[en] High radon concentrations in the indoor air in buildings used for water treatment are not uncommon. When raw water is processed in the waterworks, and the process is made in an open system, radon may escape from the water to the premises. The radon concentration of the raw water does not need to be high to give a radon escape of 2,0-50 Bq/l, which may lead to indoor air radon concentrations in the premises of 10,000 Bq/m3 to more than 50,000 Bq/m3. The waterworks are workplaces for the staff. However, it is not uncommon that other groups of employees have their jobs in the same buildings. Persons that spend long times in waterworks with high radon concentrations in the air may receive radiation doses as high as 20 mSv/a or more, which is the annual average upper limit in a consecutive five-year period for radiation workers. Waters that contain enough radon to release so much radon that it may cause high radon concentrations in the premises are groundwaters from aquifers in the bedrock and in the soil and surface waters, that has been infiltrated through deposits of sand and gravel. Surface waters that have not been infiltrated have very low radon concentrations < 1 Bq/l). This report accounts for experiences from Ludvika, Fredriksberg, Kolbaeck and Aendesta waterworks and results of radon and gamma radiation measurements in these waterworks. The report represents a part of the SSI project Inventory of industries in which radiation from natural radioactive elements is of concern. The aim of this project is to collect information on exposure to natural radiation at workplaces, and is a part in the implementation of the EU Council Directive (96119 Euratom) on Basic Safety Standards (BSS) for the protection of the health of workers and the general public against dangers arising from ionizing radiation