The Chemistry of Body Odours – Sweat, Halitosis, Flatulence & Cheesy Feet

Today’s graphic looks at the chemical compounds behind a variety of body odours – all of which we all experience at one point or another. Each is a cocktail of many different chemicals, but there are a select few that are major contributors to the distinctive aroma of each – here’s a look at some of the main players.

Halitosis

In the majority of cases, the cause of bad breath, or halitosis, is the product of bacteria in the mouth. These bacteria produce waste products, and it’s these chemical compounds that contribute towards halitosis.

The main offenders are volatile sulfur compounds, or VSCs; hydrogen sulfide has a smell of rotting eggs, methanethiol (also known as methyl mercaptan) has the odour of rotting cabbage, and dimethyl sulfide has similarly pleasant likeness to rotting cabbage, or garlic. At low levels, these compounds aren’t detected by the human nose, but it doesn’t take a great deal for them to become noticeable; 0.00047 parts per million is the threshold for the human nose being able to detect hydrogen sulfide, for instance.

As well as these main compounds, there are also some lesser compounds that can contribute, some of which are exacerbated by certain drinks or foodstuffs. For example:

• The phenomenon of ‘coffee breath’ has been linked by the Journal of Breath Research with the compound 3-mercapto-3-methylbutylformate (the odour of which is brilliantly described here as ‘catty’).
• Garlic breath is attributed to allyl methyl sulfide, a product of the breakdown of garlic, which unsurprisingly has an odour described as garlic-like.
• Eating meat & fish can also lend your breath a formidable scent. The bacteria that produce VOCs thrive on proteins, and they can also produce other compounds, such as cadaverine and putrescine, associated with the smells of rotting corpses and rotting fish respectively.

Flatulence

Sulfur-containing organic compounds are again to blame for the variety of odours produced in the digestive system. The main sulfur-containing compound here is hydrogen sulfide, followed by methanethiol & dimethyl sulfide, all of which we’ve already met. You might ask which particularly complicated scientific method was used to determine their presence – as well as gas chromatography, a slightly more rudimentary method was utilised in one study.

As part of the research, 16 subjects were fed 200g of pinto beans, then had their ‘samples’ collected via the use of a ‘rectal tube’. It gets better. These rectal tubes were then handed over to two judges, who had previously ‘proved their ability to identify’ the different sulfur-containing gases. The study relates how these judges ‘3cm from their noses, slowly ejected the gas, taking several sniffs’. They then rated the odour on a scale from 1 (no odour), to 8 (very offensive). Pleasant work…

An interesting observation of this study was the difference between the farts of men and women. Though the small sample size means it isn’t possible to draw definite conclusions, they noted that the women in the study emitted a significantly higher concentration of hydrogen sulfide, and the judges both ascribed to them a significantly worse odour. They also noted that men tended to generate ‘a greater volume of gas per passage’. Now we know.

Underarm Odour

Your underarms are home to an estimated one million bacteria per square centimetre; these convert your otherwise odourless sweat into a variety of malodourous molecules.

Underarm odour actually introduces two organic compounds that don’t contain sulfur into the mix. 3-methyl-2-hexenoic acid is widely considered to be one of the main contributors to the human ‘sweaty’ aroma, which itself has an aroma delightfully described as ‘goat-like’. 3-hydroxy-3-methylhexanoic acid contributes a cumin-like scent, whilst 3-methyl-3-sulfanylhexan-1-ol (along with other sulfur-containing compounds) provides an onion note (it is in fact present in a 75:25 ratio of two enantiomers, the lesser of which has a fruitier aroma).

Foot Odour

Again, sweat is a major player in foot odour – however, as with underarm odour, it’s due to the fact that it provides a beneficial environment for bacteria to grow, and it’s the compounds that these bacteria produce that cause the perceived odour.

Methanethiol we’ve already come across at several points; two of the other main compounds contributing to ‘cheesy’ feet are propanoic acid & isovaleric acid. Propanoic acid is described as ‘pungent, sour and rancid’, whilst isovaleric acid attracts similar plaudits: ‘cheesy, rancid & fermented.’ The presence of isovaleric acid is actually the result of a bacteria also found in some strong cheeses.

Download the graphic as an A3 PDF here.

Available to purchase as a large-sized poster here.

The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’s content usage guidelines.

References & Further Reading

• ‘Which molecules make our armpits stink?’ – Real Clear Science
• Molecule of the Month – Methanethiol – John Emsley, The Independent
• ‘3-methyl-3-sulfanylhexan-1-ol as a major descriptor for the human sweat odour profile’ – M. Troccaz et al.
• ‘Identification of gases responsible for the odour of human flatus’ – F. L. Suarez et al.
• Beating Bad Breath – Scientific American
• Foot Odour – Wikipedia