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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).