World's Smelliest Smell

World's Smelliest Smell


Methanethiol (aka methyl mercaptan) is a colourless gas with a distinctive and putrid smell.

While the organosulfur compound thioacetone is generally considered to have a more offensive smell (described as a mix of garlic, rotten cabbage and burnt rubber), the human nose can detect traces of methanethiol at concentrations as small as one part per billion, making it the strongest-smelling substance one can ever experience.

While thioacetone is difficult to make, the smell of methanethiol may already be familiar to you, because it forms a large part of the smell you may have noticed from your own urine after you’ve eaten fresh or cooked asparagus. The chemical components responsible for the change in the odour of urine show up as soon as 15 minutes after eating asparagus.

(As an aside, and by way of a fun fact inside this fun fact, French novelist Marcel Proust really, really liked the effect of eating asparagus, once exclaiming that it transformed his chamber pot into “a vase of aromatic perfume”.)

Not everybody notices a change in the smell of urine after eating asparagus and when this was first investigated, studies indicated that the digestive conversion of asparagusic acid into an expression of methanethiol after eating asparagus was a genetic trait.

However, more recent studies suggest there may also be a gene that determines whether you can smell methanethiol that has made its way into urine. It turns out that non-smellers have something called a “specific anosmia” whereby one of their 400 or so smell receptor genes is mutated and effectively turned off. This can also happen in relation to the smell of vanilla or mint … and, in our opinion, it would be more unfortunate to be denied those smells.

So, depending on who your parents were, you may or may not be familiar with a low concentration of the world’s smelliest substance. If you can smell it (and 40% of us can), scientists still aren’t entirely sure whether it’s because your genes cause you to excrete it, or your genes allow you to smell it.

Methanethiol is a byproduct of a number of industrial chemical processes and is used in the making of plastics, pesticides and animal nutrition. It has to be handled very carefully in industrial applications because it is highly flammable and highly toxic to humans at concentrations as low as 60mg/kg. There have been several fatal industrial accidents involving methanethiol; in some of which the victims were killed by fire and explosions, but some of these poor souls actually died of the stink. Not a good way to go.

Methanethiol is also used as an odorant [Ed: good word to remember] to make industrial methane less dangerous, added to methane to make it smellier and hence more likely that someone will notice a gas leak before it catches fire. A rare case of adding something more dangerous, in order to make something less dangerous, a lot safer.

Story Idea: Alan Jones (who conveniently also submitted some draft copy)




1. World's Smelliest Smell
2. Cooked asparagus
3. Model of the methanethiol molecule
4. Methanethiol
5. Photo of 
Marcel Proust by Otto Wegener, 1895
6. Video: The Truth About Asparagus and Your Pee by SciShow, 2015
7. Video: The Mystery Of Asparagus Pee by MinuteEarth, 2017

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