Asthma is basically a reactive airway disease. There's a few causes of it which include the swelling of mucosal cells, increased secretion of mucus, and constriction of the smooth muscle around the airway. About four out of every five people with asthma experience it as a result of exercise, however
, the more you exercise, the less your asthma will effect you, both in severity and number of occurrences. But don't just go start up a new exercise program and go all out unless you're cool with death.
Begin with cardiovascular work for a while before jumping into weight training. Make sure you have an inhaler with you, and take a big puff on it a few minutes before starting. Never miss your warm up. It helps gradually adapt your body to the increase in oxygen consumption, prevents premature lactic acid build up, increases your muscles temperature lowering the risk of injury and improving the elasticity of the connective tissue, and improves your neural transmission for motor unit recruitment, your coronary blood flow, and the redistribution of blood to active muscles. Basically the warm up is good. So spend about ten to fifteen minutes doing it. Work up to 20 to 45 minutes per session, 3 to 4 times per week. Then don't neglect your cool down. At least five minutes here. It will help prevent the blood pooling and drop in blood pressure that makes you light headed, muscle spasms and cramps, and helps lower specific hormones that mess with your cardiac rhythm.
After doing this for about four weeks, you can start adding some resistance training. Don't do any heavy compound movements for a while. That's lifts like lunges, leg press, squats, power cleans, dead lifts, etc. Basically the ones where you work like a banshee for thirty seconds, then rack the weight hugely fatigued sucking in tons of air. Hold off on those movements. Start with things like leg extensions, leg curls, calf raises, preacher curls, tricep pressdowns, seated rows, lat pulldowns, cable crossovers, pec deck flies, overhead press, and dumbell laterals. These won't suck the life out of you and they'll begin to build up a good muscular base. You can gradually become more advanced in your lifting, but when you start, don't go all out. Let your body adapt for a little while. The results will still come if you're consistent, don't worry. Split up your days- an upper body and a lower body day, and keep up with your cardiovascular work. If you do this, your asthma will gradually become much better, and then you can pretty much do whatever you want because it probably won't cause an attack anymore.