This week, I'm starting OSMOFOLIA's first scent study. The very first will tackle a beloved and fascinating scent that can be found all around the world: the scent of rain!
You may have heard a name for the smell of rain: petrichor.
More specifically, petrichor is the earthy scent left behind when raindrops fall on dry soil.
Whether in real life or in perfume, there can be three distinct notes in the smell of rain: geosmin, ozone, and volatile plant oils.
This week, I'll talk a little about geosmin, as well as take a look at ozone and the volatile plant oils that contribute to the scent of rain. Lastly, we'll take a look at some of the materials that perfumers could use to recreate the scent of rain, and finally, examine a rain accord!
So where does petrichor, the scent of rain, come from?
When rain droplets fall on the ground, small bubbles form and pop, releasing aerosols containing geosmin into the air.
Slower raindrops produce more aerosols, which means a light rain is easier to smell than a downpour.
This is my favorite fact about geosmin: humans can smell geosmin in concentrations as low as 5 parts per trillion. Yes, trillion! That’s equivalent to one drop of water in an Olympic-sized swimming pool.
To help illustrate just how well we’re able to smell the rain, compare sharks and humans. Sharks are so sensitive to the smell of blood that they can smell a liter of blood from a quarter of a mile away, or the distance of 2–3 football fields. Human noses are 200,000 times more sensitive to rain than sharks’ noses are to blood.
Some scientists have theorized that early humans evolved the ability to smell rain because it helped them find fresh water. (That would also explain why some species of camels are able to track the scent of geosmin from up to 50 miles away.)
Geosmin can also be found in beet peels, and it is responsible for some of the “off” smells in wine, drinking water, and fish.
[Interesting tangent: acid neutralizes the taste of geosmin, which is why lemon juice/citrus improves the taste of both beets and fish! 🍋]
But geosmin isn’t wanted in our food or water—it has an incredibly low flavor threshold of 0.00001 mg/L, which is lower than chemicals like chlorine and quinine (which are not safe for us to ingest).
Although this level of geosmin is safe for us to consume, some people experience headaches and nausea when they taste it or smell too much of it. There may be an evolutionary reason for this! If drinking water smells very strongly of geosmin, that may indicate an overgrowth of algae or bacteria in the water that would make it unsafe for us to drink.
Some cities monitor their drinking water for geosmin levels in order to properly treat the water and prevent sickness—not from the geosmin itself but from the bacteria that causes it.
But geosmin isn't just a nasty byproduct of bacterial growth. In fact, the bacteria that produce it may get an evolutionary benefit from geosmin’s scent.
The overwhelming smell of geosmin can repel creatures that would eat whatever the bacteria are growing on, like fruit flies. However, it can attract other small animals that can carry Streptomyces spores on their bodies, which helps the bacteria spread and reproduce.
Geosmin is found all over the world, so these bacteria have obviously done their job well!
However, geosmin isn’t the only chemical responsible for the smell of rain.
Ozone, or O3, is also part of petrichor’s distinctive scent.
While geosmin comes during and after rain, ozone tells our noses that rain is coming!
Depending on who you ask, ozone smells clean, metallic, pungent, electrical, or burnt. Smelling it gives the inside of your nose a little zing, almost like smelling a texture.
This form of oxygen gets its name from the Greek root “ozein”, which means to smell! Normally we don’t smell much ozone on earth. But when a rainstorm is starting, ozone may get carried down to us.
As thunderclouds build up, electrical charges in the air split nitrogen and oxygen molecules. Some of those oxygen molecules combine in groups of 3, making ozone, and the downdrafts of wind during a storm or a lightning strike carry the ozone down to nose level.
Humans can smell ozone at 10 parts per billion, which is like 3 teaspoons of water in an olympic sized swimming pool!
But when rain is coming, we’re only smelling traces of it—just like geosmin, our noses are sensitive to it most likely because the smell of ozone helps to warn us of danger.
In its pure form, ozone is deadly. Perfumers use chemicals reminiscent of ozone to help create its smell without its dangers, so that we can have the scent of rain.
But if geosmin and ozone make the smell of rain, why does rain smell different in different places?
That's because geosmin and ozone are just the foundation for the smell of rain—the actual scent picked up by your nose is influenced by what the rain touches on its way down to the ground!
VOLATILE PLANT OILS
The scent of geosmin is released into the air when rain drops bounce off the ground. When rain drops land on something else first—say, the leaves of a tree high above the soil—they pick up tiny amounts of volatile oils.
Essentially, on their way down, rain drops pick up traces of scent from other scented things.
In fact, this part of petrichor was discovered before the smell of rain even had its name! In 1891, Scientific American published an article by Thomas Lambe Phipson, who explained that “…the odour emitted… after a heavy shower of rain in the summer was due to the presence of organic substances closely related to the essential oils of plants…”
Even though rain picks up just traces of these oils, their impact on the scent of rain is huge.
So when you're smelling rain, you're not only smelling an ancient chemical process unique to Earth, you're also smelling the story of the specific environment around you. Next time you catch a whiff of rain on the horizon, celebrate this greeting from Earth to nose!
RAIN ACCORDS IN PERFUMERY
Perfumers can use all kinds of materials to make an accord for rain or petrichor. This is by no means an exhaustive list—perfumers are a creative bunch! But these are the materials that I would start with:
🌎 for earth after rain: geosmin, terrasol, hivernal neo, cascalone
🌷 for spring rain: precyclemone, hedione, geosmin, ozofleur, triplal
🌼 for summer rain: geosmin, hedione, floralozone, patchouli, cis-3-hexenyl acetate
⛈ for a torrential downpour: adoxal, geosmin, helional, cascalone, patchouli
🌧 for a light misty rain: isoamyl salicylate, helional, geosmin, hedione, veramoss, dimetol, cis-3-hexenol
Of course, any material could add an interesting twist to a rain accord. You don't even have to use geosmin! The best part of scent creation is the experimentation.
But if you're having trouble visualizing where to start, I've got a formula to share.
Without further ado... Just kidding, there's further ado. Listen, most perfumers would choke at the thought of sharing a complete formula. And to be honest, perfumery is already incredibly difficult, but this kind of secrecy just makes it lonelier, more expensive, and harder to learn.
I'll save my rant about that for another day, but in my opinion, sharing formulas is a huge step toward the democratization of scent and the decolonization of perfumery & fragrance education.
So as part of my scent studies, I'm going to share a formula or two!
Today's accord formula was not a demo formula designed for this post. It's an accord I actually use in two of the fragrances that I make & sell. It's an accord, so it's not a finished perfume on its own, but it's a great starting point to learn how these materials interact with each other.
The formula is measured in parts that add up to 65 (not 100, sorry, I know, I know). I recommend against working in drops—use a scale! If you want to smell my thought process, add each ingredient in the order that they're written. Always check IFRA safety regulations & guidelines before blending, and follow proper lab safety protocols.