"This is a considerable amount of radiation," said G. Donald Frey, a medical physicist and professor of radiology at the Medical University of South Carolina. "The limit for radiation workers in the United States is 50 millisieverts per year, but we try to keep them to less than 5 millisieverts per year."
TEPCO said it is doing all it can to protect workers' health and that it will not return to the plant any worker already exposed to more than 100 millisieverts. Japan's Nuclear and Industrial Safety Agency said Friday that several hundred workers, including non-TEPCO employees, remained on site.
After a single acute exposure of 1,000 millisieverts, people tend to start feeling nauseated and vomiting, Frey said. At 5,000 millisieverts over the course of a few hours, "people start dying."
After exposure to 150 millisieverts per day, "you're definitely in the range where you have significantly increased risk of radiation-induced cancers."
For work involving recovery and restoration in an emergency operation, the International Commission on Radiological Protection recommends no more than 50 millisieverts in any given year. But in cases where the lives of a great number of people may be at stake, the ICRP says it recommends no restriction on dose as long as "the benefit to others clearly outweighs the rescuer's risk."
TEPCO Managing Director Akio Komiri, upon leaving a news conference Friday in Fukushima at which worker exposure levels were discussed, burst into tears.
The announcement came a day after the country's nuclear safety agency adjusted its assessment of the disaster.
NISA raised its rating for the most serious issues from 4 to 5 -- putting those problems on par with those in the 1979 incident at Pennsylvania's Three Mile Island.
