The main radiation hazard is not in the actual exposure to radiation, but in ingestion (in the broad sense, including inhalation) of radioactive isotopes. Such

The main radiation hazard is not in the actual exposure to radiation, but in ingestion (in the broad sense, including inhalation) of radioactive isotopes. Such contaminants are able to remain inside the body for long times and have various long time effects that are more severe than acute radiation exposure. That is one reason why radiation (even measured in sieverts) is not really meaningful measure.