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.