REASSESSMENT OF INHALATION DOSES TO WORKERS IN AUSTRALIAN SHOW CAVES

Exposure to radon in show caves is an existing exposure situation. A survey of radon levels in underground show caves around Australia, carried out in 1994, found that most of the show caves located in South-Eastern Australia had yearly average radon levels exceeding the Australian radon reference level of 1000 Bq m-3. At the time of the original survey, the radiation doses from exposure to radon progeny of the tour guides in these caves were estimated using the epidemiologically based dose conversion factors and all dose were assessed to be less than 10 mSv per year. In February 2018, the International Commission for Radiological Protection (ICRP) published updated radon and radon progeny dose conversion factors (DCF) applicable to worker exposure to radon in show caves. These updated DCF values are based on dosimetric modelling and are sensitive to the radon progeny activity size distribution. The recommended DCF values are up to a factor four times higher than the previous ICRP recommendations. The ICRP has published data that allows for the estimation of site-specific radon progeny dose coefficients if required. A reassessment of the radiation doses to workers in Australian show caves has been made using these updated ICRP DCF values and the historical measurements of radon progeny activity size distributions in Australian show caves. Using the site-specific DCF values, it is estimated that 15% of the workers exceeded 10 mSv y-1 and 6% exceeded 20 mSv y-1. Although the total number of show cave workers in Australia is very small, the updated radon progeny dose estimates are a significant radiation protection issue for the affected individuals and their employers.