Ultraviolet Rays


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.

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.

What are the sources of ultraviolet radiation, and is the extent of human exposure changing over time?

There are both natural and artificial sources of UVR. Although there are many artificial sources of this energy, sunlight is the only natural source.

The sun emits a wide variety of electromagnetic radiation, including infrared, visible, ultraviolet A (UVA; 320 to 400 nm), ultraviolet B (UVB; 290 to 320 nm), and ultraviolet C (UVC; 10 to 290 nm). The only UVR wavelengths that reach the Earth's surface are UVA and UVB. UVA radiation is 1,000-fold less effective than UVB in producing skin redness. However, its predominance in the solar energy reaching the Earth's surface (tenfold to one hundredfold more than UVB) permits UVA to play a far more important role in contributing to the harmful effects of sun exposure than previously suspected.

Sunlight is the greatest source of human UVR exposure, affecting virtually everyone. The extent of an individual's exposure, however, varies widely depending on a multiplicity of factors such as clothing, occupation, lifestyle, age, and geographic factors such as altitude and latitude. There is greater UVR exposure with decreasing latitude. Residing at higher altitude results in a greater UVR exposure such that for every 1,000 feet above sea level, there is a compounded 4 percent increase in UVR exposure. UVR exposure increases with decreased stratospheric ozone. Other factors that influence exposure to UVR include heat, wind, humidity, pollutants, cloud cover, snow, season, and time of day.

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What are the effects of sunlight on the skin?

Marked morphologic changes in all parts of the skin, except perhaps the subcutaneous tissue, are recognized as consequences of exposure to UVR. These changes underlie the clinically observed sagging, wrinkling, leathery texture, and blotchy discoloration of skin typically associated with actinic damage. It is unclear how much exposure and how much time is required to effect these changes, although it is evident that clinically normal appearing skin can show pathologic signs of sun damage upon histologic and ultrastructural examination. It is known that individuals with fair complexions are more susceptible to this damage.

In the epidermis UVR-induced changes include aberrant tissue architecture and alterations in keratinocytes and melanocytes and functional changes in Langerhans cells. Sun-exposed epidermis becomes thickened as much as twofold compared to sun-protected skin and is disorganized, showing evidence of hyperkeratosis, parakeratosis, and acanthosis.

UVR causes unique dermal damage such as alterations in architecture, matrix composition, vascular structure and function, and cellular activities. Dermal vessels become dilated, leaky, and accumulate excessive basement membrane-like material. Inflammatory cells collect around the vessels; mast cells are increased and may show evidence of degranulation and apparent physical associations with fibroblasts. Although the nature of this relationship is unknown, it is a common observation in other disorders in which fibrosis occurs.

Tanning is the term applied to the increase in melanin pigmentation following UVR exposure. It is mediated by a combination of immediate pigment darkening (IPD) and delayed pigment darkening (DPD).

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What factors influence susceptibility to ultraviolet radiation?

Susceptibility to damage by UVR may be influenced by genetic and acquired disorders, genetic traits, age-related factors, and the use of some medications.

Genetic abnormalities can increase the susceptibility to UVR damage. These include disorders manifested in utero that may be lifelong or that may appear shortly after birth. Among them are disorders of keratinization and pigmentation. Several inherited disorders in which there is marked susceptibility to UVR in early childhood include xeroderma pigmentosum, Bloom's syndrome, Rothmund-Thomson syndrome, the porphyrias, phenylketonuria, dysplastic nevus syndrome, and the basal cell nevus syndrome.

There are also numerous and diverse acquired diseases that manifest increased light susceptibility. Significant factors that influence susceptibility to UVR damage include race, ethnicity, eye and hair color, and the tendency toward formation of freckles and nevi. Numerous systemic medications may augment UVR susceptibility. Increased UVR damage may occur with the use of oral antibiotics, antihypertensives, psoralens, immunosuppressive agents, nonsteroidal anti-inflammatory drugs, and numerous other agents

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Can ultraviolet-induced changes be prevented? If so, how?

Skin cancers in which UVR exposure plays an important role are the most common form of cancer. In 1978, there were more than 500,000 new cases of skin cancer. This is probably a substantial underestimate for 1989, because the number of office visits for NMSC has increased more than 50 percent in the past decade while the overall increase in office visits has only been 11 percent. Therefore, it is imperative to consider ways to minimize the deleterious effects of UVR.

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Are sunlight-induced adverse skin alterations treatable and/or reversible? If so, how?

Sunlight-induced adverse skin alterations include NMSC, melanoma, actinic keratoses, as well as textural and pigmentary changes characteristic of chronic photodamage. All of these cancers are treated by standard surgical techniques. Although the beneficial cosmetic effects of some of these treatments have received wide publicity, there are insufficient data demonstrating sustained improvement, reversibility of tissue pathology, or the preservation of normal skin function by those agents. There is no information regarding long-term positive, negative, or toxic effects of these agents.

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What are the directions for future research?

The following recommendations for future research are not listed according to any particular priority.
  • There is a need for updated epidemiologic data concerning NMSC in the United States. Evidence exists to suggest that the incidence of NMSC is increasing, but no new data have been obtained since 1978.
  • There is a need for more research to define the action spectra of sunlight and UVR in the pathogenesis of cutaneous melanoma and to develop better animal models for the study of this disease.
  • Studies are needed to better define the immunological effect of UVR exposure and its potential role in the development of skin cancer and cutaneous infections.
  • The risk/benefit ratio of widespread, long-term sunscreen use should be monitored.
  • More effective sunscreens and nontoxic anticarcinogenic agents should be developed as approaches to diminishing the risk of UVR exposure.
  • There is a need for epidemiologic study of individuals who use sunscreens that block UVB radiation
  • Data are needed that quantitatively assess UVR exposure in normal populations at all ages and under various conditions.
Conclusions and Recommendations
  • Human exposure to UVR from natural sunlight and artificial sources is increasing substantially.
  • UVR in sunlight is critical for vitamin D synthesis in the skin. However, it produces a variety of pathologic effects, including sunburn, pigmentary change, immunologic alterations, and neoplasia. A constellation of structural alterations of the epidermis, the dermal-epidermal junction, and the dermis is uniquely characteristic of photodamage.
  • UVR-induced changes can be minimized or prevented by the use of proper clothing, appropriate application of physical and/or chemical sunscreens, behavior modification, and awareness of photosensitizing medications.
  • There should be better education of the public with regard to the hazards of tanning parlors, and there should be greater regulation of tanning facilities to protect the public against inadvertent injury by UVR.
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Information obtained from National Institute of Health
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