After the Chernobyl accident, residents of Poland received mass distribution of this supplement in the time when the radioactive cloud was passing over them, while residents of the Ukraine and Russia did not. Subsequently, thyroid cancer and thyroid disease rates have skyrocketed in the unprotected areas, while Poland has had no similar increase in thyroid problems.
If you live within 50-100 miles of a nuclear plant, it's wise to have your own stockpile of potassium iodide on hand for each member of your family, and keep some at work, and in the car. Only take it if warnings are issued and the government instructs you via the Emergency Broadcast System to take potassium iodide, and the specific levels recommended.
For more information, see: Potassium Iodide and Thyroid Disease