I seem to do more or less what you do @Croatial I don't think I have serious sinus problems (although the optometrist has said I have blepharitis which might be connected) but I thought I would give it a try and see if it would help the headaches I get .
Instead of the cupped hand I just fill a (sterilized) cup with well boiled and cooled water and basically sniff the water out of the top of the cup with one nostril at a time.
Also ,after the water inside the nose has transited to the back of the throat and the mouth I give it an extra swirl by blowing my nose (just one nostril) with the intention of making my ears pop.
I don't think this is dangerous but I am no expert and maybe it could be harmful to delicate ear drums(?)
But the breathing feels better immediately and I get the impression that it does help the headaches although I am only 2 or3 weeks in and so can't be definitive.
My mother always used to gargle every day with salt water and claimed it kept colds away ..
Is this behaviour appropriate.
how are schools handling students who memorize books but can't actually decode


