The source of low levels of radioactive iodine-131 detected in Europe over the past few weeks was probably an isotope maker in Budapest, the International Atomic Nuclear Agency (IAEA) said on Thursday. According to the Health Ministry in Vienna, the iodine-131 had been released from the Institute of Isotopes Ltd in Budapest, which produces radioisotopes for health care, research and industry. The cause of the release is under investigation. The air collector in Vienna found no more detectable iodine-131 since the end of last week, said the department responsible for radiation protection. The natural background radiation, which every citizen is exposed in one hour is equivalent to about one hundred times the recently measured iodine-131 dose, the Health Ministry said. Over the past few days, authorities in Austria, Slovakia, Germany, Sweden, France and Poland have detected very low levels of iodine-131 in their atmospheres. The IAEA 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, while average annual background radiation is 2,400 microsieverts a year.