The skin is the most exposed organ to environmental UV and to the associated sequellae (2). Exposure to UV radiation on the skin results in clearly demonstrable mutagenic effects. The p53 suppressor gene, which is frequently mutated in skin cancers, is believed to be an early target of UV radiation-induced neoplasm (3). Although there is no direct way that the active wavelengths for the development of skin cancer in humans can be determined, there is ample indirect evidence demonstrating probable ranges. In terms of SCC in albino hairless mice, the action spectrum has been determined to have a strong peak at 293 nm with secondary peaks at 354 and 380 (4). The primary wavelength influencing melanoma risk appears to be in the Ultraviolet B (UVB) (290-320 nm) range. However, studies in fish and opossums have also shown an increase in melanoma development when exposed to UVA wavelengths (5,6). Fair skinned individuals who are more sensitive to the effects of exposure at these wavelengths are at higher risk for the development of skin cancer (7). In addition, skin cancer rates are also elevated in persons with increased artificial UV exposure through tanning salons (8).
The amount of average annual UV radiation correlates with the incidence of skin cancer (9). There is a direct correlation with BCC and SCC incidence and latitude (10). Scotto et al. (11) demonstrated a strong inverse correlation between latitude and incidence of BCC and SCC for both men and women.
In terms of melanoma, the relationship is not as clear-cut. Incidence rates for melanoma correlate in a lesser way with latitude as that for NMSC but other factors may also be involved (12). Melanoma mortality rates in the U. S. and Canada have also been shown to directly correlate with ambient UV exposure (13). The correlation of melanoma incidence to UV radiation exposure is greater when ambient UVA (320-400 nm) radiation is also included (14). High-altitude regions tend to have a higher melanoma rate that may be related to the higher UV fluences noted at these sites (15). Melanoma risk has also been noted to be directly related to annual UV flux. Fears et al. (16) demonstrated that when lifetime residential history was coupled with levels of midrange UV radiation (UVB flux) to provide a measure of individual exposure to sunlight a 10% increase in annual UVB flux was associated with a 19% increased risk of melanoma. Even in women who could develop a deep tan, a 10% increase in hours spent outdoors was associated with 5.8% increase in melanoma incidence. The association between melanoma risk and average annual UVB flux was strong and consistent for men and women. However, some of the studies examining a latitudinal gradient for melanoma risk have been somewhat inconclusive (17). Although worldwide studies have only shown a weak correlation, the association of melanoma mortality in 1950-1967 with estimates of annual erythemal solar UVB dose across the U. S. and Canada demonstrated a stronger relationship (18).
The anatomic areas that skin cancer develops on appear to be somewhat related to the average amount of UV exposure to those sites (19). The density of skin cancer is highest on the sites that are virtually constantly exposed to UV, namely, the head and neck. Skin cancer rates are low in rarely UV-exposed areas such as the scalp in women and the buttocks in both sexes (20). Melanoma tends to be found more frequently in women on the legs where more average UV exposure may occur than in men (21).
The timing and periodicity of the UV exposure appears to be important in its effect on subsequent skin cancer risk. In terms of NMSC, the long-term chronic UV exposure
appears to increase the chance of developing this cancer. Acute intermittent UV exposure elevates subsequent melanoma risk (22). Migration studies have demonstrated sun exposure early in life appears to have a greater influence on subsequent skin cancer risk than does that at a later age. Persons born in the high-UV insolation environment of Australia have a increased risk for developing skin cancer compared to those born in Northern Europe who migrated at age 10 or older (23). Several additional studies from other countries have also found that risk of developing melanoma was less in those who migrated to the country 10 or more years after birth than were those who were born there (24,25). However, a recent study has now demonstrated that excessive UV exposure later in life may be equally important to that acquired earlier. Pfahlberg et al. (26) found a very similar upward gradient of melanoma risk in exposure categories related to the frequency of sunburns comparing UV exposure occurring before and after age 15. More than five sunburns doubled the melanoma risk, irrespective of their timing in life. This study did not provide supporting evidence for the existence of a critical age interval but rather suggested that the hazardous impact of UV exposure seems to persist lifelong.