Asthma is a condition in which the airways narrow and swell and may produce extra mucus. It is characterized by variable and recurring symptoms, including airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, coughing, chest tightness, and shortness of breath. Health professionals do not know exactly what causes asthma, but genetic and environmental factors both seem to play significant roles. Some factors, such as sensitization to an allergen, may be both causes and triggers.