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Sunlight

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.

Solar flares (sunspots) also alter the amount of UVR reaching the Earth. Solar flares increase ozone concentration in the stratosphere (above 50 km) thereby reducing the amount of surface UVB. This 11-year cycle of solar flares causes as much as a 400-percent variation in UVB at 300 nm reaching the earth. When solar flares are inactive, there is a decrease in the ozone concentration, allowing increased UVB to penetrate to the Earth's surface.

There is also serious concern about depletion of stratospheric ozone by manmade chlorofluorocarbons (CFC). These extraordinarily inert chemicals are used in numerous commercial products, including aerosols and refrigerants. The U.S. Environmental Protection Agency has been charged with estimating the effects on health associated with changes in stratospheric ozone levels. In a recent risk assessment document, the Agency predicted that without controls on CFC production, there would be a 40 percent depletion of ozone by the year 2075. The Agency further concluded that for every 1 percent decrease in ozone, there will be a compounded 2 percent increase in the more damaging shorter UVB wavelengths reaching the Earth's surface. Such an increase in UVB penetration to the earth is predicted to result in an additional 1 to 3 percent increase per year in nonmelanoma skin cancer (NMSC).

Recent satellite measurements already indicate a worldwide decrease in stratospheric ozone over the last decade. Both satellite- and land-based measurements have revealed a seasonal hole in the ozone layer over the Antarctic secondary to its destruction by CFC's. Although increased surface UVB has been measured in the Antarctic, there has not yet been a measurable change in UVB as a consequence of CFC's in the stratosphere in the United States.

Over the past several decades, the average American's exposure to UVB has increased considerably due to changing lifestyles--more outdoor recreational activities, more emphasis on tanning, scantier clothing, and a population shift to the sunbelt.

The most common sources of artificial UVR exposure are various kinds of lamps that emit this form of energy. These lamps are used primarily for recreational tanning and phototherapy of skin diseases (e.g., psoriasis and cutaneous T-cell lymphoma (mycosis fungoides). UVR lamps can emit UVA, UVB, and/or UVC. Those lamps currently used for recreational tanning emit UVA primarily or exclusively. Some UVA lamps generate greater than 5 times more UVA per unit time than solar UVA radiation reaching the Earth's surface at the Equator. At these doses, "pure UVA" is likely to have adverse biologic effects. However, UVB remains a potential problem with most of these sources. Even 1 percent UVB emission from a UVA source can cause a significant increase in the potential for skin cancer.

The tanning industry is rapidly growing in the United States. Currently, more than 1 million Americans use commercial tanning facilities every day. The biggest categories of users are adolescents and young adults, especially women.

The use of artificial ultraviolet sources for the phototherapy of dermatologic diseases has increased substantially in recent years and has exposed a group of people to markedly increased doses of UVR. Epidemiologic studies of these patients have shown an unequivocal dose-dependent increase in the incidence of NMSC, especially squamous cell carcinoma (SCC).

Another potential but as yet unexplored source of artificial UVR is unshielded fluorescent bulbs used for illumination. An unresolved issue is the amount of UVA emitted by such sources and the long-term effects of this exposure. More research is needed to clarify these problems.
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