Most people are a little unclear about what is going. The by products of nuclear fission are Iodine-131 (I-131) and Cesium-137 (Cs-137). I-131 has a half life of 8 hours, which means all of it will be gone after a few weeks. It can concentrate in cow milk and trees and get onto plants, making its way into the diet. This seems to have happened in Japan, where milk and spinach from the area had 27 times the safe level. If you take potassium iodide as a preventive measure before exposure, it goes into the thyroid gland and fills up the iodine receptors so radioactive iodine won’t bind. If not, I-131 can concentrate in the thyroid gland and increase the risk of thyroid cancer many years later. This is what happened in the aftermath of Chernobyl, where local thyroid cancer rates increased. Thyroid cancer is highly treatable, however. What is on more concern is Cs-137, which has a half life of 30 years. It can cause leukemia. Fortunately so far release of Cs-137 into the environment in Japan has been very low.