Farts or fart (flatus in medical parlance), is the gas habitually released from the rectum of all humans and animals. Humans (men and women alike) normally release 0.5 - 1 liters of rectal gas per 24 hours, while ruminating animals emit such huge quantities of flatus (in their case mainly methane) that animal farting actually enters the equation of global greenhouse gases as a sizeable factor.

Swallowing air, making methane

There are two sources of flatus – external and internal. The external source is simply swallowed air, of which the oxygen (~ 20%) is normally absorbed in the digestive system, while the nitrogen (~ 80%) passes through and becomes one major component of flatus. The internal sources are various gaseous end products of the digestive process, mainly hydrogen and methane, together with small volumes of intensely malodorous sulphur-containing compounds.

Large variations

The chemical composition of flatus varies with (1) the diet (beans and other fibrous foods produce more flatus), (2) the health status of the individual’s digestive system (stomach infections often give rise to flatus, particularly of the malodorous variety), and (3) the way in which the individual eats (fast eaters gulp in a lot of air, which ends up as increased volumes of flatus).

Because of the large variations in eating habits among healthy individuals, it is difficult to pinpoint a “normal” chemical composition of farts. The available data tend understandably to be rhapsodic and are often focused on various anomalies. The world is still waiting for an all-encompassing WHO-sponsored Human Fart Project.

Free of fibers, free of farts

The following data (Tomlin, J., et al) for 10 healthy subjects (5 men, 5 women) gives a reasonably good idea of the chemical composition of fart gases. The individuals were first fed their normal diet + 200 g baked beans per day. Subsequently they were given a fiber-free diet. The results were as follows:

(A) Normal diet + 200 g baked beans:

                    volume of gas (ml) per 24 hrs
                    median           range
Total flatus        705            476 - 1491
Hydrogen            361             42 - 1060
Carbon dioxide       68             25 -  116
Methane              26              3 -  120 (3/10 subjects)
Nitrogen            213             61 -  476

(B) Fiber-free diet:

Total flatus        214
Hydrogen            ~ 0
Carbon dioxide        6
Methane             ~ 0
Nitrogen            207

Hydrogen and methane production seems to cease almost entirely when the subjects switch to a fiber-free diet. The farts then consist only of the remnants of swallowed air (nitrogen). But it is important to remember that a fiber-rich diet is much healthier, as it protects us from colon cancer and other serious health problems.

It is also worth noting in the table above that production of hydrogen in humans is much greater than methane production. In ruminating animals the situation is different.

Sulphurous odors

The actual amounts of the gases responsible for the objectionable odor of farts are very small, but unfortunately most noticeable to the olfactory organs. The following data (Suarez, F. L., et al) is based on subjects who were fed pinto beans and lactulose:

                          in flatus gas
Hydrogen sulphide         0.2  mmol/l
Methanethiol              0.04 mmol/l
Dimethyl sulphide         0.01 mmol/l

The higher the hydrogen sulphide concentration, the more objectionable the odor of farts seems to be perceived.

Global farts

In the global perspective, the chemistry of flatus doesn’t seem to be worth a fart -- provided, however, that you are not a ruminating cow or moose, in which case your bad manners are contributing to global warming.


Tawney, Timothy, R. (2002). Information on methane gas and its relation to global warming. Methane Explosion Warmed the Prehistoric Earth, Possible Again.: http://www.merck.com/pubs/mmanual/section3/chapter32/32a.htm

Tomlin J, Lowis C, Read NW: Investigation of normal flatus production in healthy volunteers. Gut 1991 Jun; 32(6): 665-9

Suarez FL, Springfield J, Levitt MD: Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour. Gut 1998 Jul; 43(1): 100-4


NOTE: This writeup was prompted by a serendipitous misreading by wertperch of the fact that the French scientist Chevreul had discovered the chemical structure of fats.

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