The source of low levels of radioactive iodine-131 detected in several European countries over the past few weeks is still unclear, the U.N. nuclear agency said Wednesday.
The International Atomic Energy Agency (IAEA) first announced Friday that traces had been detected in Europe, after it was tipped off by authorities in the Czech Republic.
The IAEA has said the traces should not pose a public health risk and that it does not think the particles are from Japan's stricken Fukushima nuclear power plant after its emergency in March.
But the origin of the particles remains a mystery. The IAEA said it was working with countries to seek out the source.
Authorities from the Czech Republic, Austria, Slovakia, Germany, Sweden, France and Poland have continued to measure very low levels of iodine-131 in their respective atmospheres in recent days, the IAEA said in a statement.
The levels of iodine-131 currently being detected are extremely low.
It said that if a person were to breathe in the levels for a whole year, they would receive an annual radiation dose of less than 0.1 microsieverts. In comparison, average annual background radiation is 2,400 microsieverts a year, it said.
Iodine-131, linked to cancer if found in high doses, can contaminate products such as milk and vegetables.
Experts have said the origin of the radiation, which has been spreading for nearly three weeks, could come from many possible sources ranging from medical laboratories or hospitals, to nuclear submarines.
France's agency for radioprotection and nuclear safety (IRSN) said Thursday the levels likely originate from central or eastern Europe.
Didier Champion, head of environment and intervention at IRSN, said the possible origin could be the Czech Republic, Poland, Hungary, Slovenia, Russia or Ukraine. Austria, which borders several of the eastern countries, could also be a possibility, he said.
Austrian authorities say they have ruled out their country as the origin and have suggested that the source is a country to its east or south east.
IRSN is carrying out calculations to track down the trajectory of air masses to identify the origin of the leak.
We should have an answer by the middle of next week, Champion said, ruling out the suggestion that the leak could be from a nuclear power plant.
If it came from a reactor we would find other elements in the air, he said, adding one hypothesis the agency was working on was the possibility the leak came from the pharmaceutical industry.
Iodine-131 is a short-lived radioisotope that has a radioactive decay half-life of about eight days.
The Czech Republic's nuclear security watchdog has said it alerted the IAEA after detecting the radiation, which it thought was coming from abroad but not from a nuclear power plant. It suggested it may be from production of radiopharmaceuticals.
(Reporting by Sylvia Westall in Vienna and Muriel Boselli in Paris; Editing by Jon Hemming)