Comments to the draft manuscript
Page 78, Table 6.3
Transfer coefficient from Blood to Urinary bladder contents of 1.5 d-1 is the same as the corresponding parameter provided in Table 3.6 of Publication 134 as well as other parameters in Table 6.3. However, those parameters were determined for workers by averaging metabolic data of various substrates including many abiogenic carbon compounds. As a consequence, for the carbon in foods, the parameter value of 1.5 d-1 is physiologically inconceivably large, because most of hydrogen in nutrients should be excreted via CO2 model instead of urine pathway (1,2). There must be some sort of logics to ensure the validity of diversion of the parameter values. Additional explanations of that will help readers in their understanding.
Page 73, Fig. 5.2
The definition of OBT-1 and OBT-2 are not clear in the draft manuscript, but in any case, the installation of OBT-2 compartment in the model may lead to overestimation of dose coefficients. If OBT-1 corresponds to the OBT compartment of Publication 56 having half-life of 40 days, the dose from OBT-2 compartment is redundant. On the other hand, if OBT-1 and OBT-2 are based on physiological data adopted by Taylor in their report (3), tissue weighting factor based on its physiological characteristics should be assigned for OBT-2 for the following reasons. Arner et al. (4) estimated the half-life of carbon in adipose tissue in humans more than a year, which is comparable to the half-life of tritium in OBT-2 compartment. In animal experiments, Takeda (5) showed that tritium in adipose tissue remains longer than in other organs and tissues. These findings suggest that at least some portion of OBT-2 consists of tritium in adipose tissue which is not radio-sensitive. The dose coefficient for workers calculated by this model without tissue weighting factor for OBT-2 would be overestimated. In case of members of the public, further overestimation could be occurred since foods contain much lipid which tend to be stored in adipose tissue. An additional explanation of the definitions of OBT-1 and OBT-2 would be helpful to readers.