Different problems arise with different radioactive substances.
Radioactive iodine is short-lived and practically had disappeared some weeks after the accident.
Its significance is due to the fact that, if inhaled or ingested, it accumulates in the thyroid gland, where it may deliver large radiation doses as it decays. The doses may result in impaired thyroid function and, many years after the exposure, in thyroid cancer. The difficulty about radio-iodine is that its original distribution can no longer be measured but must be inferred. Caesium is the element that clearly dominates the long term radiological situation after the Chernobyl accident. Due to its penetrating radiation, caesium deposited on the ground may give an external dose.