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 Web Site