- 30 the general population the permissible level is 100 strontium units; i.e., the amount of radioactivity from Sr9° in the diet should not exceed the ratio of 100 micro- microcuries (P20| disintegrations per minute) of Sr9° for each gram of calcium. Often the strontium units that are determined for single items in the diet are used as the eriteria for hazard, but more properly the strontium units should be determined from the Sr9° and calcium in the entire diet. Sr9° has been used here as an example only, because it will be shown later that very little Sr9° is found in marine organisms. The MPC values for the radioisotopes listed in Tables I and VI vary by a large factor, but the range in values for the 240 isotopes in Handbook 69 is even greater, one million. This large range in MPC values is important to note because it is obvious that an evaluation of hazard cannot be based solely on the amount of radioactivity in the sample without chance of a great error. Proper evalu- ation of hazard depends upon the determination of the amount of radioactivity as well as the identification of the radioisotopes in the sample. A concept of hazard is sometimes created, psycho-