Traces of Iodine-131 were spotted in Norway, Finland, Poland, Czech Republic, Germany, France and Spain in January, but the public were not immediately alerted.
These radioactive particles are produced by atomic bomb explosions or nuclear disasters such as Chernobyl or Fukushima.
They appear to be emanating from Eastern Europe, but experts have not been able to say exactly what produced them.
Astrid Liland, head of emergency preparedness at the Norwegian Radiation Protection Authority, told the Barents Observer that the health risk was very low – which was why she did not raise the alarm after detecting Iodine-131 during the second week of January.
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“We do measure small amounts of radioactivity in air from time to time because we have very sensitive measuring equipment,” she said.
“The measurements at Svanhovd in January were very, very low. So were the measurements made in neighbouring countries, like Finland. The levels raise no concern for humans or the environment. Therefore, we believe this had no news value.”
However, she was unable to say where the particles have come from.
They may have been released by accident by a nuclear reactor or a medical facility where they are being used to treat hyperthyroidism and thyroid cancer.
The particles could also been produced by Russian nuclear submarines.
Scientists detected the highest number of particles in Poland, although still not enough to spark a public health emergency.