Biokinetics of inhaled radioactive methane in rats: a pilot study.

Current dosimetric models for radioactive methane assume 1% of inhaled methane is absorbed, all 3H activity is converted immediately to [3H]H2O, and 14C activity is converted immediately to [14C]CO2 (50%) and organically bound carbon (50%). Results of a pilot study using rats to test these models suggest the models overestimate uptake but underestimate organic fixation of 3H and 14C, especially in liver. Also, the biokinetic properties of organically bound 3H and 14C in liver were markedly different from other tissues. Preliminary dose estimates based on observed uptake and organic fixation of label suggest current methane models likely overestimate radiation doses from radioactive methane by 3- to 10-fold.