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Introduction Radon
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 The Earth's crust contains among others, for the radiation exposure of humans less important natural radionuclides uranium-238, uranium-235, thorium-232 and potassium-40. The radioactive noble gas radon-222 (half-life: 3.8 days) is part of the uranium-238 decay chain and is built up by radium-226. It is colourless, odourless and tasteless. Radon and its short-lived decay products contribute most to the radiation exposure of the German population from natural radiation sources (full text in German only).
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Health Effects by radon in homes
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 The adverse health effects of exposure to radon are caused primarily by damage due to alpha-particles. The main health hazard from radon exposure is an increased risk of lung cancer. Epidemiological studies showed that the exposure-response relation seems to be linear without evidence of a threshold. It is estimated that about 5% of the lung cancer deaths per year are attributable to radon in homes in Germany.
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Radon in soil
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 Radon is constantly generated in soil by the radioactive decay of radium. The radon concentration in soil air greatly influences the radon migration into a building. The ratio of the radon concentration in room air to the concentration in soil air typically amounts to 1 to 5 ‰. A map of radon concentrations in soil air in Germany is the basis for assessing the regional radon situation.
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Radon in buildings
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 Radon migrates from soil below the buildings into the interior. Radon concentrations resulting in buildings depend on the radon generation in the ground, the soil properties relevant for radon transport and the method of construction. Additional contributions are from radon exhalation from building materials and outdoor radon. The annual mean concentration indoors amounts to 50 becquerel per cubic metre (full text in German only).
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Measures for the protection from elevated eadon concentrations in buildings
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 It is reasonable to refurbish buildings with a radon exposure above an annual mean value of 100 becquerel per cubic metre compartment air. Such values are mainly measured in areas/regions with high natural radon concentration in the soil. Persons staying in rooms with enhanced radon contents in the air over years, have a higher risk of contracting lung cancer. It is often sufficient to take small measures to reduce the radon concentration.
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Measurement methods for the assessment of radiation exposures due to radon-222 in houses
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Passive or direct indicating instruments can be used for the measurement of radiation exposures due to radon-222. Diffusion chambers with nuclear track detectors are one type of passive instruments. Their indication is direct proportional to the accumulated radiation exposure to radon during the measurement period. Therefore these instruments are particularly suitable for long-term measurements. Direct indicating instruments are useful to measure the actual value of the radon concentration and to acquire its time-variation (full text in German only).
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The Radon Calibration Service Laboratory at the German Federal Office for Radiation Protection (BfS)
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 The Federal Office for Radiation Protection (BfS) operates an accredited calibration service laboratory of the German Accreditation Body (DAkkS) for the measuring quantities "activity concentration of radon-222 in air" and "potential alpha energy concentration of the short-lived radon decay products" (PAEC) with standards directly based on the national standards (primary standards). Routinely the calibration of measuring devices of various design is carried out at request. In addition defined concentrations of radon and its decay products are provided in closed volumes for scientific issues.
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