U.S. Nuclear Regulatory Commission
Question 27: Do licensees have to post controlled areas
(outside the restricted area) as airborne radioactivity
areas if derived air concentrations (DAC) are exceeded?
Answer: Yes, if the airborne radioactivity is indoors.
If the airborne radioactivity is outdoors, the answer
depends on the particular situation. In certain situations
the licensee may need to identify and delineate an outdoor
airborne radioactivity area. For example posting would be
required in a small area, accessible to workers, in the
immediate vicinity of a vent on the outside of a building,
exhausting air containing concentrations of radioactive
materials in excess of the DACs specified in Appendix B to
10 CFR Part 20. (References: 10 CFR 20.1003, 10 CFR