UNCLASSIFIED The very rapid clearance of radioactivity from the G.I. tract contrasts with the slower biological decay observed in the respiratory system. Two rate constants are also sufficient to describe the loss from the respiratory system. The initial rapid loss of material (Ay = 2. 31/day) probably reflects the rapid upward movementof material in the respiratory tract, while the second component (2 = 0. 040/day) may be associated with the slower loss of particles which had been "fixed"! in the alveolar tissue or leaching of Sr from the deposits in the lung. . The slowest rate of clearance of Sr by any tissue is, of course, that from the skeletal system. The two components of this curve describe an initial fairly rapid loss (\j = 1.54/day) and a secondary slower turn- over (Az = 0. 019/day) of the bone-fixed Sr. Radiotoxicity of Inhaled Strontium Aerosol--Relative Dose to Organs The dose delivered to tissues by beta particles emitted from radioactive isotopes can usually be estimated by relatively simple methods when the tissue concentration of the isotope is known. In this experi- ment the gamma emitter Sr85 (carrier free) was used as a tracer for estimating the tissue concentration of the beta-emitting Sr?9, Since the range of beta particles in tissue is usually a few millimeters, the dose from deposited isotopes is confined largely to the organ containing the material, particularly for an organ with dimensions large with respect to beta particle rangel@, Calculations of tissue dose from beta particles are essentially estimates of the energy made available by the decay of a quantity of the isotope per gram of tissue. The isotope concentration divided by an appropriate constant permits direct conversion to a unit of dose!@, Thus, dose as used here refers to the concentration of the beta~emitting Sr?0 per gram of tissue since there is a direct relationship between the concentration of the beta emitter and the radiation dose received by the tissue. The ''relative dose" refers to the ratio of doses received by two organs. In this paper relative doses received by organs are expressed in terms of the dose received by the skeleton. Calculation of the relative dose is based on several assumptions. First, the sr85 gammaactivity per gram of tissue is assumed to re- flect accurately the Sr90 concentration. Secondly, it is assumed that the radioactivity is evenly distributed in the organs. Calculation of dose is also based on the assumption that the energy emitted in an organ jr 1 ran Ie Io rs ” a , [wm |m rm k kt Pad a Pt rm rq yo eed us a pia I> Ic 12 Ia Ike -15.