to man. Lesser programs are in effect to look at other nuclides, .both natural and artificial, and these are only mentioned briefly. Potassium-40 (Natural) | ‘Potassium-l0 is a natural component of the element potassium and its radioactivity amounts to about 800 pCi/gram of potassium. The average man contains approximately 140 grams of potassium, so there are about 100,000 pCi of potassium-40 in the body. The measurements of body potassium are very widespread because the data can be obtained when measuring wholebody cesium-137 from nuclear fallout. The potassium concentration, how- ever, is controlled by the body and varies within narrow limits, as shown. .in the diagram. body weight. tion. The total potassium content is proportional to the lean There is no variation with time or with geographical loca- The average man with 140 grams of potassium in his body would be t ‘“_ =, a2 oS wr ”, N ° “POTASSIUM CONCENTRATION IH @X/ kg ~~ w . represented by the horizontal line in the diagram. 1 0 LL} 1 ! 2am wb © 4% _ AGE WW YEARS ! 6 WN . Diagram from Anderson and Langham, Science, 1.30, 713 (1959)