Background measurements are required both for continuous monitors and for passive detectors requiring laboratory analysis. Users of continuous monitors must perform sufficient instrument background measurements to establish a reliable instrument background and to act as a check on instrument operation. These measurements are made with instruments exposed to very low radon concentration environments (such as outdoor air), or they can be blanks (unexposed passive detectors). There are two types of background measurements:
Laboratory control blanks
Laboratory control blanks are used to establish background levels that are then subtracted from samples before calculating sample concentrations.
Field control blanks
Field control blanks are used to uncover problems of shipping and storage. If a constantly high reading above the lower limit of detection (LLD) is found by the field control blanks, this high background must be considered a bias and be subtracted from the individual values reported for the other devices in the exposure group. Blank passive detectors should be labeled and submitted as normal samples to verify the quality control of the laboratory.
Users of passive detectors should employ field blanks equal to approximately five percent of the detectors that are deployed, or 25 each month, whichever is smaller. These controls should be set aside from each detector shipment, kept sealed in a low radon environment, labeled in the same manner as the field samples to preclude special processing, and returned to the analysis laboratory along with each shipment. The field blanks measure the background exposure that may accumulate during shipment and storage, and the results should be monitored and recorded. The recommended action to be taken if the concentrations measured by one or more of field blanks is significantly greater the LLD is dependent upon the type of detector.