I have never smoked in my life but, up until a couple of years ago, was doing up to 80 gigs a year, often in very smokey pubs. Some of these gigs were up to 4 or 5 hours. Like I think JulieB said above, I realised these gigs were actually making me feel very sick, probably a combination of a full day's day job, a couple of drinks, the excitement and adrenaline of playing, lack of sleep working into the wee hours and most of all, 4 or 5 hours of secondhand tobacco smoke.
As I couldn't change the policy of the venues, and the level of smoking here in Belgium is incidentally far higher than anything I've encountered in the UK or the States and the attitude of smokers much more aggressive, the only option I had was to cut out those types of gigs completely. It was a joy to play in a couple of sessions last time I was in Boston as both pubs were non-smoking so far as I could see.
I now do far fewer gigs but in places I know are smoke-free or, at least, well-ventilated. I regret this but there seems little else I can do if I'm going to keep my health.
I'm not against freedom of choice in this matter but it does seem unfair that non-smokers have little choice other than to leave in many instances.
Respectfully yours,
mcmoo