Asthma, a condition usually caused by allergic reactions to substances in the environment, affects many people. The allergic reactions cause the bronchioles to spasm. Hence, the flow of air into and out of the lungs becomes impaired. For some unknown reason, the flow of air out of the lungs is more impeded than the flow of air into the lungs. Hence, the person with asthma often finds it more difficult to expire (expel the air) than to inspire. Furthermore, such labored breathing, after many years, often results in the asthma-sufferer having a barrel-shaped chest.
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