Any radiation incident on the human body exceeding 2 grays is likely to result in death. The more radiation the sooner the death. Anyone irradiated to this level or above will likely get very sick, but then temporarily recover (the latent phase) before sickness returns followed by probable death. The greater the irradiation, the quicker the initial symptoms manifest themselves, the shorter the latent phase, and the sooner the secondary sickness stage afollowed by death.

The bone marrow, or more specifically the stem cells within the bone marrow are the most susceptible to damage by radiation. The stem cells give rise to new blood cells, specifically neutrophils and platelets, within the body. Without neutrophils, the body would quickly succumb to opportunistic infections, for it is the neutrophils that first respond to any infection, mobilising the immune system. The platelets help blood to clot; without them any wound would bleed continuously. The bone marrow and stem cells are impaired by a dose of just a half gray, but will recover. A dose above 5 grays would permanently eliminate bone marrow stem cells, which would be fatal. [This is why a part of a patients bone marrow is removed before radiation therapy for cancer, and then replaced after the therapy].

Above 5 grays and the gastro-intestinal tract is affected. Radiation damages any rapidly dividing cells, such as those lining the gut. Damage here can result in gut bacteria entering the bloodstream resulting in septic shock. This usually has dire consequences.

The central nervous system is damaged at exposures exceeding 10 grays, and death is certain with or without medical treatment.

Dose (grays) Immediate Symptoms Latent Phase Full-Blown Illness Prognosis
0.1 - 0.5 Mild Nausea Days to Weeks Slight decrease in blood cell count Almost certain survival
1.0 - 2.0 Mild to moderate nausea and possible vomitting 3 - 6 hrs after irradiation and lasting up to 1 day 10 - 14 days Loss of appetite, fatigue, reduced blood-cell counts Survival rate ~90%
2.0 - 3.5 Nausea and possible vomitting 1 - 6 hrs after irradiation and lasting up to 2 days 7 - 14 days Hair-loss, moderate to severe bone-marrow damage, severe risk of infection Fatality Rate 35 - 40%
3.5 - 5.5 Nausea and vomitting 30 mins after irradiation and lasting up to 2 days 7 - 14 days Hair-loss, internal bleding, severe bone-marrow damage with high risk of bleeding and infection, slight gastrointestinal damage Fatality Rate 50% within 6 weeks
5.5 - 7.5 Severe nausea and vomitting 30 mins after irradiation and lasting up to 2 days 5 - 10 days Hair-loss, internal bleding, severe bone-marrow damage leading to complete failure of blood system, high risk of infection, moderate gastrointestinal damage Death likely within 3 weeks
7.5 - 10 Excruciating nausea and vomitting within minutes lasting several days 5 - 7 days Severe gastrointestinal and bone marrow damage Death almost certain within 3 weeks. Complete recovery impossible.
10 - 20 Immediate excruciating nausea and vomitting and fatigue 5 - 7 days Severe gastrointestinal, bone marrow and lung damage Certain death within 5 - 12 days
20+ Coma None Certain death within hours Certain death