I often have people ask me how they should breathe. It's a big question with lots of variables, and it depends on many things. The one thing that is constant, the one thing that I always say immediately is, "Breathe through your nose every breath." The response is usually, "I always breathe through my nose," and maybe you do. But there are lots of times when even people who think they are nose-breathers will move to breathing through the mouth: when you are talking, when you are exercising, when you are cleaning the house, when you are in the shower, when you are sleeping — it varies from person to person of course.
Why should you breathe through your nose? The nose is built for breathing. It is a part of your innate immune system, identifying intruders, capturing them and killing them, and then giving a heads up to your adaptive immune system. It filters the air coming into your body. The nose warms (or cools) and humidifies the air you breathe so your sensitive lung tissue gets air that is just right for it. And the nose (in the paranasal sinuses at the back of the nose) also is where 60% of the Nitric Oxide in your body is produced. If you aren't nose-breathing, you aren't taking that Nitric Oxide (NO) into your body.
What does Nitric Oxide do? Many things. Since it was discovered in 1995 that we produce NO in our paranasal sinuses, over 60,000 studies have been done on it. NO is a vasodilator, a bronchodilator, is anti-microbial, and it helps regulate neurotransmitters like GABA and serotonin. Right now there are clinical trials happening at the University of Alabama at Birmingham on Nitric Oxide to treat severe COVID-19.
Although in these kinds of trials and in medical settings the amount of NO that is being given is far greater than what we produce in our noses, we can increase our nasal production of NO by humming. If we hum the vibrations increase our NO production by up to 15 times (that's 1500%).
Try nose-breathing. Notice when you are doing it, and notice when you are not. See if you can nose-breathe all the time, or even more often than you do, and notice how much better you start to feel.
Try this breathing exercise:
Sit quietly and gently nose-breathe for a few minutes. Then gently cover one nostril. (I like to cover the nostril with my thumb, palm up, fingers resting on my face. You can even rest your elbow on a table if you'd like.) Continue gently breathing — don't increase or decrease the rate or length of your breaths — and hum on your exhales. Do this for a few minutes on one side, inhaling through one nostril and humming with the exhale (keep your mouth closed the whole time), and then switch sides. Really feel the vibrations in your face as your hum. See if you can do 5 minutes on each side for a total of 10 minutes (but don't worry if you don't have 10 minutes to devote to this). Don't breathe big breaths — if you start to feel light-headed at any time, you are breathing too large a volume of air. Stop, slow the breath down, and try again.