Besides obtaining some appropriate dose potassium iodide and taking 2 or possibly 3 doses of it, if we get radioactive debris carried to North America by the winds you will want to be careful about breathing in the dust.
Let me emphasize that it is very unlikely that large enough amounts or high enough concentrations of radioactive dust will reach North America from the present disaster in Japan. It would take some amazingly unusual weather conditions to blow such dust to North America.
However, for those still concerned about that, and in the highly unlikely event that it occurs, a simple particulate breathing mask should prevent any significant dust inhalation. A common N95 particulate respirator mask, which is available at most hardware stores and also at any place that sells industrial protective clothing and gear will do. It will be important to avoid going outside until the exposure is gone. But if you do have to go outside, then the particulate mask and an inexpensive plastic rain poncho will provide the protection you need.
Again, I want to emphasize that it is unlikely that we will see sufficient radioactive particulate matter blown all of the way to North America to be concerned about breathing in the dust.
Dangerous though they are, there is a big difference between a nuclear reactor accident or even a full melt-down and a nuclear weapon detonation. In the detonation there is very extreme heat and a mushroom cloud, which carries the radioactive matter so high into the atmosphere that widespread and rapid spread by wind is a certainty. In a nuclear reactor power plant event, the release of heat and radioactive contaminants is much slower, and the heat and energy is insufficient to carry the particles nearly as high into the atmosphere.
Also, I have gotten lots of calls and e-mails from people in North America asking what to do if they cannot find any of the potssium iodide recommended in my previous post. (The stores in many areas are long since sold out and may not have it in stock for some time.)
So here is an alternate way to protect yourself: Get some simple, good ole Betadine from the drug store. This is the same stuff that is used topically to disinfect skin around wounds, prior to surgery, etc. It is still available at most drug stores. DO NOT DRINK IT!! It is not safe for anything other than topical application. It is poisonous to drink !!
However, when painted onto the skin, a significant amount of the iodine in it is absorbed right through the skin. Here is the recommendation: For an adult, assuming you have the most standard Betadine, which is about a 2% tincture of iodine, measure out about 8 milliliters. One measuring teaspoon such as those used for measuring when cooking is approximately 5 milliliters, or ml. So you will need a little more than one and one half such measuring teaspoons of the Betadine. Paint the entire amount onto the abdomen and allow it to dry. The amount of iodine that will be absorbed through the skin will be just about right. Repeat this for up to 3 doses, separated by 24 hours. In all other respects pertaining to timing and amount, follow the recommendations in my previous post.
For children and/or very small adults, reduce the amount of Betadine used. The cutoff point for the 8 ml amount is 130 to 150 pounds. If much less than that, reduce the amount of Betadine proportionally.
Also, there are other brands of iodine-based topical disinfectant. Some have different amounts of iodine and use different concentrations. If you cannot get the 2% Betadine, or other 2% tincture, you will have to adjust the amount applied accordingly.
To your health!!