Breathing is an activity many people take for granted. An average person takes almost 25,000 breaths a day and processes more than 10,000 liters of air. When the respiratory system functions properly, this work is done automatically. When things go awry, however, a wide range of conditions can develop that lead to outcomes varying in severity from transient to chronic and from mild to fatal. Furthermore, respiratory disorders are widespread, affecting people at all stages of life. In fact, the National Center for Health Statistics estimates that in the United States more than 3.6 million children and 22 million adults suffer from chronic respiratory diseases.