Some asthmatics also respond to physical stimuli such as exercise or a change from breathing warm air to cold air. The result is that patient feels short of breath and there may be an audible wheeze due to airway narrowing which can cause severe breathing difficulty, which in severe cases, can certainly be fatal. One of biggest problems in discussing asthma, and this is particularly true when discussing asthma in context of diving, is that spectrum of severity is extraordinarily wide.

There are three main concerns about asthma and diving. First, asthma may make divers more likely to suffer a dive-related illness. We are all taught that most important rule in diving is to breathe normally and to never hold your breath. If a diver ascends while holding his breath, expanding air can damage delicate lung tissue, and air may be introduced directly into blood, travel to brain and cause an arterial gas embolism (AGE).

There is concern that an asthmatic may suffer narrowing or blocking of small airways during a dive, and that expansion of any trapped air during ascent may lead to same problem. There is also concern that use of reliever medication, such as Ventolin, prior to diving may cause lungs to be less efficient at filtering out venous nitrogen bubbles we all have after dives. These bubbles may then circulate through lungs and reach arteries where they might, in theory, be more likely to contribute to development of decompression illness.

Second, it is recognised that an asthma attack in water may severely compromise diver's safety by incapacitating him and causing an inability to function effectively. Indeed, it is hard to argue that difficulty breathing would not be a decided disadvantage if you were caught in a current that was sweeping you away from your boat.

Third, it is a plausible concern that diving itself could precipitate asthma. Asthma can be precipitated by exercise associated with diving, or by irritant effect of breathing a cold, dry gas. It is also recognised that regulators frequently leak a little salt water, and that some of this may be nebulised into a mist during breathing. This mist can irritate airways and precipitate narrowing in vulnerable individuals.