[en] The Czech Republic, with a mean equivalent volume activity of radon of about 60 Bq/m³ (which for a nonequilibrium factor of F=4 corresponds to a volume activity of radon of about 150 Bq/m³), is among countries with the highest radon concentrations in buildings. In extreme cases, equivalent volume activities of radon in homes exceeding 10⁴ Bq/m³ were observed. With respect to the effective dose, exposure to radon and its daughter products is substantially more serious than any other exposure. In the above extreme case, the effective dose is as high as 0.5 Sv annually. Therefore, geologic survey is performed in the Czech Republic at sites where a high radon risk is expected. The measurements are carried out using track detectors which are installed in buildings for a year. So far, over 100 000 houses (mostly family houses) have been measured, and equivalent volume concentration levels in excess of 200 Bq/m³ were found in 15,000 of them. In such instances, the government contributes funds for antiradon provisions. Protection against radon is included in Czech legislation