaging Chronic affects of sunlight include photoaging, actinic keratoses, and skin cancer. (See also Overview of Effects of Sunlight.) Chronic exposure to sunlight ages the skin (photoaging, dermatoheliosis... read more and skin cancer Overview of Skin Cancer Skin cancer is the most common type of cancer and commonly develops in sun-exposed areas of skin. The incidence is highest among outdoor workers, sportsmen, and sunbathers and is inversely related... read more . UV light emitted from tanning beds has been classified as a human carcinogen, and indoor tanning has been shown to increase the risk of melanoma Melanoma Malignant melanoma arises from melanocytes in a pigmented area (eg, skin, mucous membranes, eyes, or central nervous system). Metastasis is correlated with depth of dermal invasion. With spread... read more . Quite simply, there is no "safe" tan.
Adverse effects of UV exposure include acute sunburn Sunburn Sunburn is characterized by erythema and sometimes pain and blisters caused by overexposure to solar ultraviolet radiation. Treatment is similar to that for thermal burns, including cool compresses... read more and several chronic changes Chronic Effects of Sunlight Chronic affects of sunlight include photoaging, actinic keratoses, and skin cancer. (See also Overview of Effects of Sunlight.) Chronic exposure to sunlight ages the skin (photoaging, dermatoheliosis... read more . Chronic changes include skin thickening, wrinkling, and certain lesions such as actinic keratosis Actinic Keratoses Actinic keratoses are precancerous changes in skin cells (keratinocytes) that are a frequent consequence of many years of sun exposure. Diagnosis is clinical. Treatment typically includes lesion-directed... read more and cancer Overview of Skin Cancer Skin cancer is the most common type of cancer and commonly develops in sun-exposed areas of skin. The incidence is highest among outdoor workers, sportsmen, and sunbathers and is inversely related... read more . Exposure also leads to inactivation and loss of epidermal Langerhans cells, which are an important part of the skin's immune system.
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. The skin has been classified into 6 types (I to VI) in order of decreasing susceptibility to sun injury. Classification is based on the interrelated variables of skin color, UV sensitivity, and response to sun exposure (see table Fitzpatrick Skin Type Classification Fitzpatrick Skin Type Classification ). Skin type I is white to very lightly pigmented, very sensitive to UV light, has no immediate pigment darkening, always burns easily, and never tans. Skin type VI is dark brown or black, is most protected from UV light, and is a deep, dark, black-brown color with or without sun exposure. However, dark-skinned people 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 dark-skinned people are the same as those in light-skinned people but are often delayed and less severe because the melanin in their skin provides built-in UV protection.
People with blonde 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.
There is no skin pigmentation in people with albinism Albinism Oculocutaneous albinism is an inherited defect in melanin formation that causes diffuse hypopigmentation of the skin, hair, and eyes. Ocular albinism affects the eyes and usually not the skin... read more because of a defect in melanin metabolism. Patchy areas of depigmentation are present in patients with vitiligo Vitiligo Vitiligo is a loss of skin melanocytes that causes areas of skin depigmentation of varying sizes. Cause is unknown, but genetic and autoimmune factors are likely. Diagnosis is usually clear... read more 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.
Avoiding the sun, wearing protective clothing, and applying sunscreen help minimize UV exposure. Two oral supplements—Polypodium leucotomos and nicotinamide—can also provide protection.
Avoiding the sun
Simple precautions help prevent sunburn and the chronic effects of sunlight. These precautions are recommended for people of all skin types, particularly those who are fair skinned and burn easily. Exposure to bright midday sun and other high-UV environments (see Ultraviolet (UV) radiation Ultraviolet (UV) radiation The skin may respond to sunlight with chronic (eg, dermatoheliosis [photoaging], actinic keratosis) or acute (eg, photosensitivity, sunburn) changes. The sun emits a wide range of electromagnetic... read more ) 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 risk significantly, and risk is increased at high altitudes and low latitudes (eg, at the equator).
Although sun exposure helps generate vitamin D, many experts recommend maintaining adequate vitamin D levels by consuming supplements if needed rather than by intentionally increasing sunlight exposure.
Skin exposure to UV radiation can be minimized through the use of protective coverings such as hats, shirts, 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 help protect the skin from sunburn and chronic sun damage by absorbing or reflecting the sun's UV rays. Older sunscreens tended to filter only UVB light, but most newer sunscreens now effectively filter UVA light as well and are labeled "broad spectrum." In the US, 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 scale in the US for UVA protection. People should typically use a broad-spectrum sunscreen with an SPF rating of 30 or higher.
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.
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 insufficient application of the product, too-late application (sunscreens should optimally be applied 30 minutes before exposure), failure to reapply after swimming or sweating, or failure to apply every 2 to 3 hours during sun exposure.
Allergic or photoallergic reactions can occur with sunscreens that contain chemical filters and must be distinguished from other photosensitive skin eruptions Photosensitivity Photosensitivity is a cutaneous overreaction to sunlight. It can be related to photoallergy or phototoxicity and may be idiopathic or occur after exposure to certain toxic or allergenic drugs... read more . Patch or photopatch testing with sunscreen components may be necessary to make the diagnosis. This testing is usually done by dermatologists with expertise in allergic contact dermatitis Allergic contact dermatitis (ACD) Contact dermatitis is inflammation of the skin caused by direct contact with irritants (irritant contact dermatitis) or allergens (allergic contact dermatitis). Symptoms include pruritus and... read more .
Polypodium leucotomos (a natural tropical fern extract; 1 Prevention reference The skin may respond to sunlight with chronic (eg, dermatoheliosis [photoaging], actinic keratosis) or acute (eg, photosensitivity, sunburn) changes. The sun emits a wide range of electromagnetic... read more ) and nicotinamide are two 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.