XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 495 



radon and its decay products in the atmosphere at sea level averages nearly 

 double that due to the radiation from the earth's crust, and is rather more 

 than the combined effect of 7 radiation and cosmic radiation. This ioniza- 

 tion is mainly produced by a radiation. The radon content of stagnant 

 underground air or soil air is very much higher (2). 



All the types of ionizing particle under consideration in this chap- 

 ter contribute to the total ionization, but the proportional contribu- 

 tions will vary greatly from one organism to another depending on its 

 size and location. For example, over the land a rays will contribute 

 most to the ionization of unicellular airborne organisms, or the most 

 superficial cells, either on the body surface or in the respiratory tract 

 of larger organisms. Roots may be irradiated about equally by a 

 particles and electrons. In general, electrons will contribute most of 

 the ionization to the deeper layers of large organisms, but in addi- 

 tion there will be some ionization due to neutrons and to other rays 

 that are so far only known to us as components of cosmic radiation. 

 Since the discovery of cosmic rays there has been speculation as 

 to the possible biological effects of continuous exposure to this 

 radiation. In open air at sea level the majority of cosmic ray ioni- 

 zation is probably produced by electrons of very great energy, but 

 from time to time there occurs a local release of enormous amounts 

 of energy in the form of a "shower" of particles of many different 

 types. The frequency with which an organism will be exposed to 

 such particles will be greatly increased if it is placed immediately be- 

 neath a 2 inch slab of lead, more especially if the experiment is carried 

 out at high altitudes. Figge (5) observed that the latent period re- 

 quired for tumor induction in mice injected ^Adth methylcholanthrene 

 was significantly shorter in mice placed beneath a thick slab of lead 

 than in controls not kept beneath lead, and concluded from these ob- 

 servations that cosmic ray showers are cocarcinogenic. The experi- 

 ments touch upon a subject of such immense importance that they 

 need to be repeated and extended. 



The total effect at the surface of the earth of all the components 

 of cosmic radiation, of which the "showers" are only a small part, is to 

 produce 1 or 2 ion pairs per cubic centimeter per second. When 

 contributions from radioactive substances in the air are included the 

 total is only around 10 ion pairs per cubic centimeter per second. 

 This is an extremely low rate of ionization. It corresponds to an X 

 ray dose rate of 5.10~^ r. per second and to the production of only 1 

 ion pair in a cell 12 ii in diameter once a day. Cosmic rays alone 



