Tritium Dose Conversion Factor Confusion

[eh_carbaugh@ccmail.pnl.gov]

     The confusion over tritiated water dose conversion factors can be 
     cleared up rather easily without resorting to different quality 
     factors (radiation weighting factors, for you ICRP-60 types).  One 
     simply has to examine what conversion factor one is working with.
     
     The tabulated committed effective dose per unit intake factors of 
     ICRP-30 and ICRP-68 are not significantly different:
     
        ICRP-30         1.7E-11 Sv/Bq  (6.29E-5 rem/uCi by conversion)
        ICRP-68         1.8E-11 Sv/Bq  (6.66E-5 rem/uCi)
     
     Both ICRP-30 and ICRP-68 use a simple concept for tritiated water 
     inhalation:  intake equals uptake.  All tritium inhaled is assumed to 
     be absorbed.  This is essentially no different from the ingestion 
     model with an uptake factor of 1.  In other words, there is no 
     difference in the dose per unit intake (uptake) for tritiated water, 
     whether ingested or inhaled.  One could also extend this concept to 
     skin absorption (if it's absorbed through the skin, intake also equals 
     uptake).  I would suggest that the difference in these two dose 
     conversion factors is more related to computer code rounding that 
     dosimetric modeling.
     
     The confusion comes when one considers the ALI and the DAC.  Because 
     intake equals uptake, the ALIs for inhalation and ingestion are again 
     equal, both being 3E+9 Bq in ICRP-30.  
     
     The DAC is another story!
     
     The ICRP-30 DAC for tritiated water is listed as 8E+5 Bq/m3.
     
     If one multiplies the DAC by the reference man annual occupational 
     inhalation rate of 2400 m3, then one obtains an "apparent" ALI for 
     pure inhalation of 1.92E+9 Bq, instead of the listed value of 3E+9 Bq. 
      Where does the remaining 1E+9 Bq come from?...Skin absorption!
     
     In ICRP-68, the dose conversion factor (new term, "dose coefficient") 
     for inhalation is based on the assumption that tritiated water is a 
     Class SR-2 vator with complete absorption of all inhaled tritium (same 
     concept as ICRP-30, just new terminology).  Again, intake equals 
     uptake, with the result being inhalation and ingestion coefficients 
     are equal in magnitude.
     
     The open question is, "What impact will ICRP-68 dose coefficients have 
     on calculation of new ALIs?"  Will the next tritiated water ALI for 
     inhalation exposure continue to include a 1/3 component for skin 
     absorption as did ICRP-30, or will it be a pure inhalation ALI? I 
     don't know the answer to that one, but would speculate that the old 
     approach will stand.
     
     If you are interpreting data from urine sampling as opposed to air 
     sample concentration, the issue may be irrelevant because the dose per 
     body water concentration is the same.  (Editorial comment:  that's why 
     I prefer tritium dose assessment for direct comparison with primary 
     limits, rather than intake assessment for comparison to secondary 
     limits.)
     
     If I've missed the boat somewhere on this, please let me know.  I'm a 
     newcomer to ICRP-68 also, and still in a learning mode!
     
     eh_carbaugh@pnl.gov
     
     Gene Carbaugh (509) 376-6632
     Pacific Northwest National Laboratory