” 18 of carbon. The C-14 produced in thermonuclear weapons testing is usually _ expressed as a percentage increase over the natural level. The eoncentration of carbon-14 in the normal carbon compounds of the body follows any chunge in the concentration of carbon-14 in the environment with a time lag ot one or two years. Thus there have been only a few measure- ments of carbon-14 in man and attention has been directed towards measurements in air. The following table shows the percentage of excess carbon-14 resulting from thermonuclear weapons testing. The data for 1968 are nat yet available. Inventory of C-14 in Tropospheric Air ~ Northern Hemisphere (Data Abstracted from UNSCEAR Reports) 1956 7 _ ‘ 8 . 5 1l 16 9 ou 1 25 1960 2 3 A 5 6 7 % over normal | . 23 30 65 92 90 78 65 There is no indication of any variability in the concentration of carbon-14 with age or with geographical location over the United States. Radium-226 (Natural) adiun-226 in man comes largely from the diet except for a few locations where the water contains high concentrations of radium. Fairly extensive measurements on human bone are available for three cities, New York, San Francisco und San Juan. The respective values of radium are 35 pCi, 29 pCi