Initial reports of iodine-131 measuring a few microbecquerels per cubic meter came out of the Czech Republic, Poland, Slovakia and Austria.
French officials later said the radioactive isotope was also detected at four monitoring stations across the north and east of the country.
France’s Institute for Radiological Protection and Nuclear Safety (IRSN) said that since iodine-131 is so short-lived, with a half-life of only eight days, it is unlikely to have come from Japan’s crippled nuclear power plant at Fukushima.
The agency speculated that the source could be a reactor used to generate electricity or conduct research, or a plant using iodine 131 to make medical devices.
“Although the presence of iodine-131 over national territory is quite exceptional ... the level of concentrations that have been observed are of no risk for public health,” IRSN said in a press release.