Answer to Question #199 Submitted to "Ask the Experts"

Q

What is the current occupational exposure limit for ²³⁹Pu and how was it established?

A

The current annual occupational dose equivalent limit for ionizing radiation for adults in the United States, as prescribed in the rules of the US Nuclear Regulatory Commission (10 CFR 20), the rules of the US Department of Energy (10 CFR 835), and the regulations of the various states is the more limiting of the following:

Total effective dose equivalent: 5 rem (0.5 Sv); or dose equivalent, to any individual organ or tissue (other than lens of the eye): 50 rem (0.5 Sv). Further elaboration on these limits can be found in 10 CFR 20.1201 (Code of Federal Regulations, title 10, Part 20.1201), in 10 CFR 835.202, and in corresponding sections of the various state regulations. The case of the dose resulting from the intake of radioactive material needs further explanation. When the radioactive material remains in the body for an extended period of time (that is, has an "effective half-life" of several months or more), due to a long physical half-life or slow biological turnover, the radiation dose will be delivered over a period of time greater than a year. This dose that will eventually result from the intake is referred to as the "committed dose." For the intake of radioactive material, the annual dose limit applies to the 50-year committed dose expected to result from the given year's intake. In dealing with the possible intake of a radionuclide such as ²³⁹Pu, it is operationally convenient to have a secondary limit in terms of the quantity of the radionuclide that may be taken into the body without exceeding the committed dose limit. This limit is known as the Annual Limit of Intake or ALI. The ALIs for ²³⁹Pu, as presented in 10 CFR 20, are 0.8 µCi/y (30kBq/y) for ingestion, 0.02 µCi/y (0.7kBq/y) for inhalation of Class Y compounds, and 0.006 µCi/y (0.2 kBq/y) for Class W compounds. (Class Y compounds are those for which the retention in the pulmonary region of the lungs is on the order of years–PuO₂ is assigned to Class Y. Class W compounds are those for which the retention in the pulmonary region of the lungs is on the order of weeks—all commonly occurring compounds of Pu other than PuO₂ are assigned to Class W.) The ALIs are calculated from the dose equivalent limits using the available information on the biological behavior of the element, the transformation information for the radionuclide (particle and photon types, energies, and abundances), and internal dose calculation techniques.

Several references pertaining to limits used in the United States and their basis include:

Radiation Protection Guidance to Federal Agencies for Occupational Exposure; Approval of Environmental Protection Agency Recommendations, Federal Register, Vol 52, No. 17, January 27, 1967, 2822-2834 (and corrections published January 30 and February 4, 1987).

Federal Guidance Report No. 11. Limiting Value of Radionuclide Intake and Air Concentration and Dose Conversion Factors for Inhalation, Submersion, and Ingestion. EPA-520/1-88-020; September 1988, Office of Radiation Programs, US Environmental Protection Agency: Washington, DC.

Gen Roessler, Editor HPS website