Summer sunshine is beneficial for the soul…but it isn’t so good for human skin. Although some degree of sun exposure is healthful—a few minutes’ worth of sunlight is instrumental in the formation of vitamin D precursors—exposure to excessive sunlight can cause several pathologic changes in the epidermis.
Additionally, vitamin D can be supplemented, so daily sun exposure isn’t even necessary.
The sun emits a wide spectrum of ultraviolet electromagnetic radiation, but—owing to the earth’s atmosphere—only ultraviolet A (UVA) and ultraviolet B (UVB) reach the planet’s surface. The magnitude of exposure to this radiation varies with the season, geographic location, a person’s occupation and lifestyle, and the amount of shielding that covers one’s skin.
The most acute and familiar form of sun damage is sunburn. The rays that cause sunburn are mostly in the UVB range (<320 nm); such radiation is filtered out by glass and some atmospheric particulates (e.g., smoke or smog), but it will penetrate a foot of clear water, and it is reflected by snow, light sand, or other bright or mirror-like surfaces.
Sun-tanning lamps typically emit artificial light that is mainly in the UVA spectrum, leading some proponents to mistakenly claim that artificial tanning has no ill effects. However, even light sources that contain only UVA adversely affect the skin.
What Happens to Skin That is Exposed to Excessive Sunlight?
- Following exposure to UVA or UVB radiation, specialized cells called melanocytes increase their production of a dark brown pigment called melanin. Melanin creates a colored barrier that affords some protection against further sunlight exposure; melanin is also responsible for tanning.
- Ultraviolet light disrupts elastin and collagen fibers—the skin’s supportive underpinnings—lying deeper in the dermis. Over time, this causes solar elastosis, a condition characterized by thickening, creasing, wrinkling and sagging.
- Exposure to sunlight also leads to inactivation and elimination of Langerhans’ cells in the epidermis. Langerhans’ cells are immune cells that intercept foreign antigens and present them to T cells. Thus, the innate immune function of the skin is impaired following sun exposure.
The accumulation of these and other changes in skin structure and physiology account for the sun’s chronic effects on human skin.
Chronic Effects of Sunlight Exposure
- Aging: Chronic or repeated sun exposure leads to premature aging of the skin. These aging changes produce both coarse and fine wrinkles, mottling (“age spots”), roughened and leathery texture, and telangiectasia (spider veins). Such alteration in skin architecture resembles that caused by x-ray therapy for cancer.
- Actinic keratoses (AKs): These thickened, scaly lesions form as a result of many years of sun exposure; they are precancerous lesions that resemble small patches of pink or beige sandpaper. AKs have become more common in the general population; individuals with fair hair and skin are more frequently affected and usually exhibit a larger number of keratoses than darker-skinned individuals.
- Skin cancers: Total lifetime sun exposure is directly proportional to the occurrence of squamous cell and basal cell carcinomas of the skin. Even persons who experience extensive sun exposure as children and then avoid sunlight as adults are at increased risk. Sun exposure also raises the risk of malignant melanoma.
- (From The Merck Manual, 18th Edition. Reactions to Sunlight.2006:962-65)
Sunlight exposure leads to the production of vitamin D precursors in the skin, but this physiologic need can be met with only a few minutes’ exposure each day. Chronic, prolonged, or repeatedly intense exposure to sunlight causes skin damage that is permanent and additive; habitual sunbathing and artificial tanning should be avoided.
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