Theoretical organically bound tritium dose estimates

This paper illustrates a theoretical approach to estimating the dose associated with the ingestion of both organically bound tritium and tissue free water tritium relative to the ingestion of only tissue free water tritium. Organically bound tritium, specifically non-exchangeable OBT, can result in an increased dose relative to that from exchangeable organically bound tritium and tissue free water tritium because of the longer biological half-life of the former resulting in a dose conversion factor that is twice that of the latter. Non-exchangeable organically bound tritium is tritium that is bound to carbon whereas exchangeable organically bound tritium is tritium bound to oxygen, nitrogen, or sulfur. Tissue free water ranges from 85+% in most fruits and vegetables down to approximately 10% in grains. The remaining edible food mass consists, in part, of exchangeable and nonexchangeable hydrogen that is incorporated into carbohydrates, proteins and fat. The potential organically bound tritium content of several common food items was calculated knowing the amount of bound and unbound hydrogen that exists in these foods and by assuming that the hydrogen to tritium ratio is the same for the "free water" and bound hydrogen compartments. The theoretical ratio of dose from ingestion of organically bound tritium and tissue free water tritium to dose from ingestion of only tissue free water tritium was calculated to be on average within 12%, 30%, and 261% of experimentally based values for fruits and vegetables, meats and eggs, and grains, respectively. The difference is attributed to the T:H ratio being a function of the kinetics associated with the assimilation of tritium into the tissues.