The 3 factors that cause bad breath - 1) Anaerobic oral bacteria  2) Smelly sulfur compounds 3) Tongue film.

- 85 to 90% of all bad breath cases are due to stinky volatile sulfur compounds (VSC's) produced by anaerobic oral bacteria living in the film buildup on the person's tongue. | What you need to know. | What you need to do.


Link to tongue debris section.


Link to  bacteria in periodontal pockets section.

Just imagine the smell of rotten eggs, an animal feedlot, urine, feces, sweaty feet and rotting meat.

Now, imagine that all of these odors were mixed up into a single cocktail and you had to take a whiff. Well, that's pretty much what you do every time you smell someone else's breath.

We didn't just make that list up. The different compounds responsible for those kinds of stinky smells (see list below) are found in everyone's breath.

And if their concentration is high enough, you'll have bad breath. Or, if you can keep their levels to a minimum, you won't. It's pretty much as simple as that.

Where do these smelly compounds come from?

Just like humans, bacteria consume foods and in turn excrete waste byproducts.

As it happens, the waste products produced by some types of oral bacteria are smelly sulfur compounds. And it's these molecules that usually lie at the root of a person's breath problems.

Some of the foulest smells you know are caused by sulfur compounds.

  • The stench of rotten eggs is due to the compound hydrogen sulfide.
  • The stinky smell that comes from feedlots and barnyards is created by the sulfur compound methyl mercaptan.
  • The odor you associate with the ocean is partly due to the presence of dimethyl sulfide.
And while it may come as a complete surprise to you, each of these compounds is produced as a waste product by the bacteria that live in your mouth. And these molecules escape with your breath every time you exhale.
FYI: The compounds listed above are the big three sulfur compounds typically associated with bad breath.

Section references - Aylikci

Bad breath is caused by "volatile sulfur compounds" ...

Dentists refer to the foul-smelling sulfur byproducts excreted as waste by oral bacteria as "volatile sulfur compounds" (VSC's). And it's their presence in the air we exhale that others detect as "bad" breath.

[The word "volatile" describes the fact that these compounds evaporate (transform into a gas) quickly, even at normal body temperature. It's this property that explains how these compounds are able to escape our mouth so rapidly and easily.]

FYI: Hydrogen sulfide and methyl mercaptan are the VSC's primarily associated with bad breath that has an oral/bacterial origin (90% of halitosis cases).

... as well as some other stinky molecules too.

While the VSC's in our first list above are generally considered to be the primary causative agents of bad breath, other types of volatile molecules can play a role too.

Some of these are the compounds that are responsible (at least in part) for the unsavory odors listed below.

  • Cadaverine - Urine, decaying meat.
  • Putrescine - Rotting meat.
  • Skatole and Indole - Human fecal matter.
  • Propionic and Isovaleric acid - Sweat and sweaty feet.
  • Butyric acid - Human vomit.
  • Pyridine - Unpleasant fish-like odor.

It all boils down to concentration.

This wonderful mix of smelly compounds is found in the breath of all humans, no one is an exception. Fortunately, at low levels they can't be detected by the human nose. It's only when their concentration becomes elevated that others begin to notice them.

Section references - Lourith, Aylikci

VSC's are produced by anaerobic oral bacteria.

Most of the compounds that cause bad breath are the byproducts of anaerobic bacteria (more specifically Gram-negative anaerobic bacteria).

The term "anaerobic" describes the fact that these types of bacteria do best in an environment that's devoid of oxygen.

Takeaways from this section.

Research suggests that the human oral cavity may be home to as many as 500 bacterial species, most of which are capable of producing smelly compounds that can cause bad breath.

But in particular, the following types of anaerobic Gram-negative bacteria have been identified as key producers of VSC's.

  • Bacteria typically associated with dental plaque build up around teeth - Fusobacterium nucleatum, Porphyromonas gingivalis, Tannerella forsythia, Prevotella intermedia and Porphyromonas endodontalis.
  • Bacteria typically associated with debris build up the surface of the tongue - Veillonella, Actinomyces, Prevotella, Capnocytophaga and Odontomyces (in populations age 70 and beyond).
  • Bacteria that generate high levels of hydrogen sulfide - Peptostreptococcus, Eubacterium, Eikenella, Centipeda, Selenomonas, Capnocytophaga, Veillonella, Propionibacterium, Porphyromonas, and Fusobacterium
  • Bacteria that generate high levels of methyl mercaptan - Fusobacterium, Bacteroides, Porphyromonas, Eubacterium, and Treponema denticola.

It's possible that some types of Gram-positive oral bacteria, primarily Streptococci, may also contribute to VSC formation.

A proposed mode of action is one where these bacteria cleave off sugar chains from glycoproteins, thus reducing them to a more basic form of protein molecule that's then available for digestion by Gram-negative bacteria.

