Incidence of malignant melanoma of the skin in Norway, 1955-1989: Associations with solar ultraviolet radiation, income and holidays abroad

Background. Norway has the highest incidence of melanoma in Europe. This study analyses geographical variations in melanoma incidence within Norway and their association with possible aetiological factors. Methods. Data on melanoma incidence from the Norwegian Cancer Registry were used to calculate standardized incidence ratios (SIR) for the 19 counties in Norway for each 5-year period from 1955 to 1989. Multiple regression analysis was used to examine the associations between these SIR and local UVB levels, holidays abroad and income. Similar methods were also used to analyse changes in SIR between 1955-1969 and 1985-1989. Results. There was a highly significant association between melanoma incidence and UVB in each of the time periods studied. Income showed a significant positive association in the 1960s and early 1970s but not later. Foreign holidays showed a significant positive association in the 1980s, but not earlier. Changes in melanoma SIR between 1955-1969 and 1985-1989 were significantly positively associated with holidays abroad and negatively with income levels. Conclusions. Melanoma incidence in Norway is closely related to local levels of UVB radiation independently of other factors suggesting that local exposures carry significant risk. Risks would probably increase if ozone depletion led to enhanced UVB flux (estimated as 1.6% rise in incidence for each 1% increase in UVB). By the end of the study period income was no longer a significant factor but holidays abroad had started to have a detectable effect on melanoma incidence.