Iodine is a trace element that is necessary for the body to produce the thyroid hormone. It is a bluish-black, lustrous nonmetallic solid element that mainly occurs in nature under its stable form, iodine-127. Stable iodine is naturally present for instance in seaweeds, sponges, and shell fish. It is also artificially added to salt. More: There are both radioactive and non-radioactive isotopes of iodine. One of the main radioactive isotopes of iodine is iodine-131. All radioactive isotopes of iodine are produced by nuclear fission. Radioactive iodine is the largest contributor to people"s radiation dose after an accident at a nuclear reactor. Iodine-131 has a short half-life (8 days) and therefore, any presence from nuclear fallout is essentially gone in less than three months. The accumulation of radioactive iodine in the thyroid gland can cause thyroid cancer.