I have allergic asthma.It's perfumes,cigar and cigarette smoke,candles, air fresheners etc. I use a dilator before a gig and have a little poster I put up in premises that allow smoking.
"For medical reasons the artist politely requests no smoking please"
. In the 90's I had some trouble with this from hardened smokers and lost some work because of it, but others started to make the same request and now it's general practice for gigs to be no smoking.
Another problem is the curtains and drapes on stages that can be incredibly dusty! Good luck..try all things. Also good before a gig are Vocalzones, lozenges that lubricate the throat.