There’s nothing quite like hearing a fart, and knowing the painful inevitability of it
crossing your nose’s path. But, if you could react quickly enough, do you even stand a
chance to escape it’s smell? Could you outrun a fart?
If you’re trying to outrun the sound...good luck! Sounds are vibrations that travel through
particles in a wave, and their speed is dependent on the medium they’re travelling in. For
example, sound travels 4x faster in water than air, because the particles are much closer
to together, meaning they more quickly transmit vibration energy. When travelling through
air at sea level and 15℃, sound travels at 340 m/s - way faster than the fastest man
on earth, Usain Bolt, who can only move 12.4 m/s. Not even the fastest animal, the peregrine
falcon which can travel 82 m/s - could soar past the sound of it’s own toot.
But it’s the smell that most are concerned about escaping anyway, and here’s where
it gets a bit more complicated. Unlike sound waves and visible light which travel through
the air, smell is made up of the odour compounds that create the air. When you cut the cheese,
odor molecules travel through the air and up your nose where they bind to odorant receptors.
And each individual has a unique set of receptors, meaning we all perceive smell differently.
At the basic level, smells travel through the process of diffusion whereby particles
move from area of high concentration to low concentration, until reaching a state of equilibrium.
But the composition of every fart is unique, based on the food you ate, your gut bacteria
breaking it down, and the swallowed air you took in. Yup - farts are like snowflakes;
each with it’s own variance. But the average fart tends to be composed of 59% nitrogen,
21% hydrogen, 9% carbon dioxide, 7% methane, 3% oxygen, and only around 1% is made up of
the smelly stuff. Even knowing this, we would still need to know the concentration at the
source, the temperature, air pressure, and wind movement in the room - which even in
a very still room is significant. Anecdotal evidence on the internet suggests that farts
are ejected at about 3 m/s, but there is little scientific evidence to back this up.
However, if we take into account the temperature of the gas, pressure and kinetic energy of
molecules, then the kinetic theory of gasses can help predict the average speed of a gas
molecule. But since a fart is made up of many molecules, let’s just focus on one of the
smellier parts: skatole. If we use the kinetic theory of gasses, assume that the fart is
at body temperature, and then we plug in the molar mass of skatole, we calculate that the
smelly skatole may be travelling around 243 m/s! Which is obviously faster than a human
can move.
Of course, these molecules aren’t travelling in an uninterrupted line but are constantly
colliding into other air particles and physical matter. As a result the gas cloud as a whole
travels much slower still guided by diffusion. And it doesn’t take into account any air
movement like wind or the ejection direction, which could either work for or against you.
So, while you certainly can’t run faster than the absolute speed of smelly fart particles,
with a little luck, you might be able avoid the fart itself!
Want more fart science? Check out our videos “Are Silent Farts More Deadly?” and “Why
Do We Like Our Own Farts?” By clicking the screen or using the link in the description.
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