556 



RADIATION' HIOI.OGY 



nearly the riiiifijc of carcinofioiiie radiation has a slope similar to that of 

 the cancer-iiu'ideiK'c curves althoiitrh not quite so steep. The aj^reement 

 is certainly as good as might he expected, considering the numerous 

 assumptions that had to l)e made in order to carry out the analysis. The 

 least that can be said is that the expected variation in the amount of 

 carcinogenic radiation with latitude probably is sufficient to account for 

 the variation of cancer-incidence data with latitude that is indicated by 

 Dorn's (hita. Such fa(^tors as the amount of smoke, cloudiness, and dust 



100 



Pi 80 



60 



•a 



u 

 a 20 



32 34 36 38 40 



NORTH LATITUDE, degrees 



Fig. 14-11. Distribution of carcinogenic radiation and cutaneous cancer with latitude. 

 Curve I, 2.0 mm ozone; II, 2.8 mm ozone; III, incidence of cutaneous chancer, female; 

 IV, incidence of cutaneous cancer, male; V, maximum difference. (Frorn Blum. 1948, 

 based on data of H. F. Dorn and Bryan O'Brien.) 



are left out of consideration, and these may limit the amount of carcino- 

 genic radiation in specific areas, but there are no means for estimating 

 their importance. 



PREVENTION 



When we consider how the lines of evidence converge, it seems difficult 

 to reach any conclusion other than that the ultraviolet radiation of sun- 

 light is a major causal factor in cancer of the skin in the white population 

 of the United States. The evidence is against applying this conclusion 

 to the Negro population, where the incidence of skin cancer is small in 

 any event. 



If such a conclusion is to be accepted even tentatively, the question 

 of possible means of prevention of these cancers arises. Even if it were 

 feasible, it would not be desirable to counsel the complete avoidance of 

 sunlight. This is true particularly since skin cancers of the squamous- 

 and basal-cell types are among the least dangerous of malignant growths, 

 and their incidence is relatively low even under conditions which seem 

 most favorable for their occurrence, for example in the southern United 



