that the ²²⁰Rn progeny will not be an important additional source of exposure and dose.

In the measurements of the activity-weighted size distributions described above, the system used to make the activity measurements included a-spectroscopy so that ²²⁰Rn progeny could have been observed. Such activities were not observed and thus, for these homes, only ²²²Rn decay products provided exposure and dose.

Steinhäusler (1996) has recently reviewed the information available regarding thoron exposure and dose. He concludes that the extent of information on ²²⁰Rn and its decay products is analogous to the state of affairs for ²²²Rn before the large-scale national radon surveys of the past 15 years. Few health studies have been conducted on the possible health effects of inhaling thoron decay products. Among residents in high-background areas of Brazil, China, and India, statistically significant increases have been observed for chromosome aberrations. The study in China found no increase in lung-cancer for thoron exposure at a mean concentration of 168 Bqm^-3 (Wei and others 1993).

Exposure measurements for ²²²Rn based on gross-counting methods can be affected by the presence of ²²⁰Rn decay products. For most of the mines which employed the workers used in the epidemiological studies, measurements are not available to determine the presence or absence of ²²⁰Rn progeny. For the mines in the Colorado Plateau and the Grants Mineral Belt of New Mexico, negligible concentrations of ²²⁰Rn decay products have been found. There are substantial concentrations of ²²⁰Rn decay products in the mines of the Elliot Lake, Ontario (DSMA Atcon 1985), where the PAEC arising from ²²²Rn and ²²⁰Rn were approximately equal. Corkill and Dory (1984) made a retrospective study of exposure in the fluorspar mines of Newfoundland. However, they do not mention ²²⁰Rn or its decay products. A similar lack of information exists for the other mining environments. Bigu (1985) has examined theoretical models for estimating the PAEC levels arising from the presence of both radon isotopes. The relative amounts of the radon isotopes and their progeny can be estimated based on the weight ratio of ²³⁸U to ²³²Th. For the Ontario uranium mines, this ratio is assumed to be 1. Results were then calculated based on a number of models that had been previously developed to predict the airborne activity concentrations. Bigu found that a substantial number of the measurements fall outside of the theoretical bounds and there are substantial differences among the results calculated by the various available models. Thus, more sophisticated models will need to be developed to permit adequate estimation of the exposure to ²²⁰Rn progeny. Thus, it is not possible to assess the uncertainty from a concentration of ²²⁰Rn progeny to lung dose in the miner-based risk estimates.

Steinhäusler (1996) also summarized the information available on occupa-