Lung diseases have a significant impact on the health of the United States population. The most common lung diseases are asthma and chronic obstructive pulmonary disease (COPD). Both of these respiratory conditions can result in significant shortness of breath mainly caused by narrowing of the airways (the bronchi of the lungs) or, in the case of emphysema (a disease that can cause COPD), destruction of the alveoli (the sacs where oxygen is taken up by the blood). Asthma is an episodic problem, and during an attack, the muscles of the bronchi constrict, leading to narrowing of the airways. This narrowing can be so severe that shortness of breath occurs even at rest. In COPD, the smaller airways are usually narrowed because of excess inflammation and mucus production (as seen in chronic bronchitis, a disease that can cause COPD), and in addition, in some cases there is destruction of lung tissue (as seen in emphysema). So patients with COPD can be short of breath all of the time, while also experiencing exacerbations or flares related to episodic increases in small airway inflammation that can be brought on by infections or exposure to irritants.