Melanoma and sun exposure: contrasts between intermittent and chronic exposure.

The evidence relating cutaneous malignant melanoma to previous sun exposure is now very strong. Major northern hemisphere studies consistently show increases in melanoma in association with recreational and vacation activities related to intermittent sun exposure. These studies have also recorded amounts of sun exposure from such activities. Several studies suggest an increased risk related to short periods of intensive exposure in early adult life. In contrast, regular outdoor occupation confers a decreased risk in these same studies. Australian studies, in populations with much higher levels of total sun exposure, do not show such a clear distinction between intermittent and chronic exposure. The evidence is consistent with a complex relationship of melanoma risk to sun exposure, the risk being increased by intermittent exposure to levels of sun which are higher than normal for that individual, but no increased risk or even a decreased risk related to long term chronic exposure. Possible biological mechanisms for this complex relationship are discussed. In the Western Canada Melanoma Study the effects of occupational and recreational exposure are different in form and are independent. The increased risks seen with various measures of sun exposure do not appear to be systematically different for individuals who have a good tanning response as compared to individuals who do not.