The source is believed to be cesium, with a very long half-life (a generation) compared to radioactive iodine (a week).
An American gets on average 6.2 millisieverts in a year from all sources. Japanese set the threshold for natural sources at 1 millisievert.
NHK World has the story here:
Since March 23rd, the ministry has been measuring radiation levels in 15 locations more than 20 kilometers away from the crippled Fukushima Daiichi nuclear power plant.
At one location, in Namie Town about 30 kilometers northwest of the plant, 14,480 microsieverts of radiation had accumulated over the 17-day period to Sunday.
8,440 microsieverts of radiation were observed in Iitate Village.
In another location in Namie, the amount reached 6,430 microsieverts.
People would be exposed to this accumulated amount of radiation if they had stayed outdoors throughout the entire period.
