Halitosis, oral malodor, breath 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.
Though 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.
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