What does it mean when your snot is blue?

Answer 1

Clear snot is straight mucus, which is the secretion of the mucus glands of the nasal cavity. This is normal, and everyone should have a small amount of clear snot on a regular basis - this is what traps the large dust particles and keeps them out of your lungs.

Green and yellow snot is mucus mixed with dead inflammatory cells; the color is hard to correlate with a particular inflammatory cell, as the majority of them will be dead neutrophils. Colored snot tends to indicate an infection in the nasal cavity, although a slight yellow color to snot can be normal.

Snot changing colors from clear to green to yellow means you were free of infection, then you had an upper respiratory infection, then it was cleared up by your body's defenses.