The time course of${}^{224}{\rm Ra}$,${}^{212}{\rm Pb}$, and212 Bi retention and220 Rn exhalation was measured in two groups of mice after a single intraperitoneal administration of${}^{224}{\rm Ra}$. The retained220 Rn fraction was calculated using two approaches: (1) From measured activity of220 Rn in exhaled air and from${}^{224}{\rm Ra}$ retention data. (2) From the measured${}^{212}{\rm Pb}$ or212 Bi retention and using a theoretical time course of the retention of these radionuclides; it was assumed in this case that the retained220 Rn fraction is constant within the duration of the experiment, i.e., during the first 3 wk after${}^{224}{\rm Ra}$ administration. The retained fractions found using the two approaches are in good agreement equaling 60-70% in both groups of mice; thus the fraction exhaled amounts to 30-40%.

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