GEOGRAPHIC DISTRIBUTION OF MULTIPLE SCLEROSIS 143 



erations are the generally greater level of radiation from terrestrial sources 

 of naturally occurring radioactivity- as compared with that from cosmic 

 radiation (Libby, 1955; Neher, 1957; Solon et al., 1960) and the lack of 

 an altitude effect for multiple sclerosis. 



Since direct effects of cosmic rays at the earth's surface cannot be impli- 

 cated, it is of interest to examine the latitude distribution of atomic nuclei 

 made radioactive by cosmic ray events. These radionuclides include T (the 

 hydrogen isotope, tritium), Be^ Be^", C\ Na^^ P^-, P^^ S^^ and CF^ (Suess, 

 1958). The production rate for cosmic ray-induced radionuclides is said to 

 be greater by a factor of four at the poles than at the geomagnetic equator, 

 following the latitude effect for the neutron component of cosmic rays (Suess, 

 1958; Kaufman and Libby, 1954; Simpson, 1951). Their concentration at 

 the surface of the earth, however, is dependent on several additional factors, 

 including half-life, diffusion in the atmosphere and the oceans, and meteoro- 

 logic factors. Thus, the half-life of C^* (5,570 years) is so long that diffusion 

 processes tend to minimize any variation of its concentration with latitude. 

 Diffusion effects should be relatively small for elements with short half-lives, 

 P^- for example (14.5 days). 



In any event, it would not appear possible that the latitude distribution of 

 any cosmic ray-induced radionuclide would be as pronounced as that for 

 multiple sclerosis. A latitude effect for the distribution of fall-out from heavy 

 cosmic ray primary nuclei might be considered additionally (Wesley, 1960). 



Consideration of the problem of the latitude distribution of radioactive 

 nuclei associated with cosmic rays is even further complicated by the fact 

 that some of these nuclei are produced in the explosion of atomic and hydro- 

 gen bombs. Thus, the amount of T that has been produced by hydrogen 

 bombs is comparable to or larger than the total inventory of natural T on 

 the surface of the earth, and artificially produced C^* had by 1957 increased 

 the C^* content of atmospheric CO2 by about 10% (Suess, 1958). Although 

 there is a pronounced distribution with latitude for fall-out debris, at least 

 as indicated by Sr^" in the northern hemisphere (Fig. 12), the variation is 

 with geographic latitude and not with geomagnetic latitude. It should of 

 course be remembered that multiple sclerosis was well-known as a clinical 

 entity long before the era of bomb testing. 



Linear vs. Nonlinear Dose-Response Relationships 



The above considerations concerning comparisons of the geomagnetic 

 latitude distribution of multiple sclerosis with those for various cosmic ray 



^Gentry et al. (1959) have recently found that areas with increased rates of con- 

 genital malformations in New York State appear to be associated with increased levels 

 of background terrestrial radiation. 



