Mouth Breathing

Mouth breathing has been shown to result in poor growth of the jaws and leading to orthodontic problems (ref).
The way mouth breathing influences jaw growth is by preventing the tongue from resting in the roof of the mouth (the palate).
If a person is mouth breathing it is impossible for the tongue to be resting in the palate as it should.
It appears that when the tongue rests in the palate, it resists the light inward and downward force from the cheeks and lips with the result that the upper jaw grows larger, more forward, wider and higher.
It appears that if the tongue is not resting in the palate, the upper jaw grows smaller, further back, narrower and lower.
Chronic mouth breathing generally results in the lower face growing longer vertically and the normal rest posture is with the mouth open (ref).

Mouth breathing can be a result of an obstruction in the nasal passages such as a deviated septum, enlarged turbinates, or polyps.
The soft tissue in the nasal passages can simply be enlarged due to inflammation related to allergies or the tissues can be boggy and enlarged due to underuse.The tissues in the nasal passages are generally more compact and healthier when there is continuous nasal breathing.
Enlarged adenoids can also restrict nasal breathing and trigger mouth breathing.
When an infant or young child experiences a serious upper airway respiratory infection, mouth breathing occurs out of necessity. Once the infection clears up nasal breathing can return but it does not automatically return. The mouth breathing that was used out of necessity for the duration of the infection can remain as a habit.

Air that is breathed in through the nose is warmed, humidified, filtered, and partially disinfected before it passes the adenoids, tonsils, and the throat to the lungs.
When this air reaches the lungs it is cleaned of contaminants, is at body temperature and humidity, and is easy on the lungs.
The condition of the air breathed in through the nose is far less likely to trigger asthma symptoms that air breathed in through the mouth.
Air breathed in through the mouth is relatively cold, dry, and contaminated as it passes the tonsils and enters the lungs. This compromises the health of the lungs and increases the risk of respiratory infections.