Radioactive isotopes affecting the human body, illustration. Different isotopes have different effects as they penetrate the body to different depths. Caesium-137 is a by-product of nuclear fission in nuclear power stations and nuclear weapons. Similar to potassium, it forms water-soluble compounds and is concentrated most in soft tissues. It has a half-life of about 70 days. Strontium-90 is produced in nuclear fallout from weapons testing and accidents. It has a half-life of 28.9 years and can reach bones and cause bone cancer. Iodine-131 has a half-life of 8 days. It is also a major nuclear fission product. It becomes concentrated in thyroid tissues, where it may cause cancer. Polonium-218, with a half-life of just over three minutes, is part of the decay chain from radon, a radioactive gas that is breathed into the lungs.