During a nuclear event, radioactive iodine can be released into the air. Your thyroid absorbs iodine, whether it’s safe or radioactive. If you don’t block it with potassium iodide, your thyroid may take in radioactive iodine. Short term, this can cause swelling, pain, and inflammation. Long term, it raises your risk of thyroid cancer and permanent damage. So when should you take it? If you’re outside the fallout path by hundreds of miles, don’t. If you’re within 50 miles and downwind, you likely