The UKHSA radon detectors with polyallyldiglycol carbonate sensing material produced from 2010 onward were used to confirm that they are purely radon detectors only with negligible interference from thoron presence. The statistical analysis of results, after following standard detector processing procedures, showed that such devices can only detect 2.4% ± 0.3% of the total thoron activity concentration compared to the calibrated reference device. It was also demonstrated that thoron, unlike radon, can only travel a very short distance from the source to be effectively measured. Based on the above, it has been concluded that interference of thoron in radon measurement is negligible (within the statistical measurement error), and hence can be ignored in standard indoor and outdoor measurements.