Tc! Te j= "3 \- [a [wm > Ic (2 10 tc INTRODUCTION The general problem of determining the degree of internal hazard to a biological organism resulting from inhalation of radioactive fallout is so complex that it can be treated only by breaking the problem down into simpler and more easily analyzed aspects. The complexity of the problem derives from the large number of factors which influence the uptake, re-~ tention and elimination of radioactive particles by a biological system. ~~ Determination of the radiotoxicity of inhaled materials can readily be separated into two aspects: the nature of the aerosol itself, and the reaction of the biological organism to the particular aerosol. In the production of fallout, the radioactive particles become attached to a carrier material. The physical and chemical characterization of the aerosol, therefore, includes not only determination of the chemical form of the | fission product itself but also the physical and chemical nature of the carrier with which it is associated. The nature of the carrier material and its physical association with the radioactive isotope influences the metabolism of the inhaled aerosol. The reaction of the organism to the radioactive aerosol can be considered in terms of (1) the routes of entry into the body (respiratory system and/or G.I. tract); (2) the uptake and distribution of the particles in the body; (3) their retention and eventual clearance. It is obvious that the biological reaction is greatly dependent on the characteristics of the particular aerosol to which the organism is exposed. Further, the reaction of the organism is not a simple fixed process but varies considerably with the physiological state of the organism. The radiotoxicity of inhaled radioactive fallout has been studied in terms ' of the metabolism of specially prepared simulants of fallout. 5,6 The rate of uptake, the distribution in the body tissues, and the ultimate fate of particles which gained entry into the body were measured. During the course of these experiments it was observed that following an inhalation -l- EE ?T &BERFE&?.:iEr.g EF FF & &@ ea ra UNCLASSIFIED