More than 2,000 rad:
Death is a certainty. At doses above 5,000 rad, the central nervous system (brain and muscles) can no longer control the body functions, including breathing and blood circulation. Everything happens very quickly. Death occurs within days or hours. Nothing can be done, and medical care is for comfort only.
1,000 to 2,000 rad:
The probability of death increases to 100% within one to two 2000 rad weeks. The initial symptoms appear immediately. A few days later, things get very bad, very quickly since the gastrointestinal system is destroyed. Once the GI system ceases to function, nothing can be done, and medical care is for comfort only.
150 to 1,100 rad:
Severe blood changes will be noted and symptoms appear immediately. Approximately two weeks later, some of those exposed may die. At 300-500 rad, up to one half of the people exposed will die within 30 days without intensive medical attention. Death is due to the destruction of the blood forming organs. Without white blood cells, infection is likely. At the lower end of the dose range, isolation, antibiotics, and transfusions may provide the bone marrow with time to generate new blood cells, and full recovery is possible. At the upper end of the dose range, a bone marrow transplant may be required to produce new blood cells.
50 to 150 rad:
Slight blood changes including temporary drop in production of new blood cells will be noted and likely symptoms of nausea, fatigue and vomiting for one or two days.
5 to 50 rad:
Slight blood changes may be detected by medical evaluation
Less than 5 rad:
No immediate observable effects