Asthma is caused by inflammation, or swelling of the lining of the airways in your child's lungs. In a child with asthma, the lining of the airways is always a little inflamed but when exposed to an infection, irritants such as cold air, chemicals, perfume or smoke, or allergens like dust or pollen, the lining of the airways swell. The airways contract and produce more mucus. These changes in the airway make it difficult for your child to move air in or out of the lungs.