Nitrogen: The Main Component of a Fart
When people think of intestinal gas, they think of methane, hydrogen sulfide, maybe CO₂. What surprises most people: the by far largest proportion of a fart — between 59 and 79% — consists of nitrogen (N₂), a colorless, odorless gas that has nothing to do with bacterial fermentation. Nitrogen comes from an entirely different source and explains some of the most puzzling properties of intestinal gas.
Where Does the Nitrogen in a Fart Come From?
Nitrogen in intestinal gas originates almost exclusively from swallowed air (atmospheric air is 78% nitrogen). While eating, drinking, talking, and especially while chewing gum or drinking through straws, a person swallows 0.5 to 2 liters of air per day that enters the digestive tract. Some of it is expelled as belching, while the rest passes through the stomach, small intestine, and large intestine and leaves the body as a component of flatulence. Since nitrogen is physiologically almost inert — it is neither metabolized by gut bacteria nor absorbed to any significant degree by the intestinal mucosa — it passes through the entire digestive tract virtually unchanged. The high nitrogen content explains why many farts barely smell: the bulk is an odor-neutral carrier gas.
The Complete Composition of Intestinal Gas
Typical human intestinal gas is composed as follows: nitrogen (N₂) 59–79%, carbon dioxide (CO₂) 9–29%, hydrogen (H₂) 0–21%, methane (CH₄) 0–26% (only in methane producers), oxygen (O₂) 0.1–2.3%, and trace gases below 1% — including hydrogen sulfide (H₂S), mercaptans, ammonia, and volatile fatty acids. The variability is enormous: depending on diet, microbiome, and individual physiology, the composition of a single fart can differ greatly from another. What matters for odor are exclusively the trace gases — particularly H₂S, dimethyl sulfide, and methanethiol (methyl mercaptan).
Why Does Nitrogen Affect Loudness?
Nitrogen directly contributes to the loudness of farts, even though it is odor-neutral itself: a high nitrogen content means a larger gas volume passing through the sphincter at once, causing greater vibration and therefore a louder sound. This is why the classic 'loud but odorless' fart is physiologically consistent: it consists primarily of nitrogen and CO₂ from swallowed air — odor-neutral but voluminous. The 'quiet but deadly' fart, by contrast, contains higher concentrations of fermentation gases, particularly H₂S. This principle underlies the 'silent but deadly' phenomenon.
Oxygen in the Gut: An Underestimated Topic
Although oxygen's share in intestinal gas is small at 0.1–2.3%, it has great significance for the ecological balance of the gut. The large intestine is a nearly anaerobic (oxygen-free) environment — oxygen is toxic to most bacteria living there. The small amount of oxygen that enters the gut with swallowed air is quickly consumed by mucosal cells and facultatively anaerobic bacteria. The ratio of oxygen to other gases can be used as an indicator of gut health: elevated oxygen content in intestinal gas is associated with inflammatory conditions (colitis), where immune cells use oxygen to combat pathogens.
Did you know?
- 59–79% of a fart consists of nitrogen — swallowed atmospheric air that passes through the gut.
- Nitrogen is odor-neutral and contributes little to smell — but accounts for most of the volume and loudness.
- Only the trace gases making up less than 1% of total volume are responsible for the odor.
- The oxygen content in intestinal gas is a potential biomarker for gut health.
Fun Fact
Fred Suarez and his colleagues at the University of Minnesota conducted a study in the 1990s in which they gave subjects a specially designed gas-producing meal and then analyzed every single fart over 24 hours using gas chromatography. Participants had to sit on a device that captured the gas while farting. Overall result: an average of 14 farts per person per day with an average of 33 mL per fart. The most prolific farter produced 53 gas events in a single day.
Sources
- Suarez, F.L. et al. (1997). Identification of gases responsible for the odour of human flatus. Gut, 41(1), 100–106.
- Levitt, M.D. & Bond, J.H. (1970). Volume, composition, and source of intestinal gas. Gastroenterology, 59(6), 921–929.
- Tomlin, J. et al. (1991). Investigation of normal flatus production in healthy volunteers. Gut, 32(6), 665–669.