The 1985 committee concluded that inhalation exposures had only a “minor impact on total doses expected” (NRC, 1985b). The committee’s concerns about methods of estimating inhalation doses thus did not appear to be important with regard to the potential for significant doses to atomic veterans. The view that inhalation doses generally were overestimated in the NTPR program and were unimportant was echoed in a later study (IOM/NRC, 1995).
The present committee’s evaluation of methods used in the NTPR program to estimate inhalation doses focuses on the question of whether the methods are likely to provide credible upper bounds of possible doses (see Section IV.E.4). The committee’s evaluation is divided into three parts. The first part discusses assumptions used in estimating inhalation doses that, in the committee’s opinion, tend to result in overestimates of dose. The second part discusses assumptions that, in the committee’s opinion, tend to result in substantial underestimates of inhalation doses, and it also considers assumptions that have substantial uncertainty and the importance of that uncertainty in obtaining credible upper bounds of inhalation doses. The third part summarizes the committee’s evaluation of methods of estimating inhalation doses used in the NTPR program.
The committee found that several assumptions used to estimate inhalation doses in the NTPR program should tend to result in overestimates of possible doses. In the following discussion, assumptions related to estimating inhalation dose coefficients (equivalent doses to specific organs or tissues per unit activity of radionuclides inhaled) are considered first and are followed by assumptions related to estimating inhalation exposures (intakes of radionuclides in air); these are the two components of models used to estimate inhalation doses (see Section IV.C.2).
 In exposure scenarios in which inhaled particles are assumed to be respirable (that is, when a particle size, AMAD, of 1 μm is used), organ-specific inhalation dose coefficients used in the NTPR program often (but not always) are higher than values for the same particle size currently recommended for use in radiation protection of workers by ICRP.
An AMAD of 1 μm often is assumed, for example, in scenarios involving suspension of activation products in soil or resuspension of fallout particles that were deposited on the ground or other surfaces, especially when this assumption results in higher estimates of dose than would an assumed particle size of 20 μm (see Section IV.C.2.2.1).