Halitosis

Definition and scope

Halitosis (synonyms: oral malodour, fetor ex ore) denotes persistent, socially noticeable bad breath. It is often dismissed as cosmetic, but chronic halitosis is a biological manifestation of cellular and bacterial proteolysis and can carry significant psychosocial burden. Global prevalence estimates are around 30 % (EFP), with a systematic review reporting a pooled prevalence of about 31.8 %.

In 80–90 % of cases the odour is caused by volatile sulphur compounds (VSC) — primarily hydrogen sulphide (H₂S), methyl mercaptan (CH₃SH) and dimethyl sulphide ((CH₃)₂S) — formed by anaerobic proteolysis of the sulphur-containing amino acids cysteine and methionine.

Pathophysiology

The oral microbiome contains more than 700 bacterial taxa. When oxygen tension falls — for example in deep periodontal pockets (< 3 mmHg O₂), in tonsillar crypts or under thick tongue coating — the biofilm shifts from a saccharolytic to a proteolytic metabolism. Gram-negative anaerobes hydrolyse peptides and release free sulphur amino acids, which are then converted to H₂S and CH₃SH by cystathionine-β-lyase and methionine-γ-lyase.

• Hydrogen sulphide (H₂S) — "rotten egg" odour; dominates in tongue-coating malodour. Perception threshold ≈ 1.5 ppb; clinical threshold ≈ 112 ppb.
• Methyl mercaptan (CH₃SH) — "rotting cabbage"; correlates strongly with periodontal pocket depth. CH₃SH/H₂S ratio > 1 indicates periodontal origin.
• Dimethyl sulphide ((CH₃)₂S) — sweetish sulphur note; excreted via the lungs in blood-borne extraoral halitosis (hepatic, metabolic).

Classification

I. Genuine halitosis

Objectively measurable odour above diagnostic thresholds. Sub-divided into physiological (transient, e.g. "morning breath") and pathological — of which 85–90 % is intraoral and 10–15 % extraoral (airway, upper GI or systemic-metabolic origin).

II. Pseudo-halitosis

Firm subjective conviction of bad breath despite normal objective measurements. Responds well to evidence-based education and review of measurement data.

III. Halitophobia

Persistent, sometimes delusional belief of bad breath that continues after successful treatment and repeated negative measurements. Classified as Olfactory Reference Syndrome — requires psychiatric or cognitive-behavioural management.

Aetiology

Direct causes — the source of the odour

• Tongue-coating proteolysis — the posterior third of the dorsum is the principal reservoir. Anaerobes such as Prevotella intermedia and Treponema denticola convert cysteine and methionine to VSC.
• Periodontal disease activity — advanced gingivitis and periodontitis expand the subgingival anaerobic niche. "Red Complex" pathogens (P. gingivalis, T. forsythia, T. denticola) drive collagen breakdown that feeds further VSC production.
• Systemic blood-borne exhalation — diabetic ketoacidosis (acetone), end-stage renal failure (di- and trimethylamine), hepatic cirrhosis (foetor hepaticus).

Indirect causes — structural and environmental triggers

• Hiatal hernia and gastroesophageal reflux (GERD) — compromised lower oesophageal sphincter, refluxate and micro-bleeding of eroded mucosa provide protein substrate for fermentation.
• Anatomical traps — non-vital pulps, open root canals, ill-fitting dentures and tonsilloliths retain food debris and bacteria.
• Hyposalivation (xerostomia) — Sjögren's syndrome, mouth breathing and xerostomia-inducing medication remove salivary defence and promote anaerobic overgrowth.

Evidence-based diagnostic workflow

The modern consensus requires moving from the patient's subjective complaint to objective confirmation via three primary modalities:

1. Organoleptic scoring (OLS)

Despite advances in technology, OLS remains the clinical reference standard. A calibrated clinician smells exhaled breath and scores on the standardised 0–5 scale (0 = none, 5 = overwhelming).

2. Gas chromatography (OralChroma)

Objective quantification of H₂S, CH₃SH and (CH₃)₂S individually — enabling differentiation between intraoral (H₂S/CH₃SH dominant) and blood-borne extraoral (dimethyl-sulphide dominant) halitosis.

3. Portable sulphide monitoring

Handheld sulphide monitors measure total VSC and are useful for follow-up between full analyses.

Treatment

Treatment is aetiological. For intraoral halitosis the combination of mechanical tongue cleaning (tongue scraper twice daily) and periodontal therapy reduces VSC by 60–70 %. Chemical adjuncts (0.05 % chlorhexidine, cetylpyridinium chloride with zinc) add short-term effect but are not curative.

For extraoral causes we treat GI drivers ourselves — reflux, hiatal hernia, SIBO — and refer purely ENT or systemic causes appropriately.

Prognosis

When the correct cause is identified and treated, chronic halitosis has an excellent prognosis: most patients see clinically significant improvement within 4–8 weeks. Sustained results depend on maintenance of tongue hygiene and periodontal control.

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