Retention and Excretion of Inhaled 3H and 14C Radiolabeled Methane in Rats

Abstract

A radiological concern for workers at heavy water reactor nuclear facilities is the hazard presented by tritium (H) and C. Radioactive methane is one of many potential H and C containing chemicals to which Nuclear Energy Workers (NEWs) may be exposed. Current dosimetric models for H- and C-methane, recommended by the International Commission on Radiological Protection (ICRP), are based on the assumption that 1% of methane is absorbed following its inhalation. Of this 1%, all H is converted immediately to tritiated water and C is converted immediately to CO2 (50%) and organically bound carbon (50%). In the study, rats were exposed to methane standards (H-methane and C-methane) mixed with breathing air to give a final concentration of 0.27% methane and resulting in final activity concentrations of 4.2 GBq m and 0.88 GBq m for H and C, respectively. This corresponds to exposure estimates of 580 kBq g and 120 kBq g. Simultaneous exposure to H- and C-methane allowed for the direct comparison of the retention of these radionuclides and removed uncertainties concerning their relative uptake and retention. The results demonstrate that the total methane uptake from the inhaled dose was threefold less than the 1% methane uptake predicted by the ICRP dosimetric models for H- and C-methane, with the H concentration being substantially higher than anticipated in the liver. This study provided data suggesting that current ICRP dosimetric methane models overestimate the fraction of H- and C-methane that is absorbed following inhalation and assisted in providing information to better understand the metabolism of inhaled H and C radiolabeled methane.

ICRP Publication 134: Occupational Intakes of Radionuclides: Part 2

“…(1967). The model predicts whole-body retention of carbon that is moderately higher than observed in rats over the first 14 d after inhalation of labelled methane (Didychuk et al., 2014).…”

“…Carlisle et al . (2005) and Didychuk et al. (2014) investigated the extent of oxidation and organic fixation of 3 H and 14 C following inhalation of a mixture of 3 H- and 14 C-labelled methane by rats.…”

“…Approximately 70% of 3 H retained in liver and 10% of 3 H retained in other tissues was organically bound. In a more comprehensive study using the same methods, Didychuk et al. (2014) followed retention in these tissues up to 14 d after exposure.…”

“…Most of the infused methane was exhaled unchanged (Dougherty et al., 1967). In rats exposed to 14 C-labelled methane mixed with breathing air, an estimated 72% of the label that reached blood was exhaled in the first hour (Carlisle et al., 2005; Didychuk et al., 2014). Measurements over 14 d post inhalation indicated that most of the retained 14 C was organically bound.…”