take it your asthma did not warn you? mine did.
No, my asthma is weird. The only common triggers I have are colds, flu and chest infections. Also (and I picked this up from another thread where we were discussing it), I get the impression your attacks come on quite quickly. Mine come on over several days, and it can take weeks to settle back down to normal. So, I have no excuse for letting it get out of control, but it's useless as an early warning system!
Fortunately, we realised what was going on when we were both feeling lousy, aired the house and extracted the fuel from the woodburner, which involved a melted dustpan and some nasty black patches on our landlord's new tiled floor! We later found out the flue had 4 ninety degree turns in it, and it was so badly blocked you could barely even get a finger into the remaining gap. Our landlord had to pay to get the whole thing ripped out and redone, and was very aware of how lucky he was to not be sued for compensation.