TABLE V.C.5 Summary of Assumptions Used to Estimate Inhalation Doses in NTPR Program That Should Tend to Result in Overestimates of Dosea

Dose coefficients (organ-specific equivalent doses per unit activity of radionuclides inhaled)

• Dose coefficients for respirable particles (AMAD, 1 μm) often are higher than values for same particle size currently recommended for workers by ICRP.

• Dose coefficients for large particles (AMAD, 20 μm) often are higher than values based on current ICRP recommendations.

• Assumption of respirable particles overestimates dose to lung and many other organs when most inhaled materials are large particles.b

• Assumed absorption of refractory radionuclides (for example, plutonium and isotopes of yttrium, zirconium, and rare-earth elements) from respiratory or GI tract may be overestimated, especially when large particles are inhaled.

• Dose to respiratory and GI tracts may be overestimated when large particles containing alpha-emitting refractory radionuclides (for example, plutonium) are inhaled.

• Use of 50-year committed doses may overestimate relevant doses from intakes of long-lived radionuclides that are tenaciously retained in the body (for example, plutonium).c

• Dose coefficients for the lung may overestimate dose to tissues in respiratory tract where lung cancers occur.d

Methods used to estimate inhalation exposures (intakes of radionuclides in air)

• Resuspension factors applied to fallout deposited on ground or to neutron-induced activity in soil may overestimate airborne concentrations in some scenarios.e

a Assumptions are discussed in detail in Section V.C.3.1.

b For most radionuclides, assumption of respirable particles when large particles are inhaled does not overestimate dose to organs of GI tract.

c Relevant dose is dose received before disease of concern in exposed person occurs, taking into account latent period between radiation exposure and earliest onset of disease. However, as discussed in Section V.C.3.1, comment [6], use of 50-year committed doses may underestimate relevant doses in some cases.

d Most lung cancers, including cancers caused by radiation, occur in bronchial region.

e Conclusion applies mainly to scenarios in which resuspension of fallout deposited on the ground or suspension of neutron-induced activity in soil is caused by normal wind stresses or walking and other activities that do not involve vigorous disturbance of surface soil.

V.C.3.2 Assumptions With Substantial Uncertainty or Tending to Underestimate Inhalation Dose

The committee also is concerned that some assumptions used to estimate inhalation doses in the NTPR program may not tend to overestimate actual doses and thus may not lead to credible estimates of upper bounds for use in evaluating claims for compensation.

The committee’s concerns are of two kinds. The first is that, in some cases, assumptions about scenarios of inhalation exposure or estimates of parameter values probably result in substantial underestimates of possible doses, provided that other assumptions used in estimating inhalation dose are reasonable. The second concern is that uncertainties in assumptions, models, and parameter val-

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