Answer to Question #464 Submitted to "Ask the Experts"

Q

What are the biological half lives of ²¹⁰Po and ²⁴¹Am?

A

The biological half-time is the time in which one-half of the material in a specified mathematical compartment is removed by a biologic process in the absence of any input into the compartment. The rate of the biologic process may depend on the age or gender of the individual and vary between individuals. The removal rate constant, k, from the compartment (fraction removed per unit time) is related to the biological half-time, T_b, in the same manner as a radionuclide's decay constant is related to the physical half-life; i.e., k = ln(2)/T_b. The removal of many materials from an organ or tissue depends on several processes and their behavior is described using multiple compartments with distinct biological half-times. In these instances, it is not possible to speak of a single biological half-time.  The two radionuclides in question illustrate a range in complexity.

The mathematical model describing the behavior of polonium (Po) in the body discussed in Publication 67 of the International Commission on Radiological Protection (ICRP) assumes a single biological half-time of 50 days for all tissues and ages. Furthermore, the model assumes that Po removed from a tissue does not enter another tissue. The physical half-life of d ²¹⁰Po is 138.38 d and thus the effective half-time reflecting both the biological removal and loss by radioactive decay is 36.7 d. The effective half-time is the reciprocal of the sum of the reciprocals of the physical and biological half-times. The behavior of americium (Am) in the body is more complex and depends on the age of the individual. Much of the additional complexity is associated with its incorporation and subsequent removal from bone mineral. The ICRP model (see Publication 67) for Am is presented as a compartment model with the transfers among the compartments specified by rate constants rather than half-times. The transfers vary rather widely in their fractional rates. In the adult, the removal rate of Am from blood corresponds to a half-time of about half an hour (0.021 d) while that from compact bone mineral is about 23 y (8,440 d). In the model the kidney is represented by two compartments, one with a half-time of about 70 d and the other 500 d. Am leaving a compartment may be enter into another compartment (for example, Am leaving bone mineral may enter into blood), so if a tissue were observed externally its biological half-time may not be evident. In the adult, about one-fourth of the ²⁴¹Am entering blood is removed by 3.8 y (1400 d) and one half has been removed by 27 y (10,000 d). The removal rate from the body slows down with increasing time and approaches the half-time of the skeletal tissues at times remote from the intake.

Keith F. Eckerman, PhD
Oak Ridge National Laboratory