Top photograph from the Nevada Test Site, U.S.

Kennedy signed the treaty on Octo( bottom). (France ceased atmospheric testing in 1974 and China in 1980.) President John F. stopped atmospheric testing in 1963, when the nations signed the Limited Test Ban Treaty. Fortunately, the U.S., the Soviet Union and the U.K. HOOD was the largest atmospheric test conducted at the Nevada Test Site (and in the continental U.S.). The characteristic cap forms when the fireball from the explosion cools sufficiently to lose buoyancy. Mushroom clouds, such as the one from the 74-kiloton test HOOD on J(detonated from a balloon at 1,500 feet altitude), are a universally recognized icon of nuclear explosions. The legacy of open-air nuclear weapons testing includes a small but significant increase in thyroid cancer, leukemia and certain solid tumors.

Cancer investigators have been studying the health effects of radioactive fallout for decades, making radiation one of the best-understood agents of environmental injury. These tests injected radioactive material into the atmosphere, much of which became widely dispersed before being deposited as fallout. Between 19, the U.S., the U.S.S.R, the U.K., France and China carried out more than 500 atmospheric tests of nuclear weapons totaling the explosive equivalent of 440 megatons of TNT.

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