Since the mid-1990s, I have maintained a personal stock of potassium iodate medication, which protects the thyroid from radioactive iodine-131. I chose potassium iodate (KIO3) (from a company called Medical Corps) because of its longer storage life when compared with potassium iodide (KI). Additionally, some studies show that large doses of KI may cause thyroid cancer.
Either medication is only to be taken in an acute emergency, and certainly is not recommended for general everyday use or for low levels of radiation such as what we expect to see wafting in from Japan.
But radioactive iodine-131 is only one part of the problem, and I have been wondering what to do about the other radioactive elements that are produced in nuclear disasters of various sorts. The most concerning products of nuclear fission in addition to iodine-131 are strontium-90, cesium-137, and cesium-134. You would see these in any nuclear plant meltdown or following a nuclear explosion. But in a "dirty bomb" situation, all bets are off, and you could see just about any radioactive element from radium to cobalt-60 to polonium.
Noodling around on the web, I found all kinds of information and disinformation, including so-called holistic cures of very dubious efficacy.
Then I stumbled onto the US Department of Health and Human Services' excellent web site called "Radiation Emergency Medical Management - Guidance on Diagnosis and Treatment for Health Care Providers", located here. You can download the whole site as a zip file (569 MB) to your computer here, or an abridged version onto your mobile device here. I would suggest that everyone download the whole site. On the internet, you never know when something may disappear.
The site is vast, and includes information on every medical aspect of radiation contamination with full descriptions of what to do in almost any eventuality. One page shows a table of radioactive isotopes and their effects on the body, along with another table listing the various countermeasure medications to be used for each isotope. Unfortunately, several of these medications are prescription only, which means of course that in a real disaster, by the time the medications get to people, it will likely be too late.
But to protect against strontium-90 and cesium-137/134, it might be a good idea to get some Prussian Blue, and some sodium alginate (made from kelp), both of which appear to be readily available (although the medical-grade Prussian Blue, aka Radiogardase, is apparently prescription only).
Medical Corps is currently trying to get caught up on their orders for KIO3, and has suspended new orders temporarily, but they apparently still have a very large stock.
The Inspector, for about $500. These all measure dose rate -- how much radiation you are exposed to per hour. Cold War era pen-shaped dosimeters are also available that measure your accumulated dose. Again, they are not very sensitive and are more suited to a nuclear war situation, where you are trying to determine if you will be 1) not sick, 2) a little sick, 3) not going to make it.
In other words, most of the Cold War items are intended mostly for triage and deciding when you might be able to leave a shelter, not for trying to determine if you are getting a small dose that may increase your cancer risk.
JP Laboratories makes some postage stamp-sized RADsticker dosimeters that are cheap ($4), but only start to change color at 25 rads. A friend gave me one, but I really want to know well before I get that much radiation.
They also have the credit card-sized RADTriage-FIT that starts to change color at 1 or 2 rads, which is probably about right since the allowed yearly occupational dose is 5 rads, and it is generally accepted that cancer risk begins to increase around 10-20 rads. These dosimeters have limited life (a year or two), but can be frozen to increase life to 5 years or so. A good site for some of these items is KI4U, Inc.
Stay safe out there.