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Why do farts smell so bad? – Daisy, 7, New York, New York
A healthy person farts about 14 to 20 times a day. All told, that’s enough gas to fill a super stinky party balloon.
Farts are made up of gases produced by billions of small organisms called microbes, mainly bacteria, that live in your gut. These organisms have an important job: they help chew foods that your stomach can’t break down on its own. In doing so, they act like mini chemical factories that provide you with nutrients while releasing waste gases into your gut. These gases mix with the air you swallow and other gases you produce during digestion. After passing through the intestines, they come out in bursts in the form of farts.
As a microbiologist, I study communities of bacteria, called microbiomes, in various regions of the human body. Your gut microbiome plays an important role in your health. Gas production is a sign that the bacteria in the gut microbiome are doing their job properly. Farting also relieves pressure in your belly and prevents bloating and painful cramps.
So you might ask: why does a normal, healthy bodily process cause such a bad stench?
What’s in a fart?
Farts are primarily made up of the same odorless gases found in the air we breathe.
Between 20 and 90 percent of a fart’s volume is nitrogen, which makes up more than three-quarters of air. Up to 10% of a fart is oxygen, which humans need to breathe. Up to 30% is carbon dioxide, up to 50% is hydrogen and up to 10% is methane. Microbes produce most of the carbon dioxide, hydrogen, and methane, although some carbon dioxide is produced when digestive juices mix with stomach acid.
Sandra Fauconnier/Flickr via Wikimedia Commons, CC BY
But wait: These aren’t the gases that give farts their delicious array of aromas. What makes farts so smelly are other gas molecules that contain the element sulfur. Pure sulfur has no odor on its own, but when combined with other elements it produces stinky molecules.
One of the main culprits is hydrogen sulfide, notorious for its rotten egg smell. Others include methanethiol, which has a cabbage, sewage, or skunk bouquet, and dimethyl sulfide, which hits the nostrils with scents of cooked corn, algae, or seafood.
Only small amounts are needed: one molecule of hydrogen sulfide among a million other gas molecules has enough power to do the job. That’s the equivalent of one kernel of corn in a truckload of about 1,250 ears, or 10 bricks out of all the bricks that make up the Empire State Building in New York. Other sulfur-containing gases can be detected in even smaller quantities.
The source of foul-smelling gas in your gut
The main thing that determines what gases and how much of them are in your farts is the food your gut microbiome can eat.
Certain types of foods often referred to as junk foods, including candy or diet foods containing artificial sweeteners or syrups, can change the way microbes digest food. High-fiber foods like beans and grains also have this effect. These foods can cause your body to produce more farts, often bigger and louder.
However, the loudest fart isn’t always the stinkiest. It’s those that contain sulfur that have a foul odor.
Food is a key source of amino acids, nutrients that build the proteins the body needs to function and grow. Two amino acids, methionine and cysteine, contain sulfur and are abundant in meat, dairy products, eggs and, to a lesser extent, beans and grains. Some foods—including vegetables like cabbage, broccoli, Brussels sprouts, and cauliflower—contain other molecules that also contain sulfur.
When you eat foods high in sulfur, your gut microbes digest them to produce sulfur-containing gases. This can lead to putrid farts, with a specific odor tailored to your diet – as well as your overall health. In fact, persistent foul-smelling gas may be a sign of an illness, such as irritable bowel syndrome or a food intolerance or sensitivity, for which you may need to see a doctor.
Farts in nature
As you may know from experience, it’s impossible to hold in a fart until it goes away. It will come out one way or another — sometimes at speeds of up to 10 feet per second, according to back-of-the-envelope estimates, including my own. This influx of gas leaving the body causes surrounding tissues to vibrate, often causing an annoying noise.
Farting has been the butt of jokes for millennia. According to the Guinness Book of World Records, the oldest joke ever written was a popular proverb dating back around 3,900 years that warned new husbands against their wives’ farts. The popularity of fart jokes remains strong. In the 2010 film “Despicable Me,” for example, crazy tech wizard Dr. Nefario mishears his boss’s request for a dart gun, instead creating a fart gun, or Fart Blaster, which is now a popular real-life toy.
But sometimes this natural process of the body can be really disappointing, like on an airplane. Cabin pressure in flight is often lower than normal atmospheric pressure on the ground, which can cause gases to expand in your gut, causing more farts. To protect the noses of travelers stuck together in friendly skies, researchers designed seat cushions and blankets containing sulfur-absorbing substances, such as activated carbon. So far, no airline has adopted these funk-reducing technologies, but you can purchase them yourself.
Despite farts’ assault on the senses, your ability to detect them means your nose does exactly what evolution designed it to do: act as a chemical sensor for sulfur-containing gases. Humans can detect hydrogen sulfide and methanethiol at minute concentrations, even when there is less than one molecule of these compounds among billions of other molecules.
These smells can signal the presence of decaying substances and bacterial activity that can harbor pathogens that can make you sick, or deadly environments like swamps, stagnant caves, and volcano vents that can suffocate and kill you.
So the next time you let out a fart pungent enough to clear the room, don’t apologize – just say you’re performing a live demonstration of natural chemistry that took eons of evolution to perfect.
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Gn Health
