Cumulative exposure to radiation is a real killer.  The Japanese town of Namie has a cumulative (build up over time) of radiation at 17,000 millisieverts.

The official acceptable exposure rate, for people in Japan, is 1,000 microsieverts, per year.  A millisievert is 1,000 times more than a microsievert, and Namie has hit 17,000 millisieverts since the reactor buildings exploded, in about four weeks.

Cumulative exposure refers to how many times you’ve been exposed to radiation, or, how long you are continuously exposed.  Certain radioactive isotopes build up in your body over time, they don’t go away, so the more times you’re exposed the worse it is.