When radiologically-restricted buildings or land areas are released for unrestricted use (i.e., without regard for residual radiological constituents), it is neither practical nor, in some cases possible, to remove all of the existing radioactivity. The difficulty becomes even greater when the residual radioactivity is commonly found in the natural background.
Radiation protection professionals struggled with this issue for a number of years, until the U. S. Nuclear Regulatory Commission (USNRC) issued its “Final Rule on Radiological Criteria for License Termination” in July of 1997. In that rule, the USNRC stated that buildings and areas may be released for unrestricted use as long as a maximally-exposed individual who occupies those buildings and areas has no potential to incur a radiation dose in excess of 25 millirem total effective dose equivalent (TEDE) in a calendar year.
The USNRC then, using conservative (i.e., selected to maximize the resulting dose) input parameters and assumptions, developed screening values that comply with the 25 millirem per year dose limit. These values have been divided into separate sections.