Asthma is a chronic lung disease characterized by recurrent breathing problems and symptoms such as breathlessness, wheezing, chest tightness, and coughing. During normal breathing, air flows freely into and out of the lungs. But when asthma is not under control, the airways of the lungs are thick, swollen, and inflamed. The airways become overly sensitive to environmental changes, and an asthma attack can happen easily. During an asthma attack, the lining of the airways swells, muscles around the airways tighten, and mucus clogs the tiny airways in the lungs, making breathing difficult.