Shari,
My son and I are both asthmatic -- mostly allergy-induced, but according to our allergy/asthma doctor anyone with asthma is susceptible to experiencing exercise-induced asthma at some point. She says this is a very common occurrence when an asthmatic is exercising in cold air (running outside, for example) or is involved in a strenuous cardio activity for long periods of time. Being around environmental allergens can aggravate the situation.
So for me, for example, I have to be careful about keeping my exercise space dust-free -- one of my worst allergies. If it's getting dusty I'll get wheezy for sure about 15 minutes into a cardio tape. I also can guarantee wheeziness if I try to run in cold weather -- sets me off every time.
My son has no dust allergy but unfortunately is allergic to pretty much everything green and pollen-filled. He plays high school soccer which is, needless to say, outdoors, very vigorous and goes on for 50-minute halves. He plays center midfielder which means he plays the entire field and is running continuously. He's in great shape but sometimes still gets wheezy and tight-chested sometimes during his games, particularly if it's high-pollen season.
Ask your own doctor about this before you try it, of course. But our doctor's instruction to us was to take one dose (two puffs) of our albuterol inhaler (brand names Ventolin or Proventil) about fifteen minutes before exercising. That's the standard/typical prescription asthma emergency broncho-dilator inhaler that you usually take every 4 to 6 hours until an asthma attack is under control. Alex carries a canister in his soccer bag and I have one in my exercise space. Our doctor also said that if we are involved in high-capacity cardio and still feel tight-chested and wheezy 40 to 60 minutes after the initial dose, we could take one additional dose during exercise.
The effect of the medication is immediate and feels great if you've been feeling constricted. But albuterol, as I'm sure you know, elevates your heart rate slightly so you have to be aware of that and factor it into your perceived exertion test or heart-rate monitoring.
This must be a pretty common instruction from asthma physicians, because there are about a half-dozen strapping young dudes on Alex's varsity team who whip out those inhalers just before the warm-up at every game and at every practice. And these are beautiful, healthy young men, let me tell you. Sometimes I see those inhalers reappear at half-time, although these guys are in such tremendous cardio condition that one dose before the game seems to do the trick most of the time. I know Alex hardly ever needs to use his inhaler more than once.
Just a personal aside -- Seeing all those buff young things lined up in a row on the bench using inhalers is a disturbing sight. I think the fact that so many kids have asthma is some kind of sad commentary on our environment. Maybe it's just me, but I swear I don't remember seeing anything like this when I was growing up, or even when I was around college athletes.
My two cents' worth.

This treatment keeps Alex and me comfortable and it's what our physician instructed. But again, PLEASE clear this with your own doctor before you try it, and if you use albuterol before exercising be careful to factor that into your heart rate control.
http://www.clicksmilie.de/sammlung/sport/sport003.gif Kathy S.