Invited article: in situ comparison of passive radon-thoron discriminative monitors at subsurface workplaces in Hungary.

During a one-year long measurement period, radon and thoron data obtained by two different passive radon-thoron discriminative monitors were compared at subsurface workplaces in Hungary, such as mines (bauxite and manganese ore) and caves (medical and touristic). These workplaces have special environmental conditions, such as, stable and high relative humidity (100%), relatively stable temperature (12°C-21°C), low or high wind speed (max. 2.4 m s(-1)) and low or elevated aerosol concentration (130-60,000 particles m(-3)). The measured radon and thoron concentrations fluctuated in a wide range among the different workplaces. The respective annual average radon concentrations and their standard deviations (in brackets) measured by the passive radon-thoron discriminative monitor with cellulose filter (CF) and the passive radon-thoron discriminative monitor with sponge filter (SF) were: 350(321) Bq m(-3) and 550(497) Bq m(-3) in the bauxite mine; 887(604) Bq m(-3) and 1258(788) Bq m(-3) in the manganese ore mine; 2510(2341) Bq m(-3) and 3403(3075) Bq m(-3) in the medical cave (Hospital Cave of Tapolca); and 6239(2057) Bq m(-3) and 8512(1955) Bq m(-3) in the touristic cave (Lake Cave of Tapolca). The respective average thoron concentrations and their standard deviation (in brackets) measured by CF and SF monitors were: 154(210) Bq m(-3) and 161(148) Bq m(-3) in the bauxite mine; 187(191) Bq m(-3) and 117(147) Bq m(-3) in the manganese-ore mine; 360(524) Bq m(-3) and 371(789) Bq m(-3) in the medical cave (Hospital Cave of Tapolca); and 1420(1184) Bq m(-3) and 1462(3655) Bq m(-3) in the touristic cave (Lake Cave of Tapolca). Under these circumstances, comparison of the radon data for the SF and CF monitors showed the former were consistently 51% higher in the bauxite mine, 38% higher in the manganese ore mine, and 34% higher in the caves. Consequently, correction is required on previously obtained radon data acquired by CF monitors at subsurface workplaces to gain comparable data for SF monitors. In the case of thoron, the data were unreliable and no significant tendency was seen during the comparison therefore comparison of previously obtained thoron data acquired by either CF or SF is doubtful. There was probable influence by relative humidity on the detection response; however, the effects of the high wind speed and elevated aerosol concentration could not be excluded. The results of this study call attention to the importance of calibration under extreme environmental conditions and the need for using reliable radon-thoron monitors for subsurface workplaces.