Asthma is a condition that affects the airways - the small tubes that carry air in and out of the lungs.
When a person with asthma comes into contact with something that irritates their airways, the muscles around the walls of the airways tighten so that the airways become narrower and the lining of the airways becomes inflamed and starts to swell.
Sometimes, sticky mucus or phlegm builds up, which can further narrow the airways.
These reactions cause the airways to become narrower and irritated - making it difficult to breath and leading to symptoms of asthma.
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