What actually causes bad breath? - 1) Oral bacteria  2) Smelly sulfur compounds.

- For 85 to 90% of people who have halitosis, it's caused by anaerobic oral bacteria and the stinky volatile sulfur compounds they create.


Link to tongue debris section.


Link to  bacteria in periodontal pockets section.

Just imagine the smell of rotten eggs, animal feed lots, urine, feces, sweaty feat and rotting meat.

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

We didn't just make the list above up. The different compounds responsible for those stinky smells (see 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 compounds that usually lie at the root of a person's breath problems.

Some of the most offensive 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 feed lots 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 they escape with your breath every time you exhale.

Hydrogen and dimethyl sulfide account for about 90% of the volatile sulfur compounds found in breath. (Suzuki 2012) [page references]

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

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

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

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

While the VSC's listed above are generally considered to be the primary causative agents of bad breath, the bacteria that live in our mouth produce other waste products that can play a role too.

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

  • Cadaverine - Urine, decaying meat.
  • Putrescine - Rotting meat.
  • Skatole and Indole - Human fecal matter.
  • Proprionic 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 levels become elevated that others can detect them.

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 environments that are devoid of oxygen. These are the same types of bacteria that are frequently associated with the causation of gum disease (gingivitis, periodontitis).

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 smelling compounds that can cause bad breath.

But in particular, the following types of anaerobic Gram-negative bacteria have been identified as key producers of the volatile sulfur compounds that cause halitosis.

1) Bacteria typically associated with dental plaque build up around teeth - Fusobacterium nucleatum, Porphyromonas gingivalis, Tannerella forsythia, Prevotella intermedia and Porphyromonas endodontalis.

2) Bacteria typically associated with debris build up the surface of the tongue - Veillonella, Actinomyces, Prevotella, Capnocytophaga and Odontomyces (in populations age 70 and beyond).

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

(Calil 2014, Suzuki 2012, Danser 2003)

The key is controlling the number of these bacteria that live in your mouth.

The human mouth 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 and those that don't. It's the precise balance between these two that ultimately determines the quality of a person's breath.

The role dental films play.

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

  • A layer of film as thin as 0.1 to 0.2 millimeters (about the same thickness as a dollar bill) can be oxygen depleted, thus creating the precise type of environment in which anaerobic bacteria flourish.
  • As more and more coating builds up in a person's mouth, these bacteria gain more and more available living space, thus putting the person at greater risk for having bad breath.

What's the food source for these bacteria?

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).

When you eat, they eat.

That means as we consume foodstuffs like red meat, poultry, fish, and dairy products, the bacteria that live in our mouth get a meal too and in turn produce the byproducts that cause our bad breath.

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

Even without an obvious source, like just having eaten something, it's not hard for oral anaerobic bacteria to find a protein 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.

High-protein foods.

  • 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.)

Where do these bacteria live?

As mentioned 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) the tongue, areas in between teeth, and those spaces (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 type that often covers the back portion of a person's tongue.

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. (Yaegaki 2000)

Where on the tongue?

Think back to the breath testing experiments described at the beginning of this topic.

  • While the smell emanating from the anterior portion of a person's tongue can be unpleasant, it usually it isn't the primary source of their breath problems.
  • The most common odor-producing region of the tongue is the 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 a fundamental cause 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 appear. Some of the terms researchers have used to classify its texture include: dry, slippery, dry and rough, prickly and furred.

The amount of buildup will vary.

The level of coating found on a person's tongue may be influenced by the anatomy of its surface. For example, people who have a deeply grooved or furrowed tongue may find that they accumulate more bacterial-laden 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).

(Danser 2003)

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. (Rosing 2011)

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 plays such an important role in curing bad breath.)

The Winkel Tongue Coating Index.

When performing an evaluation, some dentists use the Winkel index to quantify their patient's status.

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).

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.

Bad-breath causing anaerobic bacteria can find a home in places around your teeth too.

When you floss, you may notice that you dredged up a foul odor or taste. This odor may be more 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 evidence of this.

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 this condition causes damage to the gums and bone that surrounds a person's teeth, which results in the formation of deep spaces called "periodontal pockets."

These recesses can be difficult, if not impossible, to clean. And that makes them an ideal anaerobic environment for the types of bacteria that cause bad breath. We discuss the role of gum disease in more detail on this page.

Our next page discusses bad breath co-factors. These are conditions and issues that tip the scales in favor of you having halitosis.



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