If you or your client has recently had a radon mitigation system installed the property should be retested for radon gas. The retest cannot be performed until at least 24 hours of closed building conditions has elapsed since the mitigation system was installed. A Radon Measurement Professional will then perform a 48 short term test to confirm that the mitigation system is operating as designed. A professionally installed mitigation is almost 100% effective. A retest will confirm this.