Most fart gas comes from swallowed air and consists largely of nitrogen and carbon dioxide, the oxygen having been absorbed by the time it reaches the anal opening. These gases are odorless, although they often pick up other (and more odiferous) components on the way through the bowel. They emerge from the anus in fairly large bubbles at body temperature. A person can often achieve a good sound with these voluminous farts, but they are commonly (but not always!) mundane with respect to odor, and don't feel particularly warm.

Another major source of fart gas is bacterial action. Bacterial fermentation and digestion processes produce heat as a byproduct as well as various pungent gases. The resulting bubbles of gas tend to be small, hot, and concentrated with stinky bacterial metabolic products. These emerge as the notorious, warm, SBD (Silent-But-Deadly), often in amounts too small to produce a good sound, but excelling in stench.