The U.S. Air Force distributes potassium iodide at the Yokota Air Base in Japan during the Fukushima nuclear disaster. (They stress at the time that it's a precautionary measure and they haven't seen increased radiation exposure at the base.)

After the Fukushima accident in Japan last year, potassium iodide emptied off the West Coast shelves faster than milk and bread before a hurricane. Why? Well, everyone knows it helps decrease your cancer risk from nuclear radiation. But how effective is it? That depends on when you take it, and what you want to protect.

Here are eight truths about potassium iodide and radiation exposure that you may not know.

1. There are many types of radioactive particles. For example, from Fukushima the main ones were iodine 131 (also called sodium iodide 131 or just plain radioactive iodine. The other one was cesium.

2. You can take potassium iodide until you turn purple and it’s not going to do a single thing to protect you against the cesium. In fact, it’s not going to protect you against any radioactive particle other than iodine.

The reason is your thyroid gobbles up iodine because it’s essential for your thyroid function. It doesn’t care if the iodine’s radioactive; it takes whatever it can get. In the order it gets it.

3. In order for the potassium iodide to help, you need to take it as soon as possible after your contact with the radioactive stuff (inhaling contaminated air, eating contaminated food )—at least within four hours. Yes, you can take it before exposure. It protects your thyroid for up to twenty-four hours.

Until you‘re out of the danger zone, repeat the dose every twenty-four hours.

4. You must take enough potassium iodide to saturate your thyroid so it doesn’t have room to take up any more.

The dosage is 130 mg for adults or 65 mg for children three and up. It doesn’t help to take more than that, and it’s likely to give you side effects. The FDA website tells what some of those side effects can be, and how to dose infants and pregnant women. Of course, stay away from it if you’re allergic to iodine.

5. Iodized table salt won’t cut it. It supplies plenty for your usual daily needs, but you’d have to take around two to three hundred teaspoons daily to saturate your thyroid.

6. Potassium iodide can only help protect the thyroid. Radioactive iodine doesn’t discriminate. Oh, sure, it likes the thyroid, but it damages the DNA of other organs just like any other radioactive particle

7. If stored in a cool, dry spot, potassium iodide tablets have a shelf life of around seven years. The liquid stays good for about five.

8. Eight milliliters (about one-and-a-half teaspoons) of topical povidone-iodine (Betadine) solution painted on the chest or abdomen may get into the bloodstream and thyroid enough to give protection. Although this advice is all over the Internet, including government sites, and people site a study, I have been unable to find the study. But if you don’t have potassium iodide and you heed the precautions from truth number 5, it shouldn’t hurt. Repeat the painting once, twenty-four hours later, if the threat is still there.