Epidemiological evaluation of sunlight as a risk factor of lip cancer.

A total of 3,169 cases of lip cancer in males and 303 cases in females were diagnosed in Finland and reported to the Finnish Cancer Registry in 1953-73. The diagnosis was verified histologically in 95% of the cases in males and in 92% in females. The mean annual age-adjusted incidence rate was 7·3/10(5) in males and 0·5/10(5) in females. The annual incidence for males has decreased since the early 1960s. The decrease involved all age groups and was not due to a cohort effect. Only a very slight decrease in the risk was observable in females. The incidence was clearly higher in rural than in urban areas, the urban/rural ratio of the age-adjusted incidence rates being 0·6 for males. A decrease in the risk with time was observable for both urban and rural populations. The risk was highest in the northern and eastern parts of the country, for both urban and rural areas. It was concluded that the decrease in the incidence of lip cancer in Finland cannot be accounted for solely by the process of urbanization. An inverse relationship was found between the mean annual amount of solar radiation and the risk of lip cancer. The results are not in accordance with the theory of the association between exposure to actinic radiation and the risk of lip cancer. The synergistic action of some other factors related to outdoor occupation, and probably smoking, would provide a better explanation for the observations in this study.