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08 February 2012

Answer to Question #362 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Internal dose calculations

The following question was answered by an expert in the appropriate field:

What is the biological half-life of ²³⁰Th and ²³²Th?

The biological half-life of ²³⁰Th and ²³²Th is the same. The International Commission on Radiological Protection gives the following information:

8,000 days for thorium in bone, and 70 percent of the uptake goes to bone.
700 days for thorium in the liver, and 4 percent of the uptake goes to liver.
700 days for thorium in all other tissues and organs, and 16 percent of the uptake goes to these locations.
The remaining 10 percent is assumed to be excreted with a half-life of 0.5 days.

These values are based on animal experiments and, although expected to be close, are not exact numbers. People vary and so do their metabolisms. For example, work in the Human Monitoring Laboratory on human volunteers has seen the biological half-life of iodine vary from 20 days to 200 days. The accepted average is 80 days. Note: Uptake refers to the amount of thorium that enters the blood. If an insoluble compound of thorium is inhaled it can stay in the lung for a long time and only slowly enters the blood stream.

Gary Kramer, PhD

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