Approved December 28, 1973
Although maximum permissible body burdens (MPBB) and maximum permissible concentrations (MPC) in air and water for natural uranium were originally expressed in units of mass for the maximum permissible body burden and of mass per unit volume for the maximum permissible concentration (Hodge et al., 1953), later usage by the National Committee on Radiation Protection (NCRP) recognized a “special” curie that was applied specifically to natural uranium and to natural thorium. For natural uranium this “special” curie was defined “…to correspond to the sum of 3.7 × 1010 dis/sec from U238, 3.7 × 1010 dis/sec from U234, and 1.7 × 109 dis/sec from U235” (NCRP, 1959; 1963),1 the ratio of activities being determined by the usual activity ratio of these uranium isotopes as they occur in nature.
Because, in practice, the “special” curie for natural uranium has led to confusion and because its use may be regarded as compromising the integrity of the curie as a unit, the NCRP recommends that use of the “special” curie be discontinued and that mass units be used to define quantities of natural uranium as they appear in the specification of MPC and MPBB values. In order to conform with this recommendation, the MPBB and MPC values for natural uranium listed in NCRP Report No. 22 may be multiplied by the conversion factor 3 × 106 micrograms per “special” microcurie. Thus, the MPC value for air for 40-hour per week exposure to soluble compounds of natural uranium with the kidney as the organ of reference, is given in NCRP Report No. 22 as 7 × 10-11 “special” microcuries per cubic centimeter. Applying the conversion factor this becomes 210 micrograms per cubic meter of air. Similarly, for insoluble compounds of natural uranium with lung as critical organ, the (MPC) is given as 6 × 10-11 “special” microcuries per cubic centimeter, which converts to 180 micrograms per cubic meter of air.
This conversion from “special” microcuries to micrograms is done here for illustration only. It connotes no specific or new endorsement of the actual figures or any implication regarding whether the primary mechanism of damage is irradiation or chemical toxicity. These subjects are under study currently and will be taken up in a future NCRP report.
In 1959 the NCRP also defined a “special” curie for natural thorium, “…a curie of recently extracted thorium is considered to correspond to the sum of 3.7 × 1010 dis/sec from Th232 and 3.7 × 1010 dis/sec from Th228” (NCRP, 1959). For reasons similar to those pertaining to the case of natural uranium, the NCRP also recommends that the “special” curie definition for natural thorium be discontinued. Inasmuch as the isotopic composition of natural thorium is not constant, the NCRP further recommends that the use of a limiting concentration for natural thorium, as an entity, be discontinued, and that the established MPBB and MPC values for each of the individual thorium isotopes be utilized instead.
Hodge, H.C., Stokinger, H.E., Neuman, W.F., Bale, W.F. and Brandt, A.E. (1953). “Maximum allowable concentration of uranium dust in air,” page 2104 in Volume 1, Book 4 of Pharmacology and Toxicology of Uranium Compounds, Div. VI, National Nuclear Energy Series, Voegtlin, C. and Hodge, H.C., Eds. (McGraw-Hill, New York).
NCRP (1959). National Committee on Radiation Protection and Measurements. Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides in Air and in Water for Occupational Exposure, NCRP Report No. 22, National Bureau of Standards Handbook 69 (U.S. Government Printing Office, Washington, D.C.).
NCRP (1963). National Committee on Radiation Protection and Measurements. Addendum 1 to National Bureau of Standards Handbook 69, Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides in Air and Water for Occupational Exposure (National Bureau of Standards, Washington, D.C.).
1The value for U235 was given as 9 × 108 disc/sec in reference  but changed in reference  to 1.7 × 109 disc/sec.