Answer to Question #7853 Submitted to "Ask the Experts"

I would like to know how accurate short-term radon detectors are, as well as long-term detectors. Can you even consider them "accurate" since they measure a decay product? I know that radon detectors can be sensitive and reliable, but what can you say about their accuracy? The laboratory methods of detection seem to be sensitive also and their results have an accuracy, at least a lower limit of detection.
 

Concerning accuracy of radon detectors, most, if not all, commercially available radon measurement devices are subjected to some sort of proficiency testing. The National Radon Safety Board evaluates radon measurement devices in cooperation with the US Environmental Protection Agency. It requires that the individual relative error of each device be less than or equal to 20.0% and that the precision error of all devices be less than or equal to 20.0%. Details of the measurement conditions and calculations are given here.

The National Radon Proficiency Program (NRPP) also offers performance testing with a requirement of 25% accuracy. Based on the requirements of these two programs, one could generalize that short-term or long-term radon detectors that have passed one of these performance tests have an accuracy of about 25% or possibly better.

Regarding decay product measurement, it is true that it is radiation from the decay products that is actually detected. However, the devices are designed to exclude ambient radon decay products from the sensitive volume, so only decay products resulting from radon that enters the sensitive volume contribute to the signal. Calibration and testing is done as a function of radon concentration, not decay product radiation.

Regarding laboratory methods, devices used primarily in laboratories such as Lucas cells can be very accurate. What needs to be considered, however, is that while any of the currently accepted methods can measure with reasonable accuracy, the sample may or may not be representative of the particular environment over the desired time period.

The Environmental Protection Agency guidelines for indoor radon are based on annual average exposure. A Lucas cell measurement, while having the capability to provide a very accurate result for the exact place and time where the sample was collected, is a very poor representation of the average annual concentration in a building. Short-term detectors (two to four days) are more representative of the environment but will miss seasonal variations. The most representative measurements of annual average concentration are those made over a full year with long-term detectors.

Thomas F. Gesell, PhD