Radiation exposure in German dwellings, some results and a proposed formula for dose limitation.

The results of our investigations in the Federal Republic of Germany on the indoor and outdoor exposure to natural radiation from gamma rays and radon and thoron daughters are presented. Indoor the median Rn-222 concentration was approximately four times higher than outdoors. A correlation analysis of the data obtained showed that indoors the equilibrium factor F is almost independent of ventilation, Rn-222 concentration and other parameters. The mean equilibrium factor was measured to be F = 0.3 in dwellings and approximately F = 0.4 outdoors. The results of our investigations on diffusion coefficients and exhalation rates showed, that the activity concentration in dwellings and in cellars can generally be explained by the radon exhalation from the building materials. Only in areas of high radon concentrations, the exhalation from the soil was a decisive factor. The mean effective dose equivalent by residence in dwellings amounted to 0.2 - 0.8 mSv/a for Rn-222 daughters and approximately 0.1 mSv/a for Rn-220 daughters. A relationship has been derived which permits the calculation of the expected average radiation exposure in dwellings by gamma radiation and by radon inhalation as function of the radionuclide concentration in building materials.