Strontium-90 is a radioactive isotope of the element strontium, which is found in trace amounts in nature. It is produced during nuclear fission processes, such as those occurring in nuclear reactors and during atomic bomb explosions. Strontium-90 has a half-life of about 29 years, meaning it takes that long for half of a sample to decay into a stable form. Due to its chemical similarity to calcium, strontium-90 can be absorbed by living organisms, particularly in bones and teeth. This can pose health risks, including an increased likelihood of cancer, making it a concern in areas affected by nuclear accidents or fallout.