inhalation doses, the NTPR program relies to a large extent on an assumption that resuspension factors used in some scenarios result in substantial overestimates of airborne concentrations of radionuclides to which most participants were exposed. That is, data on relative activities of radionuclides produced in a particular detonation, the assumption of no fractionation of radionuclides except for removal of noble gases, data on external exposures or exposure rates obtained with film badges or field instruments, calculations of exposure rates per unit concentration of radionuclides on a surface or in the air, and assumed breathing rates used in dose reconstructions usually are not intentionally biased in an effort to yield overestimates of inhalation exposure. But assumptions about resuspension often are intended to be “high-sided” to an extent sufficient to compensate for possible errors and uncertainties in all other factors that enter into a calculation of inhalation dose, including the inhalation dose coefficients discussed in the following section.
Inhalation dose coefficients (equivalent doses to specific organs or tissues per unit activity of radionuclides inhaled) used in all dose reconstructions are 50-year committed doses. That is, they represent the total dose received over a period of 50 years after an acute intake. The use of committed doses takes into account that an acute intake of a radionuclide may result in a dose that is received over many years after intake, and the 50-year period for calculating committed doses represents an average life expectancy of a young adult. For some radionuclides, the entire 50-year committed dose is received within a short time after an intake. For example, the committed dose due to intakes of 131I, with a half-life of 8 days, is received within a few months. However, in cases of intakes of long-lived radionuclides that are tenaciously retained in the body, such as plutonium, the dose to an organ or tissue of concern can be protracted throughout the rest of life, with only a small fraction of the committed dose received in each year. In all dose reconstructions, 50-year committed equivalent doses are assigned to the year in which exposure occurred regardless of the dependence of the total dose received on time after an intake (the time dependence of the dose rate).
All inhalation dose coefficients used in dose reconstructions are calculated with two kinds of models:
Biokinetic models that describe the deposition, retention, translocation, and absorption of inhaled radionuclides in the respiratory tract or ingested radionuclides in the gastrointestinal (GI) tract and the transfer, deposition, and retention of absorbed radionuclides in different organs and tissues of the body.
Dosimetric models that are used to calculate equivalent doses to different organs or tissues (the target organs) per decay of a radionuclide in each source organ (site of deposition or transit through the body).