Finnish authorities are investigating a mysterious "highly exceptional" spike in levels of radioactive caesium-137 detected over Helsinki, officials said today.
Nuclear safety regulator STUK said that while the radioactive surge last week posed no danger to human health, it was keen to get to the bottom of the cause.
"The detection was highly exceptional but from a nuclear safety perspective this level of caesium does not have any effect on human health," Tarja Ikaheimonen, the head of radiation surveillance at STUK, told AFP.
She said 4,000 microbecquerels of the radioactive isotope caesium-137 per cubic metre of air were detected between March 3 and 4, which is a thousand times more than usual but only one-millionth of a level that would require people to shelter themselves from the radiation.
Radiation from the 1986 Chernobyl nuclear power plant disaster in Ukraine continues to be detected in Finland occasionally, when heavy winds lift radioactive particles from the ground into the air.
But Ikaheimonen said last week's levels were too high to be explained by the old fallout.
STUK said there was no obvious source for it as no accidents had been reported in Finland.
Finland has two nuclear power stations with a total of four reactors. One station is located in western Finland and the other east of Helsinki, but neither had reported any anomalies.
On the days of the detection, air currents blew from the east and southeast, which is where Russia and Estonia are located, but Ikaheimonen said it was too early to point fingers in any direction.
Caesium-137 is used in certain types of industrial production, hospitals and research centres.
Ikaheimonen said any accidents related to nuclear reactors or nuclear explosions were also ruled out as their radioactivity would contain a cocktail of different substances and STUK had only detected unusual levels of caesium.
The nuclear safety regulator was waiting to receive more measurements from its seven other detection spots around Finland to define the scale and duration of the radiation.
