Overview of Effects of Sunlight
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:
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 3 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 and skin cancer. 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. Quite simply, there is no "safe" tan.
Adverse effects of UV exposure include acute sunburn and several chronic changes. Chronic changes include skin thickening, wrinkling, and certain lesions such as actinic keratosis and cancer. 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. 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 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.
Avoiding the sun, wearing protective clothing, and applying sunscreen help minimize UV exposure.
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 Overview of Effects of Sunlight : Ultraviolet (UV) radiation) should be minimized (30 min or less), even for people with dark skin. In temperate zones, UV ray intensity is less before 10 am and after 3 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, most experts recommend maintaining adequate vitamin D levels by consuming supplements if needed rather than by intentional 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 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 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, and sticks. Self-tanning products do not provide significant protection from UV exposure.
Most sunscreens contain several agents that function as chemical screens, absorbing light or providing a physical screen that reflects or scatters light. Sunscreen ingredients that absorb UVB radiation include cinnamates, salicylates, and PABA derivatives. 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.
Other sunscreens, called sunblocks, contain zinc oxide and titanium dioxide, which physically reflect both UVB and UVA rays (thus blocking them from reaching the skin). Although previously very white and pasty when applied, micronized formulations of these products have significantly improved their cosmetic acceptability.
Sunscreen failure is common and usually results from insufficient application of the product, too-late application (sunscreens should optimally be applied 30 min before exposure), failure to reapply after swimming or exercise, or failure to apply every 2 to 3 h during sun exposure.
Allergic or photoallergic reactions can occur to sunscreens 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 done by dermatologists with expertise in allergic contact dermatitis.