U.S. Nuclear Regulatory Commission
Question 13: Why was a 2-hour half-life chosen as a time of reference for noble gases or short-lived radionuclides, as used in 10 CFR Part 20, Appendix B and its footnotes?
Answer: As indicated in Footnote 2 to Appendix B, the radionuclides that have half-lives of less than 2 hours "might include a significant contribution from external exposure." "Significant contribution from external exposure" in this footnote means that the contribution to the dose equivalent from external irradiation exceeds that from inhalation. Two hours is the half-life value below which the contribution to the dose equivalent from external exposure exceeds that from inhalation for virtually all radionuclides.
A more detailed explanation is provided below. For a given radionuclide, the ratio of the dose from external irradiation to that from internal irradiation (from inhalation) depends on the half-life of the radionuclide, the characteristics of the radiations emitted in the decay of the radionuclide, the physical and chemical properties of the radioactive material containing the radionuclides, and the physiological response of the body to intakes of this material. However, considering the effect of half-life alone, and in general, the value of this ratio increases as the half-life decreases. The Statement from the 1983 Meeting of the ICRP includes the following paragraph:
"In ICRP Publication 30 the values of DAC for occupational exposure to short-lived nuclides (other than isotopes of noble gases) are based on the dose equivalent to organs and tissues as the result of inhalation. The Commission wishes to draw attention to the fact that there is an additional contribution to these dose equivalents from external irradiation. In situations where short-lived materials are widely distributed in the workplace, this additional contribution may be greater than that due to inhalation by a factor that increases from about 1 to 100 as the half-life of the radionuclide decreases from 1 day to 10 min. Such contributions should be assessed as part of the external irradiation."
Actually, for radionuclides with half-lives of roughly 2 hours, the values of this factor fall within the range of about 1 to 10. Thus, for virtually all radionuclides with half-lives less than 2 hours, the value of this factor is greater than one. Values of this factor greater than one were selected as values indicating "a significant contribution from external exposure."
(References: Part 20 Appendix B Footnote 2)