You really don’t know how bad Asthma is until you’ve had it. I recently had a virally caused asthmatic period, though it seems to have resolved itself, after contracting a rather nasty bronchitis infection.
I remember wandering through the streets of Brighton, and being felled by an Asthma attack. I didn’t even know what it felt like or what it was before that. I had to use an inhaler for a while, and I’m a little susceptible to cigarette smoke.
During those days, I could have used a spirometer, esp. one to measure peak flow on a regular basis, so that I could figure out which were likely to be better or worse days. It would have helped if I had charted the results, too.
Oh, that article in Wikipedia has peak flow information for various ages, genders, and heights. So you’d get some cross references for your own results. It seems I don’t need it now, but a couple of years ago, it would have been QUITE useful.