Strontium 90

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STRONTIUM 90. When anyone discusses the harmful effects of atomic fallout, that discussion primarily relates to strontium 90 (90Sr), an isotope created during both nuclear detonations and nuclear-power reactions.

The parent chemical, strontium (Sr), is a silver-yellow alkaline-earth metal with physical and chemical properties similar to calcium. Strontium itself serves as a popular ingredient in fireworks and signal flares. The element also exists in the form of four stable and twelve unstable isotopes, including 90Sr.

A nuclear explosion creates 90Sr, one of the most lethal of the 300 radioactive products formed during detonation. Such a detonation can send a mushroom cloud thousands of feet into the atmosphere and carry radioactive isotopes thousands of miles, depending on wind trajectories. Precipitation can then deposit the isotopes on soils, waterways, vegetation, and cities in areas covered by the cloud. Because of 90Sr's chemical similarity to calcium, plant and animal tissues readily assimilate the isotope. At this stage, 90Sr may enter the human food supply, mainly in milk. The hazard occurs because the body treats 90Sr as though it actually were calcium, and the isotope subsequently concentrates in bone where it damages stem cells of the bone marrow as well as the body's immune functions. Strontium 90 has a half-life of 28.1 years, which means that it emits radioactive energy for that period of time. It is particularly dangerous for growing children and is believed to contribute to bone cancer and leukemia.

Scientists observed the effects of 90Sr at the end of World War II (1939–1945), when the United States dropped atomic bombs on the Japanese cities of Hiroshima (6 August 1945) and Nagasaki (9 August 1945); the two bombs killed in excess of 200,000 people. Prominent atomic physicists such as Enrico Fermi and Niels Bohr, both involved in the Manhattan Project that assembled the first two atomic bombs, began to advocate for peacetime applications of the new technology in the form of nuclear power.

Atmospheric nuclear weapons testing in Nevada began in January 1951, and by 1953, studies detected radioactivity levels in humans and animals, especially the presence of 90Sr. The following year, the U.S. Atomic Energy Commission began to measure 90Sr in healthy adult New York City residents who died in accidents. A 1958 study of 90Sr in 60,000 baby teeth in the St. Louis area revealed that levels of the radioactive element rose steadily during atmospheric nuclear weapons testing, except during a testing moratorium between 1959 and 1961. By 1962, the U.S. Public Health Service established an expanded program of 90Sr analysis in thirty-four U.S. cities. Levels peaked in 1964, shortly after the Limited Test Ban Treaty passed, and levels dropped rapidly after that.

Various U.S. governmental agencies began terminating 90Sr studies between 1970 and 1982, despite the fact that 90Sr concentrations in the urban areas of the northeastern United States remained at the 1957 levels. These concentrations suggest that another source of radioactivity—possibly nuclear power reactors—has added to 90Sr levels in the American environment. Researchers have linked radioactive emissions from reactors to increased childhood leukemia rates in the United States and abroad.


Makhijani, Arjun, Howard Hu, and Katherine Yih, eds. Nuclear Wastelands: A Global Guide to Nuclear Weapons Production and Its Health and Environmental Effects. Cambridge, Mass.: MIT Press, 1995.

Miller, G. Tyler, Jr. Living in the Environment: Principles, Connections, and Solutions. 9th ed. Belmont, Calif.: Wadsworth Publishing, 1996.

Miller, Richard. Under the Cloud: The Decades of Nuclear Testing. New York: Free Press, 1986.


See alsoNuclear Power ; Nuclear Weapons ; Physics: Nuclear Physics .

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Strontium 90

A radioactive isotope of strontium, produced during nuclear fission . The isotope was of great concern to environmental scientists during the period of atmospheric testing of nuclear weapons . Strontium 90 released in these tests fell to the Earth's surface, adhered to grass and other green plants, and was eaten by cows and other animals. Since strontium is chemically similar to calcium, it follows the same metabolic pathways, ending up in an animal's milk. When a child drinks this milk, strontium 90 becomes incorporated into their bones and teeth. With a half-life of about 28 years, strontium 90 continues to emit radiation throughout the individual's lifetime.

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stron·ti·um 90 • n. a radioactive isotope of strontium, used in radiotherapy. It is one of the chief products of the fission of uranium 235 and can pass from fallout into plants and animals and hence into human tissue, where it is concentrated in bones and teeth.

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