For those allergic to iodine, they should try and minimize their exposure to radioactive iodine in other ways, such as being first to evacuate or trying to acquire and use a gas mask with the specialized filter for radioactive iodine and being especially careful about later avoiding foods possibly contaminated with it.
Couple facts to keep in mind, though, many who were allergic to iodine downwind of Chernobyl took KI anyways and endured the rash, etc. cause they saw that as less of a risk than thyroid cancer, but anaphylactic shock should not ever be risked without a doctor's personal instruction and care to treat it ready at hand.
Also, keep in mind that, for anybody much over the age of 40 the odds of thyroid cancer fully expressing itself in their lifetimes is much lower than younger people and especially children. So, if much over 40 and allergic to iodine, you should be less eager to risk an allergic reaction by taking KI (potassium iodide) or KIO3 (potassium iodate).
And, lastly, if there's a lot of radioactive iodine around that you'll be ingesting and/or inhaling, and you are allergic to iodine, then you are going to be at risk to have an allergic reaction anyways just from that radioactive iodine. So, choice might come down to accepting risk of allergic reaction either way, so it might be better to do it with KI or KIO3 and not also have additional risk of later thyroid cancer, too.
BUT, ANY DECISION TO USE KI or KIO3 BY ANYONE ALLERGIC TO IODINE SHOULD ONLY BE MADE BY YOUR PERSONAL PHYSICIAN AND WITH ASSETS TO TREAT ANAPHYLACTIC SHOCK CLOSE AT HAND. TALK TO YOUR DOCTOR NOW, WELL BEFORE ANY NUCLEAR EMERGENCY, TO DETERMINE BEST COURSE OF ACTION FOR IF/WHEN NEEDED IN THE FUTURE.
BTW, everybody acquiring KI or KIO3, allergic to iodine or not, should ask their doctor now if there is any reason at all that anybody in their household should not use KI or KIO3 if/when needed. There are some few medical conditions and medications that KI or KIO3 could be a complication of and you need to get your doctors assurance now that there won't be any problems before any nuclear emergency makes doing so impossible. That's just another part of your prudent preparations, especially if you are thinking ahead enough to have acquired KI or KIO3 to begin with.
More info about KI and KIO3 and thyroid cancer from radioactive iodine here...
www.ki4u.com
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Shane