I have asthma, and use a bronchodilator before diving. I experience exercise-induced asthma about once every two years, if that, and have had no problems in 40 dives. Colo. When it was suggested that the dive community reconsider its stance on asthma and diving at Divers Alert Network's 26th diving accident and hyperbaric medicine course in May 1995, a hot debate ensued.
Asthmatics have bronchial airway obstruction, some more than others.
While breathing compressed air, an asthmatic with obstructed airways can suffer pulmonary barotrauma or a burst lung on ascent. The recommendations that you read on my web page come from the Undersea and Hyperbaric Medical Society's workshop on asthma. I believe that they are conservative and for safety's sake, should be used. It's always a mistake to omit information from your certifying agency and your dive instructor. It doesn't mean that they are fixed in cement and cannot be altered if it can be shown that you are not in any danger.