The influence of changing exposure conditions on measurements of radon concentrations with the charcoal adsorption technique

The charcoal adsorption technique is now widely used to measure indoor radon concentrations. Deployment periods range from two to seven days. The passive nature of the activated charcoal allows continual adsorption and desorption of radon, and the adsorbed radon undergoes radioactive decay during the exposure period. Therefore, the technique does not uniformly integrate radon concentrations during the exposure period. The technique can be calibrated to yield precise results for exposures during relatively constant radon concentrations. During a deployment period of several days in a home, however, radon concentrations can vary by a greater magnitude than under controlled laboratory conditions. Charcoal canisters were exposed in an environmental chamber to controlled radon concentrations, and temperature and humidity conditions were varied during the exposure period. The collection efficiency of the charcoal was investigated for a variety of exposure conditions and exposure periods. The conclusions of these investigations led to a change in exposure time from 96 h to 48 h in the United States Environmental Protection Agency charcoal adsorber measurement programme, and an improved understanding of the limitations and capabilities of the charcoal adsorption technique. (author)