Very low levels of radioactive iodine-131 have been detected in the Czech Republic and other parts of Europe.
But the International Atomic Energy Agency says the particles are not believed to pose a public health risk.
The agency said on Friday it did not believe the source was the Fukushima plant in Japan.
The IAEA said the Czech Republic was the first to inform it of raised levels.
It said iodine-131 was a short-lived radioisotope with a radioactive decay half-life of about eight days.
Hungary, Slovakia, Austria and Sweden have also reported traces at very low levels that do not pose a health risk.