The key cause of cancer, on a cellular level, is known to professional oncologists for more than 70 years. Dr. Otto Warburg got a Nobel Prize for this discovery. He wrote in his article "The Prime Cause and Prevention of Cancer" (1966), "Cancer, above all other diseases, has countless secondary causes. Almost anything can cause cancer. But, even for cancer, there is only one prime cause. The prime cause of cancer is the replacement of the respiration of oxygen (oxidation of sugar) in normal body cells by fermentation of sugar..." Anaerobic metabolism in malignant cells appears due to tissue hypoxia (low tissue oxygenation). Dozens of more recent studies confirmed that tissue hypoxia is the prime cause of cancer.
What does mouth breathing have to do with body oxygenation? There are at least three independent mechanisms how mouth breathing reduces body oxygenation and, therefore, promotes cancer.
First, when we breathe through the mouth we cannot utilize nitric oxide, a powerful hormone that is generated in nasal passages. Nitric oxide is a vasodilator (it expands arteries and arterioles) improving blood supply to tissues. In fact, it was recently discovered that popular drug nitroglycerine prevents heart attacks due to its conversion into nitroglycerine which dilates blood vessels, and hence, reduces heart rate and blood pressure.
Second, mouth breathing reduces carbon dioxide content in the lungs, blood and all tissues of the human body. Why? Because the nasal route is much longer and narrower, while oral route is short and wide. This causes hypocapnia (low CO2 in the body) resulting in vasoconstriction and the suppressed Bohr Effect (red blood cells cannot efficiently release oxygen in tissues due to low tissue CO2 values). Even slight, hardly noticeable mouth breathing immediately causes these two CO2-realted effects both of which directly contribute to cellular hypoxia.
Third, we spend most of time indoors where air has viruses, bacteria, dust, dust mites, paints, and hundreds other chemicals. Our nasal passages, when moist, can trap up to 99% of these airborne particles and substances. These airborne objects simply get stuck on the layer of mucus in sinuses. Then the mucus is drained into the stomach for dis-activation by digestive enzymes. Mouth breathing makes these processes impossible. Dust, dust mites, viruses, bacteria, and numerous chemicals, all travel straight into bronchi, bronchioles, and alveoli (tiny airsacs in the lungs). This creates stress for the immune system, kidneys, and liver.
One can even practically measure body oxygenation (or the degree of tissue hypoxia) and, hence, the effects of nasal and mouth breathing. How? Breathe through the nose for 20-30 minutes. Then after your usual exhalation, hold your breath, but only until the first signs of stress (you should not push yourself at all). This number accurately reflects freely available oxygen in the body. Then breathe through the mouth for 20-30 minutes and repeat the test and compare results. Practice show that breath holding time for mouth breathing can get reduced by 5-10 s.
This is exactly the effect that takes place in millions of people when they start to breathe through the mouth, especially during early morning hours (4-7 am). Body oxygenation drastically drops to about 15, in some cases, less than 10 s. In cases of tumours, less than 20 s of oxygen means tumour growth, while less than 10 s means chances of metastasis. Reversal of tumours is possible when the oxygenation is about 35 s or more and this is impossible for mouth breathing since mouth breathers have less than 30 s, usually less than 20 s of oxygen in the body.
In order to prevent mouth breathing during the night, Russian MDs who practice the Buteyko self-oxygenation therapy, explain to their patients the technique that is called "Mouth taping". Apply small amount of cream or Vaseline on your lips (to prevent tape's sticking) and tape your mouth either along, but better across and bend the end of the tape (for easy removal in the morning). You might be surprised about your improved health and well-being the next morning.
If the nose gets blocked during the daytime, use the breathing exercise "How to unblock the nose". Many internet articles and web-pages have description of this exercise. In 1-2 minutes most people can unblock the nose and resume their nasal breathing using a combination of comfortable breath holds and sensation of light air hunger.