Until now I believed that breath exhalation through the mouth is common for most human beings, and this fact is implicit in several technical contexts of which I can think (sports, music, medical checks...). But recently a friend was surprised when I claimed this, and conducted a small poll around himself, giving a 100% of people inhaling and exhaling both through their noses in normal breath (then I was surprised!).
This issue is not without practical interest in these COVID-19 days, because when using a surgical mask (which filters the air going out but not the air coming in), people exhaling from their mouths and not covering their noses with the mask are still giving more or less the same protection to others, so if most people actually exhale through the mouth, this common "faulty" usage would actually not be so dangerous.
Can someone point to any scientific paper having evidence or statistics about normal mouth exhalation against normal nose exhalation?