Ok. You have asthma. I get that.
The OP never said anything other than the fact that he sings a lot and hates smoke. I've worked in smoky clubs before and carried a large fan on the road to alleviate that problem. I asked the OP the question, not you Spade. Truth is I never enjoyed 4 hours onstage in those environments even when I was a smoker.
When it comes down to it we all have to weigh risk/reward for a lot of what goes into being working musicians.
This is actual QFT material here. You have a choice to play in places that allow smoking. Don't like it, then don't play there. Can't find a band willing to bend to your rules, well then you don't get to play at all. I'm not a smoker and even I get this.

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