Breath odor originating in the mouth is largely produced on the tongue rather than on the teeth. The reason lies in the surface structure of the tongue and the chemistry happening on it.
The odor compounds are sulfur gases
The strongest oral odor compounds are volatile sulfur compounds, produced when bacteria break down proteins containing sulfur-bearing amino acids.
They are detectable at very low concentrations, which is why a small amount of bacterial activity produces a noticeable effect on breath.
Because the compounds come from protein breakdown, the raw material includes shed cells, food residue and components of saliva itself.
The tongue surface shelters the bacteria producing them
The upper surface of the tongue is covered in papillae, creating grooves and crypts. That texture holds far more surface area than a smooth membrane would.
Those recesses are protected from the rubbing of food and the flow of saliva. Bacteria and debris accumulate there and are not cleared mechanically.
The back third of the tongue is the least disturbed region, and it is where the coating that generates odor most commonly builds up.
Oxygen availability shapes which bacteria dominate
Sulfur-producing species generally favor low-oxygen conditions. A thick tongue coating creates exactly that environment beneath its surface.
Disrupting the coating exposes the deeper layers to oxygen and removes the accumulated substrate at the same time.
This is why mechanical cleaning affects odor while rinsing alone often has a shorter effect. A rinse does not readily penetrate an established coating.
Why dryness makes odor worse
Saliva dilutes and clears odor compounds continuously. When flow falls, the compounds accumulate rather than being carried away.
Salivary flow drops during sleep, which is the reason morning breath is nearly universal and usually resolves after eating and drinking.
Persistent dryness through the day has the same effect but continuously, and it is frequently linked to medications or a medical condition.
When odor is not coming from the tongue
Gum disease, decayed teeth, poorly cleaned appliances and food trapping between teeth all generate odor from other sites in the mouth.
Some causes lie outside the mouth entirely, including conditions of the sinuses, airway, digestive system or metabolism. These present differently and are not resolved by oral cleaning.
Odor that persists despite good oral hygiene is a reason to see a dentist, who can determine whether the source is dental or warrants a medical referral.