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As anyone who's ever sneezed can tell you, it's caused by irritation in the nose. And the answer to a question on this very SE attests that "sneezing is a reflex, partially autonomous, to clear the nasal cavity of particles that don't belong there."

That being true, the sneeze mechanism is a little bit strange: if you hold your hand up in front of your face when you sneeze, you'll feel that all of the rapidly-expelled air that ought to be cleaning out your nasal cavity is coming out your mouth, not your nose. In fact I don't think I've ever felt air coming out my nose when I sneeze.

So what's going on there?

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    $\begingroup$ Your premise is wrong: si.wsj.net/public/resources/images/… $\endgroup$ – kmm Aug 19 '15 at 17:37
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    $\begingroup$ @kmm: Is it? I see a huge spray of droplet-stuff coming out of the mouth and a tiny bit of barely anything coming from the nose. $\endgroup$ – Mason Wheeler Aug 19 '15 at 17:38
  • $\begingroup$ To say that "all of the rapidly-expelled air" comes out of hte mouth is not accurate. Maybe the amount that comes out of the nose is sufficient. $\endgroup$ – kmm Aug 19 '15 at 17:52
  • $\begingroup$ If you keep your mouth closed, the sneeze will come out of your nose. From an evolutionary perspective, the mouth being open might help to reduce pressure which is good for preventing an aneurysm. $\endgroup$ – Alex W Aug 19 '15 at 21:53
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I've pondered this question too. It's very clear that when we sneeze, the expelled air comes mostly from our mouths. However, at least as far as my own sneezes are concerned, some of the air comes from my nose as well. I know that if I pinch my nose closed as I sneeze, my ears hurt and pop. If my nose is sniffly, I usually notice mucus come out. Our soft palate partially lowers and our tongue rises as we sneeze, in order to divert a portion of the air up into our noses. That position of our tongue is why our sneezes make a CHHH sound. If people intentionally keep their mouths wide open as they sneeze, that could explain why nothing comes from their nose, as there's nothing to divert the air.

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A good book to learn as background is called 'Why We Get Sick: The New Science of Darwinian Medicine'.

In a sense you can consider us vectors for some microorganisms. When you cough you spread germs to other individuals (other vectors). Thus, it would be evolutionarily advantageous for a microorganism's genes if they were to infect more vectors.

How many germs are spread from the nose rather than the mouth? I don't know. However, expulsion of germs from your mouth will reach further than from your nose. It might be that there is a greater density of germs coming out of your nose, but that they don't travel as far in the air.

The above is slightly speculative, though, and I could speculate more but from what I can tell that is to be kept to the comments. (I can't comment due to <50 rep).

TL;DR More germs being spread from the mouth due to increased capacity for infected droplets and increased potency in terms of expulsion - advantageous evolutionary mechanism for spreading germ's genes.

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