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