Beta-to-gamma dose ratios, such as those used to estimate beta dose from exposure to a contaminated ground surface, are not used in cases of exposure to descending fallout. Instead, beta dose is estimated by using calculated dose coefficients, which give equivalent dose rates from electrons per unit concentration of radionuclides in air,^{4} combined with the duration of exposure and composite beta-spectrum radiation energies associated with a reconstructed gamma exposure or film-badge reading. The calculated beta dose is added to the upper-bound gamma dose for the corresponding period. The approach is to be tailored case by case for any person exposed to descending fallout. The dose coefficients assume uniform suspension of radioactive material in an infinite air source, and the current method is said to overestimate the corresponding beta dose because it includes no provision for shielding from electrons or for self-attenuation by fallout particles.

Composite energy spectra from mixed fission-product beta particles were calculated as a function of time after a detonation, and the resulting beta energies were binned and combined with calculated beta-particle dose coefficients for air immersion (Kocher and Eckerman, 1981). The resulting composite dose coefficients for the assumed mixtures of radionuclides were calculated as a function of time after the detonation in units of rem y^{−1} per μCi cm^{−3} of air. The dose coefficients were tabulated by Barss (2000) for specific periods after detonation, because descending fallout typically lasted anywhere from less than a few hours to 1 day. The composite dose coefficients for assumed mixtures of fission products in descending fallout are illustrated graphically in Figure IV.B.8. Multiplication of the composite beta dose coefficient by the corresponding total activity concentration results in an initial beta dose rate that can be integrated over the time of exposure to give the total beta dose. The resulting beta dose is summed with the upper-bound gamma dose over the same period of time to yield the skin dose due to exposure in air. Clothing modification factors can be applied as appropriate. Barss also describes special methods for the case of being in contaminated air in an aircraft performing cloud sampling or tracking activities and provides an approach for the case of immersion in water. For aircraft, it is appropriate to account for beta-particle backscatter from the interior of the fuselage. The approach for submersion in water is similar to that for air, but it accounts for the different densities of water and air.

Barss (2000) correctly indicates that for skin contamination (for example, from fallout or resuspended radioactive soil), the film-badge gamma dose is a