Answer to Question #12635 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Can you please let me know why there are two different values for committed effective dose per unit intake of radioactivity (Sv Bq-1) for the worker and the adult member of the public (for the same radionuclide, same aerodynamic diameters, same f1 value). I am referring to the values listed in International Commission on Radiological Protection (ICRP) Publication 119.
I assumed that since the radionuclide will behave in the same manner inside the body whether it is a worker or an adult member of the public, Sv Bq-1 value must be the same.
Your inference that the committed effective dose per unit intake of material should be the same whether we are talking about a worker or a member of the public is fundamentally correct. Naturally, such equivalence would only apply if the radioactive material is taken in via the same route—(i.e., inhalation or ingestion) and is in the same chemical and physical form, that the same database has been used to obtain radionuclide decay data for both cases, and that the same radionuclide distribution models and metabolic models are being applied to both workers and members of the public.
In large part, these assumptions are met for the adult workers and adult members of the public, and the values that appear in the ICRP publication 119 are generally very much the same, although not always exactly so. For example, I reviewed values for four radionuclides taken into the body in particulate form—60Co, 131I, 137Cs, and 239Pu. The ingestion values for both workers and adult members of the public were the same, but be aware, that the values for the public for a given radionuclide apply to the particulate form with the highest f1 value (fraction of material expected to be absorbed from the gastrointestinal tract), whereas the values for the worker are individually included for forms with different f1 values. The inhalation values differed slightly between workers and members of the public for the four radionuclides I checked, with the largest difference I observed being 6.4 percent between the public value and the worker value for Type M plutonium (5.0 x 10-5 Sv Bq-1 for the public and 4.7 x 10-5 Sv Bq-1 for workers). For particulate inhalation, the publication lists public values for all three absorption categories, F, M, and S, even though one or more of these classifications may not be realistic for likely forms of the particular radionuclide. The worker values do not include values for unrealistic forms. One must then be a bit careful when comparing values to ensure that the same categories are being compared.
The small differences in the dose coefficients that are observed between some of the published values for workers and the public may be the result of differences in certain input values, differences in calculational methods that have been used, and it is possible that rounding errors could play a part in explaining very small differences. The ICRP Publication 119 relies heavily on previous ICRP Publications 67 (Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 2 Ingestion Dose Coefficients); Publication 71 (Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 4 Inhalation Dose Coefficients), and Publication 68 (Dose Coefficients for Intakes of Radionuclides by Workers). If you check these references you will find that for many cases the values that appeared in them for given radionuclides are the same as those that appear in ICRP Publication 119. The ICRP acknowledges in Publication 119 that there had been some errors made in some past calculations for specific radionuclides, and these were corrected in Publication 119. You might want to take a quick look at pages 17–20 of Publication 119, which discuss some differences/corrections from earlier values.
From my experience, the differences I have observed between the adult members of the public values and those for workers have been well within the overall uncertainties expected for internal dose estimations. I do not believe it is possible to conclude that any specific value for the public is better or worse than, or in many cases statistically different from, its counterpart value for workers. I hope this is helpful.
George Chabot, PhD