Iodine is a chemical element that is found in many foods, and it's also added to salt to help prevent goiters. The thyroid gland needs iodine to function properly, and too much or too little iodine can cause health problems. When there is a nuclear accident, radioactive iodine can be released into the air. If people breathe in this radioactive iodine, it can be absorbed by the thyroid gland and cause cancer. Taking an iodine pill helps to prevent this from happening by filling up the thyroid gland so that it can't absorb any more iodine, including the radioactive kind.