Beyond perfumes and air fresheners there is a far stranger olfactory catalogue: smells with names of their own, flowers that smell of corpses and aromas nobody can buy. A tour of the most curious ones.
Petrichor: the smell of rain on dry earth
That smell that appears with the first drops after weeks of heat has a name: petrichor. It comes from a mixture of plant oils that build up in the soil and, above all, from geosmin, a compound produced by soil bacteria of the genus Streptomyces.
What is striking is the sensitivity: the human nose detects geosmin at extremely low concentrations, on the order of parts per trillion. It is the same compound that gives beetroot its earthy taste and that spoils a water or a wine when it turns up where it should not.
The smell of space
Astronauts who have carried out spacewalks describe a distinctive smell on their suits when they come back into the airlock: something between hot metal, ozone and welding fumes. The most widely cited explanation points to oxidation reactions when the equipment is brought back into an oxygen-rich environment. It is not a smell you can buy anywhere.
Bibliosmia: the smell of old books
Warm, with a vanilla-like, papery base. It comes from the chemical breakdown of paper and its components — lignin among them — which gradually releases compounds such as benzaldehyde, furfural and vanillin. Literally the smell of time passing over an object.
The flower that smells of carrion
The Amorphophallus titanum, or corpse flower, produces what is probably the worst smell in the plant kingdom: rotting flesh with hints of rancid cheese. It is no accident: it imitates the smell of a dead animal to attract the carrion insects that pollinate it. It flowers only very occasionally and for just a few days, and when it does, queues form at botanical gardens to smell it.
The smell of money
Banknotes have a recognisable smell that comes from the combination of inks, cotton and linen paper, and the trace of thousands of hands. You will find articles claiming that this smell makes us less generous, citing psychology experiments; it is worth taking with a pinch of salt, because much of that literature has had replication problems.
Impossible smells, bottled
There is a whole perfumery niche devoted to recreating unlikely smells: wet earth, mountain air, new car, freshly cut grass, vinyl. They work because the nose does not distinguish between "a pretty smell" and "a smell with a story": any smell attached to a vivid memory will do.
And what has this got to do with scenting a space?
More than it seems. All these examples point to the same thing: what makes a smell memorable is not that it is pleasant, but that it is recognisable and consistent. A scent repeated in the same place ends up meaning that place.
You can see the range in the fragrance catalogue, or read how all of this works from the inside in how the sense of smell works.
