I am not asking for medical advice. I would simply like to know if there is any factual basis for the advice always given in discussions of deep breathing to inhale through the nose and exhale through the mouth.
I got like 3 million hits on the search, here is the first one-
Inhale through your nose and exhale through your mouth. It’s not just something you do in yoga class – breathing this way actually provides a powerful medical benefit that can help the body fight viral infections.
The reason is that your nasal cavities produce the molecule nitric oxide, which chemists abbreviate NO, that increases blood flow through the lungs and boosts oxygen levels in the blood. Breathing in through the nose delivers NO directly into the lungs, where it helps fight coronavirus infection by blocking the replication of the coronavirus in the lungs. But many people who exercise or engage in yoga also receive the benefits of inhaling through the nose instead of the mouth. The higher oxygen saturation of the blood can make one feel more refreshed and provides greater endurance.
From anatomy class in college I recall that the shape of our pharynx heats and / or humidifies air as we inhale through the nose.
That way the snot goes in instead of flushing out.
That appears to be multiple reasons to inhale through the nose, and I’ll add that I gather air inhaled through the nose is filtered before it gets to the lungs. (first I’ve heard of the nitric oxide business, though; is that attested elsewhere?)
However, why exhale through the mouth? I didn’t see any reason given for that part. Why not exhale and inhale both through the nose?
WAG: because exhaling through the nose dilutes the beneficial stuff from your nasal cavity, so that you get less of it when you inhale through the nose.
I don’t have an answer, but I remember getting the advice in physical rehab therapy, “Smell the roses, blow out the candles.” It was a picturesque way of saying it.
From cradle to grave.
Conclusions from a small exercise study:
“As there are benefits and drawbacks associated with either type of breathing mode (nasal or oral), the ultimate preference is at the discretion of the participant.”
Effects of Nasal or Oral Breathing on Anaerobic Power Output and Metabolic Responses - PMC
Having to consciously think of the “proper” way to breathe gets into Ondine’s Curse territory and I say the hell with it.
An old golf joke is to mess with someone’s swing by asking whether they inhale or exhale at the moment of impact. (Me, I hold my breath!) 