The Radon Equilibrium Factor and Comparative Dosimetry in Homes and Mines

The equilibrium factor, a measure of the degree of radioactive equilibrium between radon and its short-lived radioactive decay products, is often assumed to be 0.4 for risk assessment. While this is usually a reasonable assumption, there are important exceptions. In particular, in active mines where ventilation rates are high, the equilibrium factor can be much less than 0.4. In such cases, if lung dose rate is computed based on measured activity size distributions and normalised to radon concentrations, comparisons to lung dose rate in other environments must take into account possible differences in the equilibrium factor. In the comparative dosimetry section of the BEIR VI Report, this critical step was omitted. Thus, the ratio of home lung dose rate to diesel miner lung dose rate is actually 0.44, not 1 as stated in the Report. This oversight may have major public policy implications.