Sigurd, I, too, have late onset asthma, which can act up right in the middle of a song if there's a trigger, such as accidentally having a drop of saliva start to go down the windpipe, being around "air fresheners" or perfumes or cigarette smoke, or inhaling a particle of dust while singing.
For the most part, my asthma is under control if I take my maintenance medicines (beclomethasone dipropionate, Quvar, etc.) regularly instead of relying on (and thus overusing) the albuterol rescue inhalers.

There is a new medicine I tried recently called Advair, which requires only 1 puff b.i.d., and which COMBINES the fast acting and slow acting meds. I found it very effective.

Each case is different, of course, so I don't know if there's a medicine that will solve your problem. I hope so.

One thing I haven't tried yet but am very tempted to is those little air filters that are battery operated and can be worn around the neck. The idea is that they don't try to clean the air in the whole room, just the air right by your nose. Maybe they would be helpful against irritants and allergens like industrial cleaners, air fresheners, perfumes, animal dander, smoke, etc. Anyway, they don't cost much.

One other thing I have found helpful is to make sure I inhale through my NOSE while I am singing. (Maybe you already do this naturally, but I have a tendency to mouth breathe while singing, especially when trying to catch air quickly.) When I get an unexpected attack mid-song, it is almost invariably when I take in air through my mouth.

Anyway, I wish you luck in dealing with your asthma, and I share your concern.

Genie