U.S. Nuclear Regulatory Commission
Question 38: Can the results of bioassays alone be used to determine if the licensee must sum internal and external doses under Part 20?
Answer: No. Summation is required if the licensee is required to monitor for both external and internal doses. The results of bioassays alone cannot be used to determine if the licensee must monitor internal exposures or sum internal and external dose under 10 CFR Part 20. Monitoring for internal is required for adults "likely to receive" in a year an intake greater than 10% of the limit. Determination of what an individual is likely to receive is a prospective assessment of intake. Bioassay is a retrospective assessment of intake. Future intakes are not necessarily the same as past intakes. However, bioassay data may be used together with other information as a basis for the prospective intake assessment. For example, if the uses of radioactive materials in a facility are not going to change significantly and bioassays of individuals employed in the facility have shown that no one has ever received an intake greater than 10%, then one might reasonably conclude that no one is "likely to receive" an intake in excess of 10% of the limit.
(Reference: 10 CFR 20.1202)