Halitosis, oral malodor, breath odor, mouth odor, foul breath, fetor oris, fetor ex ore, or most commonly bad breath are terms used to describe noticeably unpleasant odors exhaled in breathing – whether the smell is from an oral source or not. Halitosis has a significant impact — personally and socially — on those who suffer from it or believe they do (halitophobia), and is estimated to be the 3rd most frequent reason for seeking dental aid, following tooth decay and periodontal disease.Loesche WJ, Kazor C. Microbiology and treatment of halitosis. Periodontology 2000. 2002;28:256-79. PMID 12013345.

General

In most cases (85-90%), bad breath originates in the mouth itself.Rosenberg M. The science of bad breath. Sci Am. 2002 Apr;286(4):72-9. PMID 11905111. The intensity of bad breath differs during the day, as a function of oral dryness, (which may be due to stress or fasting), eating certain foods (such as garlic, onions, meat, fish and cheese), obesity, smoking and alcohol consumption. Because the mouth is dry and inactive during the night, the odor is usually worse upon awakening ("morning breath"). Bad breath may be transient, often disappearing following eating, brushing one's teeth, flossing, and rinsing with specialised mouthwash.

Bad breath may also be persistent (chronic bad breath), which is a more serious condition, affecting some 25% of the population in varying degrees. It can negatively affect the individual's personal, social and business relationships, leading to poor self-esteem and increased stress.

Mouth

Although the causes of breath odor are not entirely understood, most unpleasant odors are known to arise from proteins trapped in the mouth, which are processed by oral bacteria. There are over 600 types of bacteria found in the average mouth. Several dozens of these can produce high levels of foul odors when incubated in the laboratory.

The most common location for mouth-related halitosis is the tongue. Large quantities of naturally-occurring bacteria are often found on the posterior dorsum of the tongue, where they are relatively undisturbed by normal activity. This part of the tongue is relatively dry and poorly cleansed, and bacterial populations can thrive on remnants of food deposits, dead epithelial cells and postnasal drip. The convoluted microbial structure of the tongue dorsum provides an ideal habitat for anaerobic bacteria, which flourish under a continually-forming tongue coating of food debris, dead cells, postnasal drip and overlying bacteria, living and dead. When left on the tongue, the anaerobic respiration of such bacteria can yield either the putrescent smell of indole, skatole, polyamines, or the "rotten egg" smell of volatile sulfur compounds (VSCs) such as hydrogen sulfide, methyl mercaptan and dimethyl sulfide.