Tobro:

First, a lot of what goes in your mouth gets into your sinuses, in
bulk or by diffusion. Remember shooting milk out your nose
when someone tells a very funny joke when you are trying to swallow?
A lot of sinus problems are caused by a thick mucus layer blocking the
main drain down the back of your throat.
Do a search on "osteomeatal complex" and "nasal valve" and
"adenoids".
One senerio evidently evolves something like this:
Apparently we all start out with a set of lympth tissue up where the
airpath bends downward to join the windpipe and esophagus.
This tissue is called the adenoids and can harbor all kinds of
bacteria. The adenoids are also supposed to be gone by 13 or so, but
if they are inflamed or inhabited that seems not to happen. Anyway,
the biofilm in the adenoids creeps a bit farward everytime a virus
kills the cilia and goblet cells in your nasal epithiealium, without
being much noticed or causing much more than post nasal drip until it
gets to the constriction, the narrow place, the nasal valve.  When it
gets to the nasal valve, there is not much room.  It plugs airpath the
drainage from you frontal and maxilary sinuses, and those fill up with
your mucus and bacterially produced goo.
When this happens, you can most easily open an airpath from behind.
Vitiman C and MSM and licorice cause the adherent goo in the back of
your throat to at least partly liquify so that you can sniff and spit
it out. This goo from about 2 to  30% solids it swells as it hydrates
and detaches like a sponge , once you have gotten to the point where
it has obstructed your breathing, there are layers and layers there,
and getting it off is like removing 30 coats of old paint, one at a
time.
You can't blow this stuff out the front you have to sniff it out the
back frist.
Soak Soak Sniff Sniff Spit Spit, Blow. repeat repeat repeat.  Clear
the main drain.
If you do that you can finally at least establish an airpath that will
stay open long enough to let you get some sleep, irrigate to wash more
goo out and get medicine in , and tackle most tasks of daily life
without having to breath through your mouth.