Asthma is a lung disease. A person with asthma has red and swollen bronchial tubes with a lot of mucus. As a result, the bronchial tubes get narrower than normal, thus making it hard to breathe.

Besides this, the bronchial tubes are highly sensitive to irritants like pollen, tobacco smoke, dust, etc. When an asthmatic person inhales an irritant, the tubes become much more swollen and more mucus is produced. This causes further narrowing of the tubes. This narrowing makes exhalation harder than inhalation. Hence, carbon dioxide rich air gets trapped in the person's body and he does not get sufficient supply of fresh oxygen. As a result, the person can have severe shortness of breath and a feeling of tightness in the chest. When an asthmatic person experiences such problems, he is said to have an asthma attack.