The first rain of the season falls for a few minutes and the smell is already up. Anyone who grew up where the rainy season is distinct recognises it, and almost nobody can name it until someone tells them.
That smell is called petrichor. The component most often mentioned in it is geosmin, a compound produced by bacteria living in the soil, alongside plant oils that accumulate in the soil and on rock. When rainwater lands, these substances are released into the air and rise with the water vapour.
Geosmin and the human nose
Geosmin is produced by certain soil bacteria, of which the most studied group is Streptomyces. The human nose is very sensitive to it, sensitive enough that a tiny amount in the air is already noticed. That is why the smell of earth after rain often arrives before the feeling of damp on your skin.
You meet the same compound elsewhere: a freshly washed root vegetable, a pond-raised fish, or a glass of water kept a long time in a concrete tank. People call it an earthy or muddy smell, and generally find it pleasant outdoors but unpleasant when it turns up in food.
Why the smell only rises when it rains
On dry days these substances sit still in the pores of the soil and in the dust on rock. A raindrop landing on dry ground pushes the air out of those pores, carrying tiny scent-bearing particles with it. Light rain after a long dry spell usually gives the clearest smell; very heavy, prolonged rain thins it out because the ground is already wet.
The term petrichor was proposed in a study published in the journal Nature in 1964. Since then it has been the most compact name for an experience that almost every culture has its own word for.
What this has to do with scent indoors
In our work with scent we have found that the damp-earth note is the hardest one to place correctly. It makes a forest scent feel real, but any more than necessary and the whole jar tips towards mud. This is an observation from the ZUNGZA workshop, not a standard ratio for every formula.
A few questions you often send us
Is the smell of earth after rain harmful
Smelling this in outdoor air is an ordinary thing and not a sign of danger. If an earthy smell appears in drinking water or in food, that is a different matter and the water or food source should be checked.
Why some people cannot smell it
Sensitivity to each scent compound differs from person to person, and some people barely register geosmin while the person next to them finds it very clear. This is normal, in the same way that the threshold for tasting bitterness is not the same for everyone.
Can this smell be recreated in a candle
You can suggest the feeling by combining earth, moss and damp-wood notes, but it is hard to call it recreation. The smell after rain is tied to humidity, to temