Smell this.

Coffee, ham & eggs, baby’s skin, cherries blossom? What about the smell of rain?

Have you ever thought about how we detect smells? How the hell do we know and determine if it is a good smell or a bad smell? Let’s start from the begining.

To smell an odor, you need to inhale some air through your nose, of course.

Then, the inhaled air carries that particular odor molecules.

Once in the nose, the molecules then dissolve into a mucous membrane called the olfactory epithelium.

In the olfactory epithelium the molecules are able to spread out and bind to receptors on the tips of dendrites of olfactory neurons.

Once molecules bind, the olfactory neurons start firing signals to the olfactory bulb in the brain.

The olfactory area in the brain is closely connected to the amygdala and the hippocampus.

The amygdala is involved in emotion and the hippocampus is involved in memory. Therefore, the sense of smell is linked very closely with emotions and memories.

A smell can remind us of a certain memory or feeling and thus force us to connect the smell to something we perceive as positive or negative/pleasant or unpleasant. Moreover, some smells which all humans regard as bad may be due in part to evolution.

For example, the smell of rotten food and the smell of mildew may be unpleasant to us because they are warnings of danger.

Since the science of smell has just recently spiked scientists’ interest, the relationship between the molecular composition of an odor molecule and how we perceive the smell of the molecule is still largely unknown.

Now, let’s talk about PETRICHOR. Have you ever heard of it?

THE SMELL OF THE RAIN.

Coined by scientists Isabel Joy Bear and Richard Thomas in their 1964 article Nature of Argillaceous Odour, published in the journal Nature.

The word was coined from Greek petros, meaning “stone”, and ichor, meaning “the fluid that flows in the veins of the gods.

You might ask yourself — how and why rain even has a smell, if it’s only consisted of water molecules? We kinda know since ever that water has no smell?

Of course, rain itself is scent-less. But moments before any rain event, an “earthy” smell known as petrichor does permeate the air.

Most of us call it fresh, clean, slightly musky, but overall – pleasant.

That rain odor we are perceiving is not actually the rain itself, but what is released when a rain drop hits the surface of earth.

Petrichor’s main ingredients are made by plants and bacteria that live in the ground.

In short, petrichor is a combination of fragrant chemical compounds.

Some are from oils made by plants. The main contributor to petrichor are actinobacteria. These tiny microorganisms can be found in rural and urban areas as well as in marine environments. They decompose dead or decaying organic matter into simple chemical compounds which can then become nutrients for developing plants and other organisms.

A byproduct of their activity is an organic compound called GEOSIMIN which contributes to the petrichor scent.

Geosmin is a type of alcohol, somewhat like rubbing alcohol. Alcohol molecules tend to have a strong scent, but the complex chemical structure of geosmin makes it especially noticeable to people even at extremely low levels. Our noses can detect just a few parts of geosmin per trillion of air molecules.

Now, geosmin is even becoming quite common as a parfume ingredient.

It’s a really potent material and it smells like concrete when the rain hits it.

There’s something very primitive and very primal about the smell of geosimin.

Even when you dilute it down to the parts per billion range, humans can still detect it. Believe me.

Yet, we also have an odd relationship with geosmin — while we are very drawn to its scent, many of us dislike its taste.

Even though it is not toxic to humans, the tiniest amount can completely put people off mineral water or wine when just a dash of geosimin is present in them.

Back to petrichor.

During a prolonged period of dryness when it has not rained for several days, the decomposition activity rate of the actinobacteria slows down. Just before a rain event, the air becomes more humid and the ground begins to moisten. This process helps speeding up the activity of the actinobacteria and even more geosmin is formed.

When raindrops fall on the ground, especially porous surfaces such as loose soil or rough concrete, they will splatter and eject tiny particles called aerosols. The geosmin and other petrichor compounds that may be present on the ground or dissolved within the raindrop are released in aerosol form and carried by the wind to surrounding areas. If the rainfall is heavy enough, the petrichor scent can travel to moisten and alert people that rain is soon on the way.

The scent eventually goes away after the storm has passed and the ground begins to dry. This leaves the actinobacteria lying in on the ground for – wait for this – helping us know when it might rain again.

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