Inhalation of soluble Class D materials usually results in relatively low doses to tissues of the respiratory tract and relatively high doses to other organs and tissues. As the solubility decreases, doses to tissues of the respiratory tract increase and doses to other organs and tissues decrease, although there are some exceptions in organs of the GI tract.

Inhalation dose coefficients that apply to radionuclides in oxide form are used in all dose reconstructions (Egbert et al., 1985), on the basis of the expected insolubility of radioactive materials in fallout and the insolubility of oxide forms of many radionuclides compared with other chemical forms. That assumption usually results in the highest estimates of dose to the lung and organs in the GI tract, but in these cases, it often results in lower estimates of dose to other organs and tissues compared with doses based on an assumption of a more soluble chemical form.¹⁴ However, there are exceptions, including isotopes of strontium for which the oxide form is assumed to be Class D rather than Class Y (ICRP, 1979a). For some elements, including iodine, the same clearance class is assumed for all chemical forms.

On the basis of numerous studies in animals, alpha particles are known to be more effective than photons and electrons in inducing biological responses (NCRP, 1990). That is, if the same absorbed dose of alpha particles and of photons or electrons is delivered to an organ or tissue, the probability that a cancer or other stochastic radiation effect will result is substantially higher in the case of exposure to alpha particles. The biological effectiveness of alpha particles is an important concern, for example, in estimating equivalent doses due to inhalation of plutonium, which is an alpha-emitter.

Two sets of dose coefficients for inhalation (and ingestion) of alpha-emitting radionuclides were calculated by the ORNL group (Dunning et al., 1979); one set assumed a biological effectiveness of 10 for alpha particles compared with photons and electrons, and the other assumed a biological effectiveness of 20. However, only the dose coefficients calculated by the ORNL group that incorporate the higher biological effectiveness of alpha particles (20), thus giving higher equivalent doses, are included in the FIIDOS code (Egbert et al., 1985) and have been used in dose reconstructions since then. This assumption conforms to the current recommendation by ICRP (1991a), and is incorporated in all dose coefficients used in dose reconstructions that are obtained from ICRP Publication 30 (ICRP, 1979a).