ABSTRACT
The estimated values of time-integrated dose equivalent are different if one assumes continuous intake, such as is the case for longer-term environmental exposures, instead of instantaneous intake, which is the assumption made for most standardized computational procedures. This paper presents the solutions of the first order kinetic equations for the ICRP 30 models of the respiratory system (RS) and the gastrointestinal tract (GI) for continuous activity intake. These solutions are applied to compute the cumulated number of disintegrations for different intake and integration times in order to show the effects of the assumptions of instantaneous intake vs. continuous intake on dose. For pulmonary clearance class D compounds instantaneous and continuous intake functions result in dose estimations agreeing to within 10% (when normalized to total intake) for integration times larger than 8 d for both RS and GI, for the case of radionuclides with radiological half-lives ranging between 8 d and 10(7) y. For 141Ce (class W) for both RS and GI the temporal intake function affects the dose by less than 10% for integration times greater than 280 d. In this case, the results for GI are less sensitive to the type of intake assumed than those for RS: the ratio of the number of transformations with the two intake assumptions has values of 0.8 or less for intake times of less than 7 d for GI and for intake times of less than 153 d for RS. For radionuclides such as 89Sr and 90Sr (class Y), the magnitude of the dose is very sensitive to the assumption made regarding the intake and has a strong dependence on the radionuclide half-life: for a 10% accuracy in dose evaluation for a continuous intake treated as instantaneous, the integration time must be greater than 650 d for 89Sr and 18,000 d for 90Sr. For all cases studied, the assumption of instantaneous intake overestimates the integrated dose for a continuous intake. More accurate internal dose evaluations may be made by consideration of the intake function shape when dose integration times less than approximately 10 y are of interest and non-instantaneous intakes are believed to have occurred.
