According the US-EPA protocol, when a home is purchased it should be tested for indoor radon using a short-term (2-7 day) device like a container of activated charcoal, and the indoor radon concentration should be less than 4 pCi/L. When a home is tested because long-term occupancy is likely (e.g., many years), the test is commonly done using a long-term (e.g., 3-month) device like a container of film that can record the tracks produced by alpha tracks generated by radon and its immediate radioactive decay products. For long-term occupancy, the US-EPA recommends that the indoor radon concentration be less than 2 pCi/L. In our study of over 1000 homes, using both short-term activated charcoal detectors and long-term alpha-track detectors, we found that at the 70% confidence level, when trying to estimate the average indoor radon concentration over an entire year, an uncertainty of +/- 90% had to be applied to single activated charcoal detectors and +/- 30% to single alpha-track detectors.
Mushrush, George; Mose, Douglas; and Simoni, Fiorella
"Measurement Uncertainty Of Activated Charcoal And Alpha-Track Indoor Radon Detectors,"
Proceedings of the Annual International Conference on Soils, Sediments, Water and Energy:
Vol. 11, Article 14.
Available at: http://scholarworks.umass.edu/soilsproceedings/vol11/iss1/14