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What was the force they feared? When a nuclear bomb explodes, the triggering process called fission—the splitting of atomic nucleii—produces some 200 radioactive products that are quickly sucked up into the troposphere and the stratosphere. Some of these fall to earth quickly, causing dangerous fallout around the blast area; others drift around the earth in the troposphere, like the clouds of radioactive ash from recent Russian tests; still others—the great majority—may stay in the stratosphere for months or even years before dropping to earth. A "dirty" nuclear explosion is one that depends heavily on the fallout-producing fission process (used as a trigger for H-bombs), especially when it is exploded so low that it sucks up dirt, which causes radioactive materials to fall to earth more quickly than in a nuclear high-air burst.
Most of the fallout from the Russian tests will not return to earth until late winter or spring. By that time, most of the particles will have lost their radioactivity, but others will still be active. The active fallout is dangerous because of its abilities to emit gamma rays and high-speed particles that can destroy living tissue. Unlike local fallout, which falls on those downwind from the test site and penetrates the body externally, delayed fallout enters the human body through food supplies, particularly milk, meat and vegetables.
The chief villains in fallout are three radioactive isotopes known as strontium 90, cesium 137 and iodine 131. Strontium 90 and cesium 137 remain active for 28 years, iodine 131 for only eight days. Strontium 90 is the greatest threat of all because it concentrates in the bones of children, where it is believed to cause bone cancer and leukemia. In its brief life, iodine 131 is suspected to cause cancer, chiefly in children's thyroids. Cesium 137 may course through the entire body, sowing the seeds of possible future mutations.