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From Wikipedia:
Nitrogen is the primary gas released. Methane and hydrogen, lesser components, are flammable, and so flatulence is susceptible to catching fire. Not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained bacteria capable of producing methane[5]. Similar results are found in samples of gas obtained from within the rectum. The gas released during a flatus event frequently has a foul odor which mainly results from low molecular weight fatty acids such as butyric acid (rancid butter smell) and reduced sulfur compounds such as hydrogen sulfide (rotten egg smell) and carbonyl sulfide that are the result of protein breakdown. The incidence of odoriferous compounds in flatus increases from herbivores, such as cattle, to omnivores to carnivorous species, such as cats. Flatulence odor can also occur when there is a number of bacteria and/or feces in the anus while being expelled. A small amount of solid or liquid fecal matter in fine particulate aerosol form may also be expelled, and included, along with flatulence.
Intestinal gas is composed of 90% exogenous sources (air that is ingested through the nose and mouth) and 10% endogenous sources (gas produced within the digestive tract). The exogenous gases are swallowed (aerophagia) when eating or drinking or during times of excessive salivation (as might occur when nauseated or as the result of gastroesophageal reflux disease). The endogenous gases are produced as a by-product of digesting certain types of food.
Flatulence producing foods are typically high in polysaccharides (especially oligosaccharides such as inulin) and include beans, lentils, milk, onions, radish, sweet potatoes, cheese, cashews, broccoli, cabbage, Jerusalem artichokes, oats, yeast in breads, etc. In beans, endogenous gases seem to arise from oligosaccharides, carbohydrates that are resistant to digestion. These pass through the upper intestine largely unchanged, and when they reach the lower intestine, bacteria feed on them, producing copious amounts of flatus (McGee 1984 pp.257hahaha8211;8).In the case of those with lactose intolerance, intestinal bacteria feeding on lactose can give rise to excessive gas production when milk or lactose-containing substances have been consumed.
Interest in the causes of flatulence was spurred by high-altitude flight and the space program; the low atmospheric pressure, confined conditions, and stresses peculiar to those endeavours were cause for concern (McGee, 1984 pp.257hahaha8211;8).
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