Strontium | Radiation Protection | US EPA

Strontium (chemical symbol Sr) is a silvery metal that rapidly turns yellowish in air. Strontium is found naturally as a non-radioactive element. Strontium has 16 known isotopes. Naturally occurring strontium is found as four stable isotopes Sr-84, -86, -87, and -88. Twelve other isotopes are radioactive. Strontium-90 is the most important radioactive isotope in the environment, although strontium-89 can be found around reactors, and strontium-85 is used in industry and medicine

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Strontium | Radiation Protection | US EPA