It is perfectly natural to breathe through your mouth at certain times, such as when lifting a heavy load or exercising. Breathing through the mouth most of the time, however, can cause health problems.
Mouth breathing starts when we can’t get enough air through the nose, so the mouth takes over. This forces us to breathe through the mouth out of sheer necessity, and this can become a life-long habit if not corrected.
The most common causes of mouth breathing are:
Allergies and/or food sensitivities
Enlarged tonsils or adenoids
Chronic nasal congestion
Respiratory infection
Asthma
Deviated septum
Nasal polyps