Effects of X-Rays and Atomic Radiations on Living Cells 61 



greater in the growing bones of young people than of older people. 

 However at the present time the effect is not serious as the amount of 

 strontium present in the bones of human beings has reached about a 

 thousandth of a microcurie and one microcurie is regarded as a rea- 

 sonably safe permissible amount. However, even this small amount 

 is expected to produce a certain incidence of bone cancer. 



Another well-established effect of radiation is a disease of the bone 

 marrow known as leukemia. This is a kind of cancer in which an un- 

 controlled excessive formation of white blood cells occurs. Many 

 cases of leukemia have occurred among the survivors of the atomic 

 bomb at Hiroshima and the observations recorded in the following 

 Table show that there is a definite correlation between the incidence 

 of the disease and the amount of radiation received: 



Incidence of leukemia between 

 Distance from explosion January 1947 — A ugust 1955 



(per 10,000 persons) 

 more than 2,000 metres 2 



1,500 — 2,000 metres 3-4 



1,000—1,500 metres 28 



below 1,000 metres 128 



The possible effect of ionizing radiations from radioactive sub- 

 stances on the genie material is of even greater moment than the 

 effect on the health and life of the individual. As we have seen, ioniz- 

 ing radiations are capable of giving rise to mutations and it has been 

 found that a great majority of the mutations are deleterious, i.e. result 

 in the loss of a useful character. 



It must be remembered that such mutations may remain concealed 

 for several generations. This is because all the body cells contain two 

 chromosomes, one from each parent. Thus, if one of the chromo- 

 somes is complete, the fact that the other is damaged may be hidden 

 and only come to light when two individuals with the same mutated 

 gene produce offspring. 



It has been of great importance to make an estimate of the fre- 

 quency with which mutations are likely to occur as a result of ex- 

 posure to ionizing radiations. The first estimate of this kind was made 

 by H. J. Miiller, working with fruit flies, who found that the exposure 

 of the spermatozoa of the fruit fly to only one roentgen of X-rays in- 

 creased the mutation frequency (for certain mutations) no less than 

 ten times. W. L. Russell, at Oak Ridge, has made studies on mice ex- 

 posed to ionizing radiation and found an effect at least ten times that 

 observed with fruit flies. However, the natural mutation frequency is 



