tionally exposed individuals (iron and rare-earth miners, workers in Th-processing plants and the monazite industry totaling 1557 persons). Increases in respiratory diseases, pancreatic cancer, and chromosome aberrations were found to be statistically significant. Among niobium miners in a Th-rich area, an increase in lung-cancer rates (observed/expected lung-cancer cases = 11.3) was found (Solli and others 1985). Among approximately 53,000 European and Japanese patients treated with thorium oxide injections, an increase in lifetime excess cancer risks for liver and bone cancer as well as leukemia has been observed. However, there were no excess lung-cancers. To reduce the uncertainties concerning the effects of 220Rn decay products, we will need to better characterize dosimetry of thoron progeny and obtain more data on exposures.
In this chapter we have reviewed the basis for the dosimetric extrapolation of risk from miners to the general population, taking into account differences in the aerosol characteristics, breathing rates, and the respiratory physiology of the various segments of the population (women, children, infants, as well as men). The constants have changed from the values of about 0.70 to 0.75 reported by the NAS Dosimetry Panel (NRC 1991) to values very close to 1. The differences in input parameters between the calculations are summarized in Table B-13. The primary cause of the shift in K value is attributable to the reduction in the miner breathing rate based on measurements of actual miners in Tajikistan and South Africa. The risks for infants are slightly higher than for miners, but for the other groups, the risks per unit exposure are essentially identical in homes and mines. This value around 1 is reasonable given the compensatory factors of particle concentration raising the airborne concentrations, but in sizes that are only weakly deposited in the lung. Thus, the doses per unit radon concentration are essentially the same although in each case, specific unusual aerosol conditions could substantially increase the dose per unit exposure.