fallout

fallout, 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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fallout

Descent of radioactive materials from the atmosphere to the earth. Radioactivity in the atmosphere may arise from natural causes such as cosmic rays as well as from nuclear explosions and atomic reactor operations. The explosion of nuclear weapons leads to three types of fallout: local, tropospheric, and stratospheric. The first, intense but relatively short-lived, occurs as larger radioactive particles are deposited near the site of the explosion. Tropospheric fallout occurs when the finer particles enter the troposphere, and it spreads over a larger area in the month after the explosion. Stratospheric fallout, made of fine particles in the stratosphere, may continue years after the explosion, and the distribution is nearly worldwide. Many different radioisotopes are formed during a nuclear explosion, but only long-lived isotopes (e.g., cesium-137, strontium-90) are deposited as stratospheric fallout.