Radon control in Estonia

Radon surveys started in Estonia in 1989. The Department Building Physics at the Estonian BuildingResearch Institute measured radon levels in dwellings, in building materials produced in the country,and in the soil. The indoor radon concentrations in more than 400 houses were measured under grabsamplingtechniques using Lucas cells. The highest measured radon level was 6700 Bqm-3. The resultsof these measurements made during 1989-91 showed that the main source of indoor radon is the soilunderneath buildings. No building materials with elevated radium concentrations have been found sofar.A new nation wide radon-monitoring program within the Estonian Environmental Monitoring Programstarted in 1994. The main objectives of this program were to identify radon risk areasin the country and radon risk housing construction types.The bedrock in Estonia consists of sediments from the Cambrian to Devonian periods. Thesesediments have low to normal uranium concentrations with the exceptions of the OrdovicianDictyonema black shale and phosphorous-rich Glauconite sand that are exposed mostly in the northernpart of the country. The Dictyonema shale is similar to the uranium-rich black alum shale found inSweden and Norway. Till, silt, clay, sand and gravel are the main soil types. The coastal regions ofnorthern Estonia have the greatest radon problem in the country; many coastal houses are constructedon ground that contains Dictyonema shale, either in the bedrock or as fragments in the soil.During the five year (1994 - 1998) radon-monitoring program, the indoor radon concentrations in 700dwellings that were located in the expected radon prone areas were measured. The measurements weremade with passive alpha-track detectors, the measurements period was three months during heatingseasons. The maximum measured values of indoor radon concentrations exceeded 12,000 Bqm-3. Thearithmetical mean of these measurements was 102 Bqm-3. Approximately 65% of the measureddwellings had radon levels below 100 Bqm-3. In 3 % of the houses, radon concentration exceeded 800Bqm-3 .In 1997, a national survey of radon in randomly selected dwellings across the entire country wasstarted. This survey includes radon measurements in 500 dwellings.