Question 379: Should radioactive noble gas concentrations
be excluded (a) with regard to evaluating and posting
Airborne Radioactivity Areas and (b) in determining Derived
Air Concentration-hours (DAC-hours)? The definition of
Airborne Radioactivity Area refers to areas where airborne
radioactivity concentrations exceed the DAC values or where
an individual could exceed 12 DAC-hours in a week. DAC is
defined as the "concentration of a specific radionuclide in
air which, if breathed...results in an intake of one ALI
[Annual Limit on Intake]." The values listed for
radioactive noble gases in the DAC column in 10 CFR 20
Appendix B are identified as "submersion" values that apply
to external, rather than internal, exposure. Also, there
are no ALI values listed in 10 CFR 20 Appendix B for
radioactive noble gases. From this, it appears that
radioactive noble gas concentrations do not apply to
evaluating and posting Airborne Radioactivity Areas or to
DAC-hour determinations.
Answer: (a) Radioactive noble gas concentrations should
not be excluded with regard to evaluating and posting
airborne radioactivity areas. See the discussion below.
b) Radioactive noble gases of the "submersion" class (which
have no inhalation ALI) should be excluded in determining
DAC hours for use in determining the committed effective
dose equivalent (CEDE). In other words, the DACs for noble
gases are based on external dose and should not be used to
assess internal dose.
Discussion: The following discussion relates to the
posting question. Although the definition of DAC in 10 CFR
20.1003 does not include concentrations calculated on the
basis of the external dose resulting from "submersion,"
Appendix B clearly states that the DAC values listed in
Table 1 of Appendix B "relate to one of two modes of
exposure: either external submersion or the internal
committed dose equivalents resulting from inhalation of
radioactive materials." The definition of "airborne
radioactivity area" refers to "...concentrations - (1) In
excess of the derived air concentrations (DACs) specified
in Appendix B...." Thus, the definition of "airborne
radioactivity area" includes the DACs in Appendix B that
are noble gases and that are based on "submersion." The
preamble to revised Part 20 (56 FR 23379, second and third
columns) also indicates that areas that meet the definition
of "airborne radioactivity area" because of the presence of
noble gases are required to be posted.) (References: 10
CFR 20.1902, 10 CFR 20.1003).