Flatus is flammable, as both methane and hydrogen are flammable gases. The odor in flatulence comes from hydrogen sulphide (from foods in the diet) and other sulfur- or nitrogen-containing compounds including methanethiol (methyl mercaptan). Upon lighting a match the hydrogen sulphide will ignite to form water (vapor) and sulphur dioxide. Removing the hydrogen sulphide may also remove the odor, although this has been disputed.
Nitrogen is the primary gas released during flatulence, along with Carbon dioxide which is present in higher quantities in those who drink carbonated beverages regularly. The lesser component gases Methane and hydrogen are flammable, and so flatus containing adequate amounts of these can be ignited. However, 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 archaea capable of producing methane. Similar results are found in samples of gas obtained from within the rectum.
The gas released during a flatus event frequently has an unpleasant 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, through omnivores to carnivorous species, such as cats and dogs. Such odor can also be caused by the presence of large numbers of microflora bacteria and/or the presence of feces in the rectum.
The major components of the flatus (which are odorless) by percentage are:
Flatus is brought to the rectum by the same process which causes feces to descend from the large intestine, and may cause a similar feeling of urgency and discomfort. Nerve endings in the rectum usually enable individuals to distinguish between flatus and feces, although loose stool can confuse the individual, occasionally resulting in accidental defecation, or "sharting".
Flatulence-producing foods are typically high in certain polysaccharides (especially oligosaccharides such as inulin) and include beans, lentils, dairy products, onions, garlic, scallions, leeks, radishes, sweet potatoes, cashews, Jerusalem artichokes, oats, wheat, yeast in breads, and other vegetables. Cauliflower, Broccoli, cabbage and other cruciferous vegetables that belong to the genus Brassica are commonly reputed to not only increase flatulence, but to increase the pungency of the flatus. In beans, endogenous gases seem to arise from complex oligosaccharide (carbohydrates) that are particularly resistant to digestion by mammals, but which are readily digestible by microorganisms that inhabit the digestive tract. These oligosaccharides pass through the upper intestine largely unchanged, and when these reach the lower intestine, bacteria feed on them, producing copious amounts of flatus. 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.
A marked increase in flatulence has been noted in subjects who engage in anal sex.
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. In the field of mountaineering, High Altitude Flatus Expulsion was first noticed over two hundred years ago.
Probiotics (Live yogurt, kefir, etc.) are reputed to reduce flatulence when used to restore balance to the normal intestinal flora. Live Yogurt contains Lactobacillus acidophilus which may be useful in reducing flatulence). L. acidophilus may make the intestines more acidic, thus maintaining the natural balance of fermentation processes. L. acidophilus is available in supplements (non-dairy is reputedly best). Prebiotics, which generally are non-digestible oligosaccharides, such as fructooligosaccharide, generally increase flatulence in a similar way as described for lactose intolerance.
Medicinal activated charcoal tablets (brand name CharcoCaps) have also been reported as effective in reducing both odor and quantity of flatus when taken immediately before food that is likely to cause flatulence later.
While not affecting the production of the gases themselves, surfactants (agents which lower surface tension) can reduce the disagreeable sensations associated with flatulence, by aiding the dissolution of the gases into liquid and solid fecal matter.
Preparations containing simethicone reportedly operate by promoting the coalescence of smaller bubbles into larger ones more easily passed from the body, either by burping or flatulence. Such preparations do not decrease the total amount of gas generated in or passed from the colon, but make the bubbles larger and thereby allowing them to be passed more easily.
Often it may be helpful to ingest small quantities of acidic liquids with meals, such as lemon juice or vinegar, to stimulate the production of gastric hydrochloric acid. In turn, acid ingestion may increase normal gastric enzyme and acid production, facilitating normal digestion and perhaps limiting intestinal gas production. Ingestion of bromelain- or papain-containing supplements (such as raw pineapple or papaya, respectively,) may be helpful.
Odor from flatulence, caused by the intestinal bacteria called microflora in the bowel, can be treated by taking bismuth subgallate, available over-the-counter as Devrom. Bismuth subgallate is commonly used by individuals who have had ostomy surgery, bariatric surgery, fecal incontinence and irritable bowel syndrome.
A similar product was released in 2002, but rather than an entire undergarment, consumers are able to purchase an insert similar to a pantiliner that contains activated charcoal. The inventors, Myra and Brian Conant of Mililani, Hawaii, USA still claim on their website to have discovered the undergarment product in 2002 (8 years after Chester Weimer filed for a patent for his product), but states that their tests "concluded" that they should release an insert instead.
Flatulence is not poisonous; it is a natural component of various intestinal contents. However, discomfort may develop from the build-up of gas pressure. In theory, pathological distension of the bowel, leading to constipation, could result if a person holds in flatulence.
Not all flatus is released from the body via the anus. When the partial pressure of any gas component of the intestinal lumen is higher than its partial pressure in the blood, that component enters into the bloodstream of the intestinal wall by the process of diffusion. As the blood passes through the lungs this gas can diffuse back out of the blood and be exhaled. If a person holds in flatus during daytime, it will often be released during sleep when the body is relaxed. Some flatus can become trapped within the feces during its compaction and will exit the body, still contained within the fecal matter, during the process of defecation.
Flatulence is often blamed as a significant source of greenhouse gases owing to the erroneous belief that the methane released by livestock is in the flatus. While livestock account for around 20% of global methane emissions, 90-95% of that is released by exhaling or burping. Only 1–2% of global methane emissions come from livestock flatus.
Since New Zealand produces large amounts of agricultural produce it is in a unique position of having high methane emissions livestock compared to other greenhouse gas sources. The New Zealand government is a signatory to the Kyoto Protocol and therefore attempts are being made to reduce greenhouse emissions. To achieve this an Agricultural emissions research levy was proposed and it promptly became known as a "fart tax" or sometimes a "flatulence tax". It encountered opposition from farmers, farming lobby groups and opposition politicians.
In Fresno, California, a system to harvest methane by-product from dairy cattle and convert it to usable bio-gas is being used, in a partnership with Pacific Gas & Electric (PG&E) and BioEnergy Solutions, in which BioEnergy Solutions sells the methane harvested from cows to PG&E, who then converts the methane to usable bio-gas, which is very similar to natural gas.
While the act of passing flatus is generally considered to be an unfortunate occurrence in public settings, flatulence may, in casual circumstances, be used as either a humorous supplement to a joke ("pull my finger"), or as a comic activity in and of itself.