I'm allergic to nicotine - and I don't just mean in the politically correct sense...
When I was growing up I couldn't stand any exposure to smoke, it made me vomit a lot and really quite badly ill. Now I'm a bit older my sensitivity is a little diminished, but still bad enough that I can't spend more than twenty minutes or so in an averagely smoky pub before I start developing symptoms - sore eyes, sore throat, coughing - and if I don't get home, put my clothes straight in the wash and shower it develops into flu-like symptoms, headache, nausea, dizziness, aching... it's really very unpleasant and I could do without it! At worst it can easily put me in bed for a couple of days.
Unfortunately I've never found anything that will prevent it, so all I can do is avoid exposure to smoke... which isn't always as easy as it sounds, particularly when you work in public buildings and use public transport. Hurrah for the smoking ban, that's all I can say. I'll actually be able to go spend an evening in a pub with my friends without being made ill! These smokers make a lot of noise about their civil liberties being infringed by the ban, but I wonder if they ever stop to consider those of us who don't have choices...