In contrast, however, mouth breathing "pulls all pollution and germs directly into the lungs; dry cold air in the lungs makes the secretions thick, slows the cleaning cilia, and slows down the passage of oxygen into the bloodstream." As a result, chronic mouth breathing may lead to illness. In about 85% of cases, it is an adaptation to nasal congestion,: 281  and frequently occurs during sleep. More specialized causes include: antrochoanal polyps;: 350  a short upper lip which prevents the lips from meeting at rest (lip incompetence);: 281  and pregnancy rhinitis which tends to occur in the third trimester of pregnancy. : 435

Potential effects
Conditions associated with mouth breathing include cheilitis glandularis,: 490  Down syndrome,: 365  anterior open bite,: 225  tongue thrusting habit,: 225  cerebral palsy,: 422  ADHD, sleep apnea, and snoring. In addition, gingivitis,: 85  gingival enlargement,: 85  and increased levels of dental plaque: 108  are common in persons who chronically breathe through their mouths. The usual effect on the gums is sharply confined to the anterior maxillary region, especially the incisors (the upper teeth at the front). The appearance is erythematous (red), edematous (swollen) and shiny. This region receives the greatest exposure to airflow during mouth breathing, and it is thought that the inflammation and irritation is related to surface dehydration, but in animal experimentation, repeated air drying of the gums did not create such an appearance. : 85 Breathing through your mouth decreases saliva flow. Saliva has minerals to help neutralize bacteria, clean off the teeth, and rehydrate the tissues.
Yoga
Yogis such as B. K. S. Iyengar advocate both inhaling and exhaling through the nose in the practice of yoga, rather than inhaling through the nose and exhaling through the mouth, using the phrase, "the nose is for breathing, the mouth is for eating." References
Further reading
Nestor, James (2020). Breath: The New Science of a Lost Art. Riverhead Books. ISBN 978-0735213616. External links

Effects of Mouth Breathing - WebMD (video clip)