The science behind farts

Due to an unfortunate episode involving the Lab Dog, the Progeny and I found ourselves deeply involved in a conversation on the contents of flatulence and also argued for a while over the origin of the discrete sound produced by our canine companion. This one was quite silent but deadly.

The Lab-dog

The Lab Dog in her natural environment.

The farting sound is produced by the vibration of the anal sphincters and possibly also exaggerated by a larger volume of gas being pushed up through the closed buttocks, but that is to be seen as a bonus for the naked-butt-cheeked humans.

Flatulence is produced in the intestines by bacterial fermentation.  The more volatile sulfur compounds in the end product, the riper the odor.  There are strong correlations between three particular compounds and smell. The first is hydrogen sulfide (H2S), which is the same compound that is produced by marine anaerobes (bacteria that can live without oxygen) when the water has turned anoxic (no oxygen). See my post on Dead Zones to read more about that. The second one is methyl mercaptan (MM), and the third dimethyl sulfide (DMS). In the study that provided us with this delightful information, the test persons were fed a large amount of pinto beans to provoke flatulence.

The smell of each of these compounds was delightfully described as:
Decomposing vegetables (MM), unpleasantly sweet radish (DMS) and rotten eggs (H2S).


“German peasants greet the fire and brimstone from a papal bull of Pope Paul III in Martin Luther’s 1545 Depictions of the Papacy” (credit: Lucas Cranach, the Elder)

If you eat a diet too rich in proteins, the smell of your farts will increase due to sulfur-containing amino acids. The smell can also increase if there are feces (poo) in your rectum (the last part of your intestine) or if your microbial flora is somehow overactive.

Large intestine and rectum. By BruceBlaus: staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. (Own work) [CC BY 3.0 ( licenses/by/3.0)], via Wikimedia Commons

Large intestine and rectum.
(image credit: BruceBlaus)

 Flatulence can in rare instances be caused by the swallowing of air (aerophagia) such as when chewing gum. This flatulence does not smell since it is made up of the normal composition of air. Besides the issue of aerophagia, it is extremely annoying to watch people chew gum with their mouths open. I hope they get painful flatulence.

The methane and hydrogen of the flatulence are extremely flammable, and the proportion of these two compounds can vary depending on medical conditions. If you are obese, have irritable bowel syndrome (IBS) or constipation, you have proportionally more methane than hydrogen in your farts.

It is dangerous to set your farts on fire no matter how spectacular it might look.

[Tweet “The smell of farts: Decomposing vegetables (MM), unpleasantly sweet radish (DMS) and rotten eggs (H2S).”]

It is normal to fart between 500-1500 ml gas in 24 h and about 8-20 times per day. This makes about 5-350 ml per fart. We fart more in the mornings than in the evening.

Science Safely!


...still thinks that farts are very funny.

…still thinks that farts are very funny. scared of her own farts.

…is scared of her own farts.

Tangerman, A. (2009). “Measurement and biological significance of the volatile sulfur compounds hydrogen sulfide, methanethiol and dimethyl sulfide in various biological matrices”. Journal of Chromatography B 877 (28): 3366–3377.

Tomlin, J; Lowis, C; Read, NW (1991). “Investigation of normal flatus production in healthy volunteers.”. Gut 32 (6): 665–9. doi:10.1136/gut.32.6.665. PMC 1378885. PMID 1648028

Levitt, MD; Furne, J; Aeolus, MR; Suarez, FL (1998). “Evaluation of an extremely flatulent patient: case report and proposed diagnostic and therapeutic approach.”. The American Journal of Gastroenterology 93 (11): 2276–81. doi:10.1111/j.1572-0241.1998.00635.x

Posted in Bacteria, Biology, Medicine, The Biology of Disgusting and tagged , , , , , .


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  1. Pingback: The Science Behind Period Farts - The Imaginarium

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