Because of thoron exhalation, the measurement results of radon exhalation rate using a local still method is usually larger than the true value of radon flux rate of the monitored material surface. The influence of 216Po (ThA) on radon exhalation rate can be eliminated for sensitive radon monitors. Theoretical evaluations of the influence of 212 Bi (ThC) and 212 Po(ThC') on radon exhalation rate are carried out in a sampler with diameter of 188 mm, and height of 125 mm, and supplied electrostatic field inside (generated by high voltage and electret) under following conditions: the sampling time are 1, 2, 3 h, respectively, thoron exhalation rate is 100 times of radon's. The calculation results indicate that the measurement results of radon flux rate are possible 35.5% larger than true value due to the influence of thoron for fast and multifunctional radon monitors with electret, high voltage, respectively and using CR-39 SSNTD as detector, but this influence is negligible for PCMR-1 passive and continuous radon monitor using a silicon surface barrier as detector.

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