Section references - Suzuki, Calil, Danser

The key to controlling bad breath is to control the number of causative bacteria that live in your mouth.

The human oral cavity is home to hundreds of different types of bacteria. And there's a constant battle for living space between the types that cause bad breath (which are mostly Gram-negative anaerobic ones) and those that don't.

It's the precise balance between these two that ultimately determines the quality of a person's breath.

a) The role dental biofilms play.

The accumulation of biological films (the whitish coating that forms on teeth, above and below the gum line, and on the tongue) can tip the scales in favor of odor-causing bacteria.

  • As the biofilm matures, a transition takes place where within its depths it becomes ever more oxygen-depleted, and thus an increasingly hospitable environment for anaerobic bacteria.

    (This transformation takes place as quickly as during the first few days of the biofilm's formation.)

  • As more and more coating builds up on surfaces within a person's mouth (e.g. tooth, gum tissue and tongue), these bacteria gain more and more available living space, thus putting the person at ever-increasing risk for having bad breath.
So one way of controlling the number of offending bacteria that live in your mouth is to keep the level of biofilm that's present (dental plaque) to a minimum.

Section references - Kenney

b) The role food sources play.

Most volatile sulfur compounds are the waste products created by anaerobic bacteria as they digest proteins (or more precisely, the related compounds: proteins, peptides and amino acids).

Takeaways from this section.

Most specifically, studies indicate that it's the breakdown of sulfur-containing amino acids (the building block molecules of proteins) that result in the formation of volatile sulfur compounds.

These are primarily Methionine and Cysteine. (Other sulfur-containing amino acids don't play a major role because they are not incorporated into proteins.)

What's the source of these proteins?

Even without having created an obvious exposure, like just having eaten something high in protein, it's not hard for oral anaerobic bacteria to find their preferred meal.

  • Some of the compounds that make up saliva are proteins.
  • With lax brushers and flossers, there's always the leftover food debris from yesterday's meal, and the meal before that, and the one before that ...
  • The debris that tends to build up on the surface of the tongue (see below) is protein-rich. Major components include shed skin cells, expired white blood cells, blood metabolites and of course food debris.

Section references - Porter, Aylikci

High-protein foods.

Oral bacteria eat what you've eaten.

Since the primary food source for the bacteria that cause bad breath is protein, you can help to control their numbers by limiting your intake of foods that are rich in them. More importantly, you should make sure to clean your teeth and mouth promptly after meals that have included them.

Foods that are protein-rich.
  • Meat, fish and seafood, eggs, and dairy foods (milk, cheeses, and yogurt) are all obvious examples of foods that are high in protein. Most of us get about two thirds of our daily needs from them.
  • Other sources include cereal grains (and cereal grain products), nuts, and the seeds from pod bearing plants (peas, beans, and lentils).
  • Many deserts (cakes, pies, yogurt) often are surprisingly protein-rich. (Often due to egg white or nut content.)


Make sure to clean your mouth after meals.

Keep in mind, if you've eaten a meal that contains high-protein foods, and you fail to clean your teeth and mouth thoroughly afterward How to., you've created the type of prolonged food source that's idea for the proliferation of the kinds of bacteria that cause bad breath.

Where do the types of bacteria that cause halitosis live?

As discussed above, anaerobic bacteria do best in environments that are devoid of oxygen. And due to this fact, they're typically found in the greatest numbers in the hidden recesses of the mouth. This includes:

  • The deep grooves, micro-furrows and crypts on the dorsal surface (top side) of the tongue.
  • Areas in between teeth, and those spaces (termed periodontal pockets, see below) that form as a result of gum recession and bone damage caused by gum disease.
  • They're also found in thick debris films, such as the coating that's frequently found covering the back portion of a person's tongue.

(Takeaway: You may have noticed that two of the locations in the list above involve the tongue. That's why tongue cleaning plays such a vital role in controlling bad breath. How to do it.)

Illustration stating that tongue debris is the most common cause of halitosis.

The most common cause of halitosis is debris buildup on a person's tongue.

a) The dorsum of the tongue.

For the vast majority of people, most of the bacteria that cause their breath odor live on the dorsum (top surface) of their tongue.
In fact, studies suggest that roughly 4 times as many bacteria (100 vs. 25) tend to attach to a single skin cell on the tongue as compared to other areas of the mouth.

Section references - Yaegaki

Where on the tongue?

Think back to the breath testing experiments Self-testing. we outlined on the first page of this topic.

  • While the smell emanating from the anterior portion of a person's tongue can be unpleasant, it usually isn't the primary source of their breath problems.
  • The most common odor-producing region of the tongue is its posterior (back) portion.

Image showing smelly bacteria-laden debris on the back part of the dorsum of the tongue.

