Why is it so important to inhale through our nose? There are several reasons for this. When we inhale through our nose, the hairs that line our nostrils filter out particles of dust and dirt that can be injurious to our lungs. If too many particles accumulate on the membranes of the nose, we automatically secrete mucus to trap them or sneeze to expel them. The mucous membranes of our septum, which divides the nose into two cavities, further prepare the air for our lungs by warming and humidifying it. Over time, this filtering and humidification process helps protect our lungs from the damage that would otherwise occur.
Another very important reason for breathing through the nose--one that very few people are aware of--has to do with maintaining the correct balance of oxygen and carbon dioxide in our blood. When we breathe through our mouth we usually inhale and exhale air quickly in large volumes. This often leads to a kind of hyperventilation (breathing excessively fast for the actual conditions in which we find ourselves). It is important to recognize that it is the amount of carbon dioxide in our blood that generally regulates our breathing. Research has shown that if we release carbon dioxide too quickly, the arteries and vessels carrying blood to our cells constrict and the oxygen in our blood is unable to reach the cells in sufficient quantity. This includes the carotid arteries which carry blood (and oxygen) to the brain. The lack of sufficient oxygen going to the cells of the brain can turn on our sympathetic nervous system, our "fight or flight" response, and make us tense, anxious, irritable, and depressed. There are some researchers who believe that mouth breathing and the associated hyperventilation that it brings about can result in asthma, high blood pressure, heart disease, and many other medical problems. Some people, for instance, get exercise-induced asthma, a temporary condition in which one begins gasping for air.
When you work out aerobically the whole point is to find ways to get more health benefits from your workout. Here are some questions you might ask yourself. Would you like to burn more fat during your workout? Would you like to reduce exercise-related fatigue and injury? Would you like to increase your endurance and stamina? Would you like your workout to help improve your breathing?
If your answer is "yes" to any or all of these questions, and it no doubt is, then there is one simple thing you can do: don't let yourself become "breathless" at any point during your workout. When you become breathless, you undermine your breathing coordination, burn sugar instead of fat for fuel, become tight and tense (which can promote injury), and, in general, undermine your endurance and stamina.
The simplest way to know whether you are exercising too intensely and becoming breathless is to try to speak several sentences out loud while you're working out. If you can't do it without gasping for breath, then your workout is no longer "aerobic"--it is, or is about to become, "anaerobic," which means that it is proceeding without oxygen and you are no longer burning fat for fuel. Another way to look at what has happened is that you are hyperventilating, which means that you won't get oxygen where it needed in your brain and body and you will feel as though you are out of breath, even though you may have plenty of oxygen in your blood.
A simple way to ensure that you are working out at a level that will not make you breathless is to inhale and exhale only through your nose. If you try this you will quickly discover, especially at the beginning, that you will have to work at a slower or less-intense rate during your workout. Gradually, however, your breathing coordination and blood chemistry will improve and you will be able to do more and progress more rapidly, eventually going well beyond your previous limits.