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EETI G OF MEDICAL AND ' LABORATORY DIRECTORS UNITED STATES ATOMIC ENERGY COMMlSSlON
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(Assigned t o Los Alamos ScientFfic Laboratwy) Rats have h e n exposed continuously to various cmaentrations of tritium in inspFred air for periods up t o 145 hours. centration in the air breathed wae varied from lo4 to 3 x loo2 pc/cc different experimsnts but remained essentially constant tbraughout the period of exposure In any one experiment. When present as HPO inspired air a major portion of all the activity W e d is taken up in the body fluids indicating rapid exchange and absorption of activity across resphatory membranes. less than 0.s of the laspired actiyity appears In the bcdy fluids as !EO. Apparently biological catalyzatfm of Lf2 t o ET0 takas place in the animal body. for absorption of li!T per 9e in body fluid8 t o be detected. exposure t o H!l' ur HTO fn inspired air it took about 100 hours to reach e relatively steady value f o r HTO Fn the blood. Beyond this time €IT0 excretion through the various water excretory routes balances El'O intake fnto the body. bY due t o short time variations in the water turnorrer rate. The rat feeds and drinks most of its daily turnover a t night so that the tritlum activity in body fluids is usually lavest Fn the morning and highest in If present as XI! in inspired air The range of activities studied vas not. high enough T h e equilibrium value reached varies lO-l5% during the One experiment has been done on man in wMch 3 millicuries as ID0 was ingested In 200 CC. of water, abSOrpti011 i n t o the blood stream vBa linear with time and vas c q l e t e in about 45 &Utes. The tritium was diluted u i t h 57% of the body weight suggesting that bcdy water fs prbarilr
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involved. Body uater turnmer ma measured mer a pricd of s i x days by veighing a food and fluid intake and all excretory out@. For four days water turnover was kept near nonnal (2.7 liters per day). wing the l a s t 2 days vater turnover VBB lncrsaaed t o 12.8 Uters per b y . During both periods tritium axcretian vas exponential with vator turnover. The resemoir calculated &am the rate canstant vaa 5s of the b w wight. Tritium excretion rates determinsd im sight othsr human subjects in which water t\aPover uaa measured leu8 precisely ludicate a reservoir ranging Frau 57 t o 61tg of body uaigbt. S lm llar excretion data on rata give + a l u ~ ~ ~In t h i a same range. The biologiaal half-life oi MI0 in nine human subjects varied fran 9 t o 14 d 8 y s on ad libitum water intake and was reduead to 2 l/2 days in one subject on high water intake. Ths biological hslf-llfa of ET0 in rats ranged f r o m .G t o 5 days on ad llbitum wster intake. The tritium sctivity in mter fran meat, insensible perspiration, expired water vapor, sputum, and u r l n e has been masured and fcnmd t o be essentially the same as that in water frau the blood. These data support the thesis that isotopic dil uti cr pl or concentration of tritium does not occur t o asy marked degree anywhere in the body and that trftlum ie generally distributed throughout the body in propartion t o t h e water content of each part. With this in mind it vould seem that the safety factor of 10 introduced a t the C W River Conference mfght be dropped and that tritium tolerance doses could be caloulatdd, aa it is far radiation generally, nambly on the baris of that which Would give 0.3 rep per week in the body. for a 70 kflogram man of appraxbatsly 10 milllmuriea. A8sUm bg a 2.5 This giYb6 a tolerance dose
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liter daiu water turnmar, 500 pc are excrutrd daily. To absorb t h i a much in 811 8 hour day asmrming 10 cubic meters of air hspirsd u d d E. To make allovance for the fact that same body tisaur contain as much 88 9ok uater, these tolerance aotfvitiss might be reduced by sbaut 3%; 3.5 x 10-5 pc/cc f a 5 0 or 0.035 p / c c a6 IIT.