Smelly debris accumulation on the back part of the dorsum (top side) of the tongue.

Do this check.
Go to a mirror, stick your tongue out, and take a look. You'll probably see some amount of coating on your tongue's surface (see picture).
For most people, this film, or more precisely the anaerobic bacteria that live in it, is the fundamental source of their breath odor.


The coating is typically white but can also be yellow or grey. Some parts of it may even be brown or black.

The further back you look (toward your throat), the heavier the layer will usually be. (Researchers sometimes use the following terms to categorize the appearance of the film: dry, slippery, dry and rough, prickly and furred.)

The level of buildup will vary.

The amount of coating that forms can be influenced by the anatomy of the person's tongue. For example, people who have a deeply grooved or furrowed tongue may find that they accumulate more coating than those whose surface is smoother.

Other factors can play a role too. For example:

  • Age - Older people tend to have more accumulation due to age-related factors such as:

    1) Reduced salivary flow.  2) Decreased dexterity resulting in an inability to perform tongue cleaning.  3) Changes in the type of taste buds present (some kinds trap debris more so than others).  4) Changes in the type of bacteria present.

  • Periodontal disease - People who have gum disease tend to have a thicker accumulation (and more breath problems too, see below).

Section references - Danser

The amount matters.

Research has shown that there's a direct correlation between the amount of coating on a person's tongue and the total number of bacteria that are present (more coating = more bacteria). And in fact, the tongue can be home to literally billions of individual bacterial organisms.

The Winkel Tongue Coating Grid.

Image showing the Winkel Tongue Coating Index grid superimposed on a picture of a tongue.

The greatest amount of coating is usually found in section B, followed by A and C. Sections D, E and F usually have the least.

As you can probably guess, when the anaerobic bacterial load on a person's tongue is reduced (like by cleaning it), there's usually a direct correlation (an improvement) in the quality of their breath. (That's why tongue brushing or scraping Directions. plays such an important role in curing halitosis.)
The Winkel Tongue Coating Index.
As a way of documenting their patient's status, some dentists use the Winkel index to quantify what they find. Using this system:
  • An imaginary grid consisting of 6 rectangles is mentally superimposed over the patient's tongue (see picture).
  • Each section is then rated from 0 to 2 based on the amount of coating present.
  • All of the individual scores are then totaled to give an overall rating ranging from 0 (no coating) to 12 (a very heavily coated tongue).

Section references - Danser

Illustration showing where bacteria can accumulate below the gum line.

Odors can come from bacteria living below the gum line.

b) Bacteria that live around teeth.

The anaerobic bacteria that cause bad breath frequently find a home in places around your teeth too.
The evidence.
When you floss, you may notice that your floss dredges up a foul odor or taste. This odor is often most noticeable as you floss between back teeth.

These locations are places where anaerobic bacteria have found a home. The taste and smell you get is proof of this fact.

Healthy vs. diseased gums.

Animation showing periodontal pockets that harbor bacteria that cause halitosis.

Gum disease causes deep pockets that are hard to clean.

The role of gum disease.

Even in relatively healthy mouths, anaerobic bacteria are able to find suitable (oxygen-deprived) places to live. These types of locations are however more numerous and available in the mouths of people who have periodontal disease (gum disease).
That's because advanced gum conditions result in damage to the gum and bone tissue that surrounds a person's teeth, the outcome of which is the formation of deep spaces termed "periodontal pockets."

These recesses can be difficult, if not impossible, to clean. And that makes them an ideal undisturbed anaerobic environment for the types of bacteria that cause bad breath. (For more details, this page explains the halitosis / gum disease relationship. It's role.

Learn more ...

Our next page discusses important bad breath co-factors 8 culprits.. These are issues and conditions that tip the scales in favor of your developing halitosis, or a worse case of it.

Or if you're more interested in just learning how to resolve your breath problems, this page outlines the plan needed for an effective halitosis cure. 3 goals to meet.


 Page references sources: 

Aylikci BU, et al. Halitosis: From diagnosis to management.

Calil CM, et al. Effects of stress hormones on the production of volatile sulfur compounds by periodontopathogenic bacteria.

Danser MM, et al. Tongue coating and tongue brushing: a literature review.

Kenney EB, et al. Oxidation reduction potential of developing plaque, periodontal pockets and gingival sulci.

Lourith N, et al. Oral malodour and active ingredients for treatment.

Porter SR, et al. Oral malodour (halitosis).

Rosing CK, et al. Halitosis: An overview of epidemiology, etiology and clinical management.

Suzuki N, et al. Induction and inhibition of oral malodor.

Yaegaki K, et al. Examination, Classification, and Treatment of Halitosis; Clinical Perspectives.

All reference sources for topic Bad Breath.