Why radiation sickness hits fast-dividing cells first
Radiation sickness is less a single illness than a sliding scale. The dose a body absorbs in a short burst decides which organs falter and how quickly, because ionising radiation does its worst damage to cells that divide fast, starting with the factories that make blood and the lining of the gut.
Doctors call it acute radiation syndrome, and it generally involves a whole-body dose above 0.7 gray delivered within minutes, usually from a source outside the body. Absorbed dose is measured in grays, or the older rad, where one rad is a hundredth of a gray. That unit is deliberately kept separate from the sievert, which gauges long-term risks such as cancer rather than immediate harm. Heavy doses can wreck DNA beyond repair, and tissues that renew themselves constantly suffer first.
The effects stack up in tiers. Blood tests can pick up falling cell counts after as little as 0.25 gray, though below 1 gray the person may never notice. Higher up, the bone marrow falters, leaving the body short of white cells to fight infection and platelets to stop bleeding. From roughly 6 grays the gut lining is also hit, and above about 30 grays the nervous system is overwhelmed. Bigger doses bring symptoms sooner, and the early signs, such as nausea, fatigue and fever, are common to many illnesses, so they prove little alone.
Radiation type matters too. Alpha and beta particles barely penetrate and threaten mainly when contamination lands on skin or clothing, while gamma rays and neutrons pass straight into the body. Early twentieth-century radiographers used to calibrate their machines by exposing their own hands and timing how long the skin took to redden.
The syndrome is rare, but one event can strike many people at once. The Chernobyl disaster, the worst nuclear accident, produced 134 cases. Others arose from forgotten radioactive sources, as in the Goiania accident, where material taken from an abandoned hospital killed four people, and from software faults in radiotherapy machines like the Therac-25. Astronauts beyond low Earth orbit face the risk from solar particle storms, which in the past reached levels that would have been lethal without protection.
Source: Acute radiation syndrome
