Asthma is a chronic lung condition, characterized by a difficulty with breathing. People with asthma have extra sensitive or hyper-responsive airways. During an asthma attack, their airways become irritated and react by narrowing and constructing, causing increased resistance to airflow, and obstructing the flow of the air passages to and from the lungs.

Altitude will generally have little effect on stable asthmatics. If your asthma is so severe that your blood oxygen is low, then air travel can put you in danger of reducing your blood oxygen level even further. That, combined with the dry, cool conditions generally encountered at high altitudes, could trigger asthma symptoms. In addition, if your destination is a high altitude, a city such as Denver for example, and you're unaccustomed to that altitude and have little opportunity to acclimatize, you may experience heightened symptoms. As always, discuss this with your doctor ahead of time. Now, if you're dealing with severe asthma and you've been using your bronchodilator three or more times a week over the previous year, high-altitude treks can also increase your risk of an asthma attack. As already mentioned, the dry, cool conditions generally encountered at higher altitudes tend to aggravate the disease. Climbing, in and of itself, can be a very strenuous exercise which may trigger exercise-induced asthma in some people.

The best thing to do is to discuss your trip with your doctor several weeks in advance of your departure. This will allow time to work out a personal asthma action plan for the trip. This might involve increasing your preventer treatment for several weeks before the trip to give your airways extra protection, or measuring peak flow while away to determine how altitude is affecting your lung function, or even simple things like ensuring that you have enough medication and backup medication.