Health Physics Questions and Answers - Question 426
Question 426: Are the noble gas ("submersion") DACs based on a dose of 5 rem per year or 50 rem per year? Is the submersion dose calculated at a depth of 1000 mg/square cm or 7 mg/square cm?
Answer: There is no one particular dose or one particular depth. The method for calculating submersion doses is explained in Federal Guidance Report No. 11 on pages 10, 18, 181, and 182. When air concentration is limited by submersion dose, the DAC for a particular radionuclide is the maximum concentration of that radionuclide in air that, for a 2,000-hour exposure, will result in a dose that is equal to or less than each of the applicable limits (5 rem effective dose equivalent, 15-rem eye dose equivalent, 50-rem dose equivalent to other organs and tissues, shallow dose equivalent of 50 rem to the skin). That is, the DAC for a particular radionuclide depends on which of the applicable dose limits is the most restrictive with respect to the concentration of that particular radionuclide. The dosimetric model used to calculate the DACs considers shielding of organs by overlying tissues and the degradation of the photon spectrum through scatter and attenuation by air. The dose from beta particles is evaluated at a depth of 7 mg/square cm for skin, and at a depth of 3 mm for the lens of the eye. The worker is assumed to be immersed in pure parent radionuclide, and no radiation from airborne progeny is considered. In most cases, the concentration limit for submersion is based on external irradiation of the body; it does not take into account either absorbed gas within the body or the inhalation of radioactive decay products. An exception to the preceding statement is Ar-37, for which direct exposure of the lungs by inhaled activity limits (stochastically) the concentration in air. The skin dose is limiting for Ar-39, Kr-85, and Xe-131m; the eye dose is limiting for Kr-83m. Note: There are typographical errors in the discussion of submersion doses on page 10 of Federal Guidance Report No. 11. In the fifth sentence of the paragraph beginning "Some airborne radionuclides...", the word "effective" should be added before the words "dose equivalent rate". In equation (8b), the subscript "E" should be the subscript "T."
(Reference: 10 CFR 20 Appendix B)