Solar Radiation Joseph Scotto, M.S.*	Solar radiation exposure is the chief cause of nonmelanoma (i.e., basal cell and squamous cell) skin cancer. It is also a prime factor in the etiology of cutaneous melanoma (Armstrong, 1994). Nonmelanoma skin cancers are now considered to be over 99 percent curable. Though they accounted for as many deaths in the United States during the 1950s and 1960s as did melanomas, which are rarer but far more lethal (Ries, 1990; Riggan et al., 1983), mortality rates decreased in the 1970s while melanoma death rates increased. Currently, more than 600,000 new cases of nonmelanoma skin cancer are thought to occur in the United States each year, and this number is rising. With an annual rate equal to about half the rate observed for all other cancers combined, nonmelanoma skin cancer is the most common form of cancer among Caucasians. In the South, the incidence of skin cancer exceeds that of all other cancers combined, and in parts of the North it accounts for about 30 to 40 percent of all cancers (Scotto, 1983). The relationship between sun exposure and nonmelanoma skin cancer has been clarified greatly in the past several decades (Blum, 1959; Emmett, 1973; Urbach, 1974; Kricker et al., 1994). Observers noted in the late 1800s (Unna, 1894) that sailors exposed to the sun developed "Seemannshaut," or "sailor's skin," and in the early 1900s an excess risk of skin cancer was observed among farmers. The greater risk for Caucasians exposed to sun was also observed (Hyde, 1906). By 1928, scientists were able to demonstrate the cancer-causing effects of ultraviolet radiation on the skin of laboratory animals, using both sunlight and artificial light sources (Findlay, 1928). These carcinogenic effects were produced by ultraviolet-B (UV-B) radiation in the 290- to 320-nanometer (nm) range--the same range that produces burning in human skin (erythema). UV-B exposure can alter DNA, and may also affect the immunosuppressive system (Kripke, 1990). Though latitude, or distance from the equator, generally determines the amount of UV-B radiation in a given location, altitude and sky cover are also determining factors. Atlanta, Georgia, and El Paso, Texas, for example, are in the same general latitude (32 to 33 degrees N). But El Paso, which is higher and drier, has an annual UV-B count 38 percent higher than Atlanta. The amount of UV-B received annually in Hawaii (at about 19 degrees N) is approximately 10 times that received in Alaska (at about 72 degrees N). Time of day and time of year also affect the amount of UV-B radiation in any location. The greatest amount, of course, occurs during the summer months, and 60 percent of the day's total amount occurs between the hours of 10 a.m. and 2 p.m. (or 3 p.m. DST). The safest time of day to be outdoors is when the sun's angle is less than 45 degrees above the horizon. As a rule of thumb, this occurs when a person's shadow is longer than his or her height. In 1974, Robertson-Berger (Berger, 1975) designed meters to measure surface levels of UV-B radiation. In collaboration with the NCI, the National Oceanic and Atmospheric Administration (NOAA) placed these meters at a number of National Weather Service stations in the United States. Physical data from these meters, combined with epidemiological data from NCI surveys, allowed scientists for the first time to use direct measurements of ambient UV-B to estimate the potential health effects associated with solar radiation exposure (Scotto, 1976). The most striking association was the inverse relationship between latitude and nonmelanoma skin cancer: The lower the latitude (the equator is zero), the higher the incidence. The data also indicated that nonmelanoma skin cancer is related to annual, cumulative UV-B exposure, while skin melanoma may be related to brief exposure to high-intensity UV radiation (Fears, 1977). A 1 percent increase in solar UV-B exposure may result in a 2 percent increase in the incidence rate of basal cell carcinoma, a 4 percent increase in squamous cell carcinoma of the skin, and a 1 percent increase in skin melanoma (Scotto, 1976, 1983; Fears, 1977; Rundel 1983; Green, 1978). Subsequent NCI studies have confirmed initial dose-response findings for both melanoma and nonmelanoma of the skin (Scotto et al., 1982; Scotto and Fears, 1987). The single most important factor affecting UV-B exposure is the amount of ozone (03) in the atmosphere (Cutchis, 1974). Ozone gases absorb most of the UV light in the upper stratosphere and allow only small amounts (less than .1 percent) to reach the earth's surface. There has been growing concern since the early 1970s (Roan, 1990) that man-made chlorofluorocarbons (CFCs such as aerosol propellants, refrigerants, solvents for computer chips, and insulation in styrofoam containers) are destroying ozone molecules in the stratosphere and may have caused the recent "Ozone Hole" (Farman et al., 1985) over parts of Antarctica. Current estimates predict that ozone concentration could be reduced by 6 percent or more within several decades if world levels of CFC production continue. Many nations have signed an agreement, called the Montreal Protocol, which would limit and possibly end CFC production by the year 2000. As much as a 2 percent increase in UV-B radiation may be expected for each 1 percent reduction in stratospheric ozone concentration. These concerns are now being studied by a number of federal agencies. However, though stratospheric ozone reportedly decreased between 1969 and 1987 (WMO, 1989), with highest depletion rates occurring during the winter months when UV-B levels are lowest, no increases in the annual amounts of biologically effective solar ultraviolet radiation at the earth's surface were observed between 1974 and 1985 in the United States (Scotto, 1988). Solar radiation exposure ranks high among the "lifestyle" factors associated with skin cancers. Most individuals have some choice in the amount of sunlight exposure they receive. Preventive measures (e.g., using sunscreens and protective clothing, and avoiding sunlight exposure around the noontime hours) may outweigh the effects of small relative decreases in stratospheric ozone.
continue

* From the Biostatistics Branch, Division of Cancer Etiology, National Cancer Institute, Bethesda, Maryland