hoosier:
Asthma is a tricky one unlike allergy. Many physiologies have been conducting the study about the Asthma and scuba diving. You should be careful than any other divers because this study is on going, so there is no exact guideline about how long and deep you can dive with the medication. I also know one professor who is doing the research about this subject by getting PADI research funds.
Yes, this is a good point. Althought allergies and athma are frequently present in the same patient, they are very different as far as their effect on diving. And there are very different degrees of asthma. Here is my understanding of the problem (I'm not a pulmonologist, so I'm a bit outside of my area of training here..)
Asthma is now called "reactive airway disease" because it represents constriction of the air passages (bronchospasm) in the lungs in reaction to a triggering event. Constriction causes wheezing and respiratory distress. Common triggering events are exposure to dirty air, cold air, dry air, or exercise. To some extent, we all have reactive airways- anyone will wheeze given a strong enough trigger. We call people athmatics when they are sensitive enough so that triggering events happen commonly and cause significant respiratory symptoms.
Interestingly, one of the reasons people with nasal problems (like allergies) can have problems with asthma is that the nose cleans, warms and humidifies the air we breathe. When it is blocked, you breathe less through your nose and lose this function, making the inhaled air more of a trigger. Therefore, aggressive management of allergies or sinusitis can improve athma symptoms.
When you dive, you are also breathing relatively cold, dry air (some regulators help this a little), and exercising as well. So if you already have a tendancy to wheeze (that is, if you are an asthmatic), diving can make it worse. And obviously, one of the worse places to have an astmatic attack is underwater! Such an attack can be fatal even in an emergency room, let alone while diving...
One more theoretical concern is pulmonary barotrauma. We all know that it is important to keep an open larynx while diving (more particularly, while changing depth, especially while ascending!). That is, you never hold your breath because trapped air in the lungs can cause a rupture of the lung tissue as it expands. A similar problem could happen with obstruction of the small airways due to a combination of bronchospasm and mucus plugging. Unlike the whole lung barotrauma that would happen from holding your breath (blocking the entire airway), this might just effect a small portion of one lung. Again, not clear how real this risk is, and the research is still ongoing.
All that having been said, not everyone with asthma is automatically excluded from diving. It is a question of degrees, and you would need to make this decision with your doctor.
Mike
PS if you are REALLY interested, here are a couple of article abstracts:
http://www.ncbi.nlm.nih.gov/entrez/...ve&db=pubmed&dopt=Abstract&list_uids=12617690
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=7944003