During which construction project did 250,000 workers reach their lifetime radiation limits ?
The International Commission on Radiological Protection estimates that the risk to a fetus being exposed to 10 mGy ( a unit of radiation exposure , see Gray ( unit )) increases the rate of cancer before 20 years of age from 0.03 % to 0.04 % ( for reference a CT pulmonary angiogram exposes a fetus to 4 mGy ) .