A new method for determining the free 222Rn exhalation rate from building materials is described. The sample is enclosed in a container from which the exhaled Rn is continuously purged by nitrogen gas. After 2–3 h, when the Rn level in the container has reached a steady-state concentration, the outflowing Rn is trapped on silica gel at about −190°C. About 16 h after sampling, the silica gel is analysed by liquid scintillation counting to determine the area exhalation rate. The method described has a good repeatability and reproducibility with coefficients of variation of 7.8% and 8.3%., respectively, at 5 Bq m−2 h−1. The low limit of detection of 11 mBq 222Rn offers the opportunity to quantify the exhalation rate of almost all kinds of building materials. It was found that the air humidity strongly influences the exhalation rates of building material and, therefore, should be controlled. Two typical building materials were investigated. For gypsum, an increase in the exhalation rate with increasing water vapor pressure was found, whereas for concrete, a linear decrease with increasing water vapor pressure was observed. The 222Rn area exhalation rates of 20 Dutch building materials, including some experimental ones, were determined at 50% RH, 20°C, showing a range of less than 0.02–15.8 Bq m−2 h−1. The lowest values were found for natural gypsum board, the highest for phosphogypsum blocks. Building materials containing fly ash gave area exhalation rates comparable to those of similar materials without fly ash.
©1991Health Physics Society