After the Meltdown at Fukushima, my business doubled. People wanted an adequate dietary source of iodine to insure that their thyroids would not take in radioactive iodine being emitted into the atmosphere and taken up in the food chain. I found myself counseling customers to avoid roasting seaweeds as condiments, thus volatilizing some of the iodine. Make soup, I advised. You are mostly made of water, and your digestion transacts with seaweed in the realm of water. To tell the truth, this is becoming a time of triage in a world polluted by radionuclides. I use the physics department at the University of Maine at Orono to monitor for radionuclides being emitted at Fukushima. So far, we are free and clear.
A lot of so-called “kelp” is actually rockweed! (ascophyllum nodosum) It’s best to buy whole leaf kelp, not powders or granules.
Radioactive iodine 131 has a half life of 8 days. The rule of thumb for the decay of a radionuclide back to a normal element (back to iodine 127 in this case) is ten half lives. That would be 80 days. My kelp is warehoused at least three months before coming to market, so I am certain that it does not contain radioactive iodine 131.