Bad Breath (Halitosis): Why It Smells from Mouth But Not Nose

Oral-origin halitosis is often perceived on mouth exhalation but not nasal because:

• Tongue coating – posterior dorsum harbors anaerobes producing volatile sulfur compounds (VSCs). Mouth breathing releases VSCs directly; nasal airflow bypasses the oral cavity.
• Periodontal disease – subgingival biofilms generate VSCs; expelled orally.
• Sinusitis/post-nasal drip – infected mucus drains into oropharynx, adding odor on oral exhale; nasal air may be less affected if ostia patent.
• Xerostomia – ↓ saliva → ↑ bacterial load; worse on oral breathing.
• Systemic – diabetic ketoacidosis (acetone), uremia (ammonia), hepatic fetor – detectable both routes but often stronger orally.

Management: Tongue scraping, flossing, hydration, treat periodontitis/sinusitis, medical eval if persistent.