minute particles of radioactive material produced by nuclear explosions (see atomic bomb; hydrogen bomb; Chernobyl) or by discharge from nuclear-power or atomic installations and scattered throughout the earth's atmosphere by winds and convection currents. Heavier fallout particles tend to settle to earth around the explosion site and downwind from it soon after the explosion. Lighter particles may stay in the atmosphere for years. Radioactive decay products in fallout include strontium-90, potassium-40, carbon-14, and iodine-131. They may contaminate food supplies if taken up by plants and animals or contaminate water supplies by falling into streams. If they accumulate in the human body, they can form concentrated internal sources of dangerous radiation. Fallout may thus be a cause of leukemia, bone cancer, and other diseases. It can also cause genetic damage.

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