United States Nuclear Regulatory Commission - Protecting People and the Environment

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


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


"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)

Page Last Reviewed/Updated Thursday, March 29, 2012