Effect of Radiation on Human Body (unit : gray (Gy))

100    : Unconsciousness or coma. Death within several hours.
 10    : Destruction of bone marrow, severe radiation sickness and
         reduced white blood cells and platelets. Death within 30 days.
  1    : Nausea and vomiting. Reduced cell formation in bone marrow, 
         temporary reduction in white blood cells.
  0.1  : Changes appear in lymphocytes produced by bone marrow.
  0.01 : No apparent symptoms. 

Initial Radiation Levels (unit : gray (Gy))

Distance from Hypocenter   100m   500m  1,000m  1,500m 2,000m 2,500m
Gamma rays (g)            117.0   35.0   3.93   0.487  0.071  0.012
Neutron rays               33.1   6.04  0.227   0.008  0.000  0.000

(note) The human body is said to absorb approximately one milligray (0.1 rad) of radiation each year from natural sources such as outer space, the sun and natural radioactive materials.