55-1 



RADIATION niOLOGY 



south than in the north. In tliese data the category " cancer of the buccal 

 ca\ity" inchides some tumors of the exposed parts of the lip, which prob- 

 ably accounts for the distinct north-south distribution of the incidence of 

 such tumors. Cancer of all other sites does not show this relation to lati- 

 tude. A small increase with increasing latitude among males is opposed 

 by a small decrease among females. These statistics suggest that some 

 factor which varies with latitude markedly affects the incidence of cancer 

 of the exposed parts of the body. 



120 



2 

 O 



100 



3 



a 

 o 

 c- > 



80 - 



60 





40 



to 



o 



20 - 



-- 3000 



q: 



UJ 



I 



- 2000 



1000 



32 34 36 38 40 



NORTH LATITUDE, degrees 

 Fig. 14-10. Distribution of cancer with latitude. Curve I, skin, male; II, skin, female; 

 III, buccal cavity, male; IV, buccal cavity, female; V, all other sites, male; VI, all 

 other sites, female. {From Blum, 1948, based on data of H. P\ Dorn.) 



We may examine the possibility that this factor is sunlight, but to do so 

 we must know the extent of variation with latitude of the particular com- 

 ponents of sunlight that we accuse of inducing cancer. The animal 

 experiments indicate that the wave lengths concerned are the same as 

 those that cause sunburn. Total annual sunlight does not vary with 

 latitude to nearly so great an extent as would be required to explain the 

 distribution of cancer incidence shown by Dorn's data; but the carcino- 

 genic wave lengths do show a considerable difference in north-south distri- 

 bution.'^ In order to make an exact comparison between the incidence 

 of carcinogenic radiation in sunlight at various latitudes and the corre- 

 sponding incidence of cutaneous cancer, it would be necessary to know the 

 action spectrum of carcinogenesis as well as to have complete data on the 

 shorter wave lengths of sunlight, Init it is not feasil)le to obtain the former 



'•"■The factors determining the variation of .sunlight with latitude are discussed 

 briefly in Chap. 13 and extensively by Sanderson and Ilulburt in Chap. 3. 



