Why the Nose Makes Nitric Oxide — and the Mouth Doesn't

When researchers discovered that nitric oxide was an important signalling molecule — a finding that won a Nobel Prize in 1998 — attention turned to where the body makes it. One of the surprising answers was the nose. This review documents the unusually high concentrations of nitric oxide produced in the nasal airway and the paranasal sinuses, and what that means for how we breathe.

Nasal nitric oxide isn't a curiosity. It acts as a first-line defence against viruses and bacteria, and it helps drive the beating of the cilia — the tiny hairs that sweep mucus and trapped particles out of the airway. With every nasal breath, this gas is carried downstream toward the lungs, where it helps open blood vessels and improve the uptake of oxygen.

This matters because it reframes nasal breathing as an active chemical process, not just a filter. A person who habitually breathes through the mouth skips this entire system: no nitric oxide boost, less microbial defence, weaker airway clearance. It's one more reason the project insists mouth breathing is not a harmless habit but a bypass of machinery the body was built to use.