Science Matters: Stinky science and a wind of foul flatulence

“Hey mum - I farted!”

If, like me, you’re a parent of a small child, this is probably something you hear on a regular basis. Because kids are awesome…

No really, kids are great. They’re curious. They love to explore. They love to ask questions, they love to experiment, and they want to know how everything works. 

In short, kids are fantastic little scientists. 

This column is inspired by my own cute, yet slightly stinky, little scientist… and he wants to know what farts are, and why they smell. So if you’re a bit squeamish, you might want to stop reading now.

A fart (or flatus, if you prefer the scientific term!) is excess intestinal gas escaping from the body. Throughout the day we swallow a bunch of air while we’re eating and talking.

At the same time the microbes that live in our intestines are producing gas while they ferment some of our partially digested food. Unless you want to end up bloated and with a belly-ache, all of this gas needs to come out somewhere.

Although most of us probably won’t admit it, the average person farts about 15 times a day. As well as counting farts, some scientists have actually spent their time figuring out how big farts are, and have come to the conclusion that the volume of a fart varies from about 17 mls (about the volume of a tablespoon) up to 375 mls (the volume of a can of coke).

So farting is perfectly natural – we all do it. But why do some of us fart more than others? And why are some farts odourless while others can quickly empty a room? 

It mostly comes down to what we eat, and the types of microbes that live in our digestive systems.

Scientists have studied the composition of farts (yes, okay, sometimes scientists are weird) and determined that they’re made mostly of odourless gasses.

A typical non-smelly fart will be made up mostly of nitrogen, hydrogen, oxygen, carbon dioxide and some methane.

Some foods will make us produce more of these gasses. We’ve all heard the saying about beans making you toot. It’s because beans (and cabbage, asparagus and whole grains) contain raffinose, a type of sugar which we can’t digest.

The bacteria that live in our colon can digest it, and when they do they produce a bunch of gas, including hydrogen and methane. And a lot of gas equals a lot of farts. 

While most farts are odourless, really smelly farts happen when some extra gasses are added to the mix. The rotten egg smell of some farts is a result of the bacteria in our gut producing sulphur-containing compounds, like hydrogen sulphide.

If you eat foods that contain a lot of sulphur, like broccoli, cabbage, dairy products and even red meat, then you’re likely to produce more of these gases, and have stinkier farts.

Maybe farts aren’t the most polite topic for dinner table conversation, but it turns out there’s actually some really interesting science behind all the smell.