Why intestinal gas usually has no smell
Most gas in the digestive tract is made up of gases such as nitrogen, carbon dioxide, hydrogen and, in some people, methane. These molecules have little or no odor, so their presence alone does not produce the smell people associate with passing gas. Some nitrogen and oxygen also enter when we swallow air; other gases form as gut microbes break down food that was not fully digested earlier in the tract.
The smell comes from a small chemical fraction
The odor is driven instead by trace amounts of volatile compounds—chemicals that readily escape into the air. Some contain sulfur, including hydrogen sulfide, which can smell like rotten eggs. Gut microbes produce these compounds while processing sulfur-containing material in food and other contents of the intestine.
Because the odor-producing compounds can be detected at very low concentrations, they do not need to make up much of the gas. The bulk can remain odorless while a tiny chemical fraction determines what the nose notices.
Why the smell varies
The microbes in the gut and the material they have to work on both affect which compounds are produced. A change in what reaches the large intestine can therefore change the odor, even if the total amount of gas does not change in the same way. A memorable smell is not evidence that most intestinal gas is foul-smelling; it is the nose responding to a small, potent component.
