Absorbed dose rates in air have been continuously observed at six environmental radiation monitoring stations in Maizuru and Ayabe, Japan. Increases of absorbed dose rate, where the quantity was a maximum of 18nGy/h, were found several times at one station on April 3, 2000, and could not be ascribed to any known variation of distribution of natural radioactive material such as radon progeny in the air or rainwater. The result of analysis of the energy spectrum and half-life elucidated that the unknown radiation had been emitted from a patient administered radiopharmaceutical 99mTc, who lived near the station.
