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Sunlight, Ultraviolet Radiation, and the Skin
It is ingrained in humans to love light and, indeed, since mankind's first wanderings from the caves, worship of the sun has been a fundamental tenet that many societies hold even to the present.
The properties of the sun that have inspired such reverence include its light (visible radiation) and its warmth (infrared radiation). Additional portions of the solar spectrum that cannot be perceived directly by the senses (ultraviolet) are capable of evoking both physiologic and pathologic events in the skin.
Sunlight is the ultimate source of energy and is vitally important to life as we know it. However, absorption of incident solar energy by components of the skin can cause a variety of pathological sequelae.
Until the 20th century, the sun was the predominant source of human skin exposure to energy within the photobiologic action spectrum. More recently, artificial devices capable of mimicking the emission of some or all of the solar spectrum have been introduced, compounding the opportunities and risks of ultraviolet radiation (UVR) exposure.
Despite the undeniable importance of cutaneous exposure to ultraviolet radiation for vitamin D homeostasis, there is little evidence to indicate that there are additional beneficial effects of such exposure. Indeed, overwhelming evidence exists to support the concept that the skin is damaged in many different ways by its direct exposure to natural or artificial UVR. Some exposure is virtually unavoidable over a lifetime and is dramatically dissimilar in different populations depending upon climate, geography, occupation, and recreational activities. The consequences of this exposure are also influenced by factors such as the degree of melanin pigmentation. The effects of UVR can be divided into two general types, acute and chronic. Acute effects include sunburn, and chronic effects include, among others, the development of certain forms of skin cancer. In addition, the skin is a major site of immunologic activity, and UVR is capable of affecting the immune system via its effects on the skin. The skin is also susceptible to degenerative changes evoked by chronic UVR. These changes are a major component of the constellation of physical changes perceived as skin aging but, which in reality, are due to chronic photodamage.
It is now possible to measure the effects of solar radiation on the skin, and epidemiologic studies from around the world have provided important new knowledge concerning the risks and benefits of exposure to sunlight and UVR.
Expanding knowledge about the hazards of exposure to sunlight and UVR has been accompanied by improved approaches to photoprotection, including the development of more effective sunscreen formulations. In addition, there is increasing interest in pharmacologic agents such as the retinoids that may be capable of inhibiting the development of or possibly even reversing certain chronic effects of cutaneous sun exposure.
Considerable controversy remains concerning the specific adverse effects caused by various wavelengths of UVR, the magnitude of the adverse effects, and potential strategies for their prevention and/or treatment. A Consensus Development Conference was undertaken in an effort to define the specific interactions of sunlight, UVR, and the skin as well as to identify methods for preventing and/or treating the adverse effects of UVR. Sponsored by the National Institute of Arthritis and Musculoskeletal and Skin Diseases, the Office of Medical Applications of Research, the National Cancer Institute, and the National Institute of Child Health and Human Development of the National Institutes of Health, the Food and Drug Administration, and the Environmental Protection Agency, the conference brought together physicians, scientists, and other health care professionals, along with representatives of the public on May 8-10, 1989. Following 1 1/2 days of presentations and discussions by the invited experts and the audience, members of the consensus panel drawn from the biomedical research community and the public weighed the scientific evidence in formulating a draft statement in response to several questions:
In applying the recommendations of this consensus conference, it is important to recognize that special circumstances may exist for each patient. These may include unavoidable exposures to UVR or the inability to use certain of the preventive strategies. There are clearly some areas in which final recommendations cannot yet be made due to insufficient data. In these situations, physicians must use their best clinical judgment in advising patients.
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