Asthma is caused by inflammation of the airways. These are the small tubes, called bronchi, which carry air in and out of the lungs. If you have asthma, your bronchi will be more inflamed and sensitive than normal.
When you come into contact with something that irritates your lungs, known as a triggers (e.g. dust mites, animal fur, pollen, tobacco smoke, exercise, cold air and chest infections), your airways become narrow, the muscles around them tighten and there is an increase in the production of sticky mucus (phlegm).