Abstract: A radon detection system is provided with the capability of monitoring certain environmental parameters together with certain other conditions indicative of possible tampering. The environmental data allows the calculation of an accurate projection of average long-term radon levels in the structure under test on the basis of a relatively short measurement period, while the tamper data indicates whether an attempt has been made to tamper with the detection unit or the measurement conditions during the test period. When an indication of possible tampering occurs, the system enters an accelerated data collection mode during which data is gathered that can later be used to determine whether tampering has actually occurred. The detection system may be provided with first and second detectors for separately monitoring airborne levels of radon gas and radon decay products.