VIENNA Very low levels of radioactive iodine-131 have been detected in Europe but the particles are not believed to pose a public health risk, the U.N. nuclear agency said Friday, saying it was seeking to find the source.
The International Atomic Energy Agency (IAEA), the Vienna-based U.N. watchdog, said it did not believe the radioactive particles were from Japan's stricken Fukushima nuclear power plant after its emergency in March.
Experts said the origin of the radiation -- which has been spreading for about two weeks -- remained a mystery but could come from many possible sources ranging from medical laboratories or hospitals to nuclear submarines.
The Czech Republic's nuclear security watchdog said it had tipped off the IAEA after detecting the radiation it thought was coming from abroad but not from a nuclear power plant. It suggested it may come from production of radiopharmaceuticals.
Germany's Environment Ministry said slightly higher levels of radioactive iodine had been measured in the north of the country, ruling out that it came from a nuclear power plant.
Hungary, Slovakia, Austria and Sweden also reported traces at very low levels that did not pose a health risk.
Iodine-131, linked to cancer if found in high doses, can contaminate products such as milk and vegetables.
Paddy Regan, a professor of nuclear physics at Britain's University of Surrey, said the suggestion that it may have leaked from a radiopharmaceuticals maker "sounds very sensible and totally reasonable."
He said since iodine was used in the treatment of thyroid conditions it was also likely that hospitals in many European countries would have it in their stores.
"It would be very unlikely for it to have come from Fukushima since the accident was so many months ago and iodine-131 has a brief half-life," he said.
Iodine-131 is a short-lived radioisotope that has a radioactive decay half-life of about eight days, the IAEA said.
Massimo Sepielli, head of the nuclear fission unit of Italy's national alternative energy body ENEA said any number of sources could be to blame for the readings.
"It could be coming from the transporting of (nuclear) material, it could come from a hospital ... it could even come from a nuclear submarine, even if it's a more complicated possibility ... but you can't rule that out."
Professor Malcolm Sperrin, director of medical physics at Britain's Royal Berkshire Hospital, said any link with Fukushima was extremely unlikely.
"It is far more likely that the iodine may be as a result of excretion by patients undergoing medical treatment. Whilst such patients are carefully controlled, some release of iodine into the environment may be inevitable but would certainly be well below any limits where health detriment would even begin to be an issue for concern," he said.
The IAEA said the Czech Republic's nuclear safety body had informed it that "very low levels" of iodine-131 had been measured in the atmosphere over the country in recent days.
"The IAEA has learned about similar measurements in other locations across Europe," the brief statement said.
"The IAEA is working with its counterparts to determine the cause and origin of the iodine-131."
The Czech watchdog said it had detected iodine-131 at a number of monitoring stations since late October. It said there was no health risk from the iodine.
"It was detected by our radiation monitoring network, with probability bordering on certainty the source is abroad. It is iodine-131 and we have asked the IAEA if they know what the source could be," Czech State Office for Nuclear Safety chief Dana Drabova told Reuters.
Officials in Spain, Russia, Ukraine, Finland, France, Britain, Switzerland, Poland and Norway said they had not detected any abnormal radiation levels. Romania's watchdog said there had been no incident at the country's sole nuclear plant.
Austria's Environment Ministry said small levels were measured in the east and north of the Alpine country, saying the estimated dose level for the population was one 40,000th of the dose of radiation received in a transatlantic flight.
In the world's worst nuclear accident since Chernobyl in 1986, an earthquake followed by a massive tsunami overwhelmed the Fukushima plant in Japan, causing a reactor meltdown and leakage of radiation, including of iodine.
In the days and weeks after the accident, tiny amounts of iodine-131 believed to have come from Fukushima were detected as far away as Iceland and other parts of Europe, as well as in the United States.
(Additional reporting by Kate Kelland in London and European bureaux; Editing by Janet Lawrence)