LONG TERM PASSIVE METHODS: Alpha Track, Electrets, etc.
These long term test devices provide a single numeric value that represents the average reading for a long-term test period. Their strength is to identify actual exposures regardless of building operation at different times and other factors that fluctuate radon entry into and around the dwelling.
Long term test devices are deployed for at least 91 days. However, six months to a year is recommended to insure seasonal differences in building operation are included in the measurement. More than one test device is recommended with passive devices.