For normal respiration, air reaches the lungs through the respiratory tract through the nose. The inner layer of the nasal passages secretes mucus, which has insecticidal nature. The hairs in the nasal passages prevent small insects and dust particles from entering. While breathing through the mouth, cold, dry air along with dust particles will pass through the cavity and enter the lungs. Such air can cause damage to the oral cavity and throat. Tonsils enlarge and the possibility of bronchitis increases. Gums and teeth may get damaged. There may also be damage to the Eustachian tube which may cause difficulty in hearing. Parasites of many types of diseases can enter the body.