Although these meta-analyses have yielded very similar risk estimates, the measurement of sun exposure is complex; new studies using comparable protocols in different populations with varying levels of sun exposure are needed.
One explanation offered for the rise in melanoma incidence relates to the differential effects of chronic and intermittent sun exposure; as people have replaced outdoor occupations with indoor occupations, they have engaged in more intermittent sun exposure. Data from very different settings seems to suggest that intermittent sun exposure is critical to the risk for developing melanoma.
The evidence relating lifetime cumulative exposure to melanoma risk comes from two sources: migrant studies and studies of lifetime exposure, controlling for intermittent and occupational exposure. Data from Australia and Italy show that individuals who migrate from areas of low exposure to ultraviolet (UV) radiation, such as the United Kingdom, to areas of high exposure, such as Australia or Israel, before they reach age 10 years have a lifetime risk of developing melanoma that is similar to that of people in the new country.[8,9,10] Alternatively, adolescents or older individuals who migrate from areas of low solar exposure to areas of high solar exposure have a risk that is more similar to that of people from their area of origin than to that of people in the new area. These data have often been cited as indicating that childhood sun exposure is more important than adult sun exposure in melanoma development. However, the data could also be interpreted as suggesting that the length of high-level exposure is more critical than the age at exposure. Thus, people who migrate early in life to a high-insolation region have a longer potential period for intense exposure than do those individuals who migrate later in life.
Data from Connecticut have shown that cumulative lifetime exposure to ultraviolet-B (UVB) radiation does not differ between melanoma cases and controls; rather, intermittent sun exposure is the more important risk factor. The risks related to intermittent sun exposure are even greater if this pattern is experienced both early in life and later in life. These data can also be interpreted as suggesting that sun exposure patterns are rather consistent and stable throughout one's lifetime, i.e., that individuals who receive a great deal of intermittent sun exposure during early life are also likely to receive a great deal of intermittent sun exposure during later life. Nonetheless, an intermittent pattern of sun exposure over many years appears to significantly increase melanoma risk.
The relationship between sun exposure, sunscreen use, and the development of skin cancer is also complex. It is complicated by "negative confounding," i.e., subjects who are extremely sun sensitive deliberately engage in fewer activities in direct sunlight, and they are more likely to wear sunscreen when they do. These subjects are genetically susceptible to the development of skin cancer by virtue of their cutaneous phenotype and thus may develop skin cancer regardless of the amount of sunlight exposure or the sun protection factor of the sunscreen.[12,13]