Radioactivity in the Baltic Sea

Finland has been a forerunner in the monitoring of radioactivity in the Baltic Sea. Radioactivity surveys were initiated as early as the end of the 1950s by STUK’s predecessor Institute of Radiation Physics in co-operation with the Finnish Institute of Marine Research. 

An impulse for these activities was given by the above-ground nuclear weapon tests carried out in Novaya Zemlya in the 1950s and 1960s. Gradually the surveillance expanded into co-operation between all the Baltic Sea countries. The fallout from nuclear weapon tests was distributed relatively evenly throughout the Baltic Sea catchment basin, containing mainly cesium-137 and strontium-90.

In 1986, the radioactive plume from Chernobyl was carried by winds directly towards the Baltic Sea, causing a fallout that was distributed quite unevenly.  The Baltic Sea received more of the Chernobyl fallout than other seas in the world (e.g.,  the Black Sea, the Mediterranean Sea, the North Sea and the northeast Atlantic). On the basis of the amount of cesium-137 carried in the fallout, the Baltic Sea can be classified as one of the most radioactively contaminated sea regions in the world, together with the Irish Sea.

Distribution of the cesium-137 deposition caused by the Chernobyl fallout in the Baltic Sea catchment basin

Distribution of the cesium-137 deposition caused by the Chernobyl fallout in the Baltic Sea catchment basin.


During the more than twenty years that have passed since the accident, cesium has disappeared from the water of the Baltic Sea relatively fast, as it has sunk to the bottom with sedimenting material. While the highest cesium-137 concentrations measured immediately following the accident at some locations in the Gulf of Finland and the Sea of Bothnia were approximately 5,000 becquerels per cubic metre, in 2012 the concentrations had fallen to one hundredth (< 50 Bq/m3). The Gulf of Finland has been cleared somewhat faster than the Sea of Bothnia. This is due to different sedimentation rates and more efficient water exchange between the Gulf of Finland and the Baltic Proper.

Cesium-137 concentrations in surface water in different parts of the Baltic Sea from 1975 to 2012

Sea of Bothnia
Gulf of Finland
Bay of Bothnia
Baltic Proper
Target value

Cesium-137 concentrations in surface water in different parts of the Baltic Sea from 1975 to 2012.

At the moment, most of the cesium-137 present in the Baltic Sea is buried in the bottom sediments. The regional distribution of cesium in the bottom sediment is largely in line with the distribution of the Chernobyl fallout.

The cesium-137 concentrations in Baltic Sea fish are less than 50 Bq/kg, clearly lower compared with freshwater fish. Therefore, the radiation dose received by Finns from Baltic Sea fish has remained low, being a few thousandths of a millisievert per year.

The annual report STUK-B-148 includes a summary of the results of the surveillance of environmental radiation in 2011. It also includes the results of the monitoring of radioactivity in the Baltic Sea. STUK is engaged in international monitoring of radioactivity in the Baltic Sea, carried out in accordance with the recommendation of the Helsinki Commission. The results of the monitoring are also available on the website of the Helsinki Commission.

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