Have you ever been called a mouth breather? If so, you should take offense to that. While mouth breathing in comparison to nasal breathing may not seem like a big difference, the results speak for themselves. While it is possible to breath in and out through our mouths, it puts us at a mechanical disadvantage. Nasal breathing makes it easier to activate your diaphragm (the only muscle that should be active during normal breathing) whereas mouth breathing leads to activation of secondary muscles that can contribute to anterior neck and shoulder pain. By exhaling through your nostrils, you are able to keep air in your lungs a little bit longer, which then lowers your respiratory rate, heart rate, and blood pressure. This causes a domino effect into using less energy overall and maintaining better oxygenation.
In the day and age of COVID-19 nasal breathing is even more important as it provides your body with an extra line of immune defense. The mucus membrane in our nasal passages are designed to block allergens, bacteria and viruses, whereas there is no such barrier in our mouths or throats.
An often overlooked benefit of nasal breathing is better sleep. In the book "Breathe" by James Nestor, the author goes into details of his personal experiment with strict mouth breathing vs. nasal breathing. It’s a great read that I recommend to everybody if you get the chance. To sum it up, during the strict nasal breathing component the author exhibited better brain function, better oxygenation, better sleep, ZERO sleep apnea events, better mood, lower heart rate, lower blood pressure and overall reports of better health. Sleep apnea is a major health problem in our society and by practicing strict nasal breathing, a lot of people could decrease the number of sleep apnea events throughout the night and it has been shown (after months of training) to actually reduce sleep apnea in some less severe cases.