This narrowing or obstruction is caused by:

• Airway inflammation (the airways in the lungs become red, swollen and narrow)
• Broncho-constriction (the muscles encircling the airways tighten or go into spasm)

Airway Inflammation
If a normal airway has been exposed to certain stimulus (i.e. inhaled allergen like grass pollen), it becomes inflamed, swollen and plugged with mucus. This makes the airway opening smaller and more difficult for air to get through. It’s easy to see why children, who have small airways to begin with, would have difficulty breathing if this happens.

Broncho-constriction
If the opening of a typical airway is exposed to certain stimuli (such as cold air or enthusiastic exercise), the muscle fibres surrounding the airway contract thus making the airway opening even smaller. This makes breathing for the sufferer much more difficult.