Sulfur gases from our _____ have a most unpleasant odor. But why?

Fill in the blank here: 

"Beans, beans the magical fruit, 

the more you eat, the more you ____."

Do you remember that one? It was pretty common for my cousins and I when we were children. I recall at one point thinking of the "toot" like it was a tug boat's whistle in a harbor.

Let's try this one:

"Beans, beans, they're good for your heart,

The more you eat, the more you _____."

Okay, that one's pretty direct. Fart, toot, rip gas, bust ass, cut the cheese, break one lose, let one rip, send a bouquet, blast your chair, trouser cough: we have lots of ways to describe flatulence. One that I remember from my early adolescence was "doorknob": if you farted and someone said the word "doorknob" before you could say "safety" then you had to run as fast as you could to find and then touch a doorknob while your friends would do their best to pummel you. I know, I know... the stupid things that young boys do.

What is it about flatulence that seems so darned funny in some situations and absolutely disgusting in others? For that matter, why do farts smell bad?

The answer to the latter question is pretty simple: sulfur. 

Well, that might be a bit misleading. It's been thought for some time that some nitrogen-containing organic compounds contributed some of the odor to farts (most of these would smell something like the naphthalene in mothballs), however most recent research on the odors of flatulence points to volatile sulfur compounds (VSCs) as being the primary cause of "the big stink". Articles such as those of Suarez and colleagues (1998) and Tangerman (2009) point to hydrogen sulfide, methanethiol, and dimethyl sulfides as the primary agents of those smelly fart odors.

You may already know that sulfur is my favorite chemical element (and not just because my research involves sulfur). So let's talk about why sulfur makes our "booty belches" stink.

German peasants farting at Pope Paul III from
Martin Luther's "Depictions of the Papacy" (1545)

A fart, by any other name...

The majority of the gases that make up flatulence have no odor whatsoever. The portion of our flatulence that smells, the VSCs, make up less than 1% of the gases that come out. 

The other 99% or more of a fart is composed of gases like nitrogen, carbon dioxide, hydrogen, oxygen, and methane (many people think that methane has a "gassy" odor, but that's actually not true; pure methane has no odor). This image below, from a post on the Chronicle Books Blog, shows the primary composition of flatulence:

Image of primary fart compositions taken from Chronicle Books Blog

The gases in our "thunder from down under" come from three primary sources: atmospheric gases, undigested food material, and microbial metabolisms. The lattermost are the most important in making our noxious stenches. 

Life on Earth is utterly dominated by microorganisms. We might have developed civilization and science, but microbes still beat us in numbers by far. Indeed, the number of microbial organisms living on and inside of us outnumber the cells of our own bodies by anything from 10 to 1 up to 100 to 1. A lot of the processes that occur on our skin and in our guts are driven by the microbes who live with us. There can be over 1000 different microbial species living in our guts. These organisms help us digest our food: many of them ferment undigested carbohydrates to produce molecules that we can then use. Many of the dominant gases of our flatulence come from the processes of fermentation by our gut flora, especially hydrogen, a byproduct of fermentation. 

The action of sulfur-metabolizing microbes (such as sulfate reducers) is what causes the production of the malodorous sulfur-containing gases in our guts. In many cases, these microbes will pair the oxidation of hydrogen gas with the reduction of sulfur (from sulfate and from sulfur-containing amino acids), which is why we call them hydrogenotrophs. By consuming the hydrogen byproduct of fermentation, these sulfur reducing microbes form a symbiotic relationship with our other gut microorganisms. Thanks to that symbiosis and the processes involved, our gut microbes and our bodies benefit while our friends who have to smell the stench from our trouser trumpets are not so lucky. 

Here's a GIF showing the number of microbial species that can be found in or on various parts of our bodies:

So why do sulfur gases smell so bad?

I've been working on a project studying sulfur for so long that I've become accustomed to the smell of hydrogen sulfide. We usually say that hydrogen sulfide smells like rotten eggs, but that's not quite fair: the smell of rotten eggs includes hydrogen sulfide but also has other sulfur gases that make for a more repulsive odor. In fact, the smell of rotten eggs is much closer to the smell of farts than the smell of pure hydrogen sulfide gas. Still, there's not really another good way to describe the smell of hydrogen sulfide to most people. That's probably due to the fact that when we smell sulfide gases, we tend to do our best to get away from the odor. But why?

Season 2, episode 12, of Neil deGrasse Tyson's podcast Star Talk Radio was called "Appetite for Destruction" and featured an interview with Peter Ward, a paleobiologist and astrobiologist at the University of Washington (Peter Ward's book "Life as We Do Not Know It" rekindled my personal interest in astrobiology during my early college years). Tyson and Ward spoke about Ward's then recently published idea, The Medea Hypothesis, which is his consideration of the general relationship between life and the planet. There's a point in the episode where Tyson and Ward discuss hydrogen sulfide as it relates to global climate change and mass extinctions. That's when Peter Ward says this (roughly transcribed):

"Flatulence is essentially hydrogen sulfide... a very very deadly, deadly gas. It smells bad because it is so deadly to us. We are detecting the tiniest, tiniest [amount]; very few molecules floating around [and] you know it. ...We've come from a very long history of exposure to this stuff in the past mass extinctions. We've evolved to run [from the smell of this gas]. We know just how dangerous this is; our lineage of mammals goes back 300 million years [and] we have repeatedly been gassed by this stuff. We are the survivors of a number of hydrogen sulfide gas attacks. 200 parts per million, 10 times what you can smell, will kill you; and that was the level that appears to have happened at least 12 times in the past 500 million years due to global warming." 

So did we evolve to hate the smell of hydrogen sulfide to keep ourselves safe?

We can smell hydrogen sulfide at very low concentrations. According to OSHA, something in the range of 10 parts per trillion (ppt) to 1.5 parts per billion (ppb) is the beginning of human reception of hydrogen sulfide in the air we breathe (even better than the number that Ward threw out off the top of his head). Somewhere in the range from 2 to 20 ppm is when we start feeling the ill effects of breathing too much hydrogen sulfide, while the hundreds of ppm or more region of hydrogen sulfide exposure can be fatal. 

We know that hydrogen sulfide has had an important role in the evolution of life and the biosphere and may even have been involved in the earliest origins of life on Earth. Sulfur is one of the most important elements for life, but sulfide, it's reduced form, can be quite dangerous for those of us who breath oxygen. The end-Permian mass extinction event, believed to be the greatest mass extinction event in history, occurred about 252 million years ago and may have been highly influenced by the buildup of hydrogen sulfide in the oceans. It's even been suggested that the evolution of multicellular life on Earth, during the Avalon and Cambrian Radiations, was held back for over 1.3 billion years due to hydrogen sulfide concentrations in the middle to lower zones of the oceans. Yet that doesn't mean that we've necessarily evolved to avoid hydrogen sulfide.

Many organisms that would otherwise want to avoid hydrogen sulfide have evolved various mechanisms for dealing with too much of it. I haven't found any literature that can provide direct evidence to support Ward's claim (let me know if you can!), yet it makes a lot of sense as a speculation. Hydrogen sulfide is dangerous to us, so it makes sense that we would have evolved to avoid areas where it's abundant, or even to avoid the gases that are produced from our own bodies since they contain hydrogen sulfide and other reduced sulfur gases. Until I have some more evidence in hand (I'll be reading Ward's book on the Medea Hypothesis soon) I can't say for sure that we evolved to avoid hydrogen sulfide, but that sure sounds like a reasonable suggestion.

Even though the VSCs that make our farts stink only make up <1% of the total gas that comes from our hind ends, that's still a lot more than the range necessary for us to smell it (1%, or 1 part per hundred, is also 10,000 ppm). I don't know if evolution guided us to detest the smell of hydrogen sulfide due to its danger to us, but I can say for sure that the reason our "stink bombs" are so pungent is due to the presence of reduced sulfur gases.

I suppose that leaves one more interesting question about our farts: why are most of us okay (or at least not disgusted) with the smells from our own farts but we always find the flatulence of others to be revolting? 

Well, our detestation to the aroma of the farts of others might not just be due to the sulfur gases contained within but might also be about avoiding potential dangers related to many things that smell bad to us. Here's a video from AsapSCIENCE that considers just this:

You can consider this video a bit further by checking out the review from ThinkTank on Youtube.


Fart Science

If you found this post interesting, then you might want to check out this video by Vsauce on Youtube where he considers some more fart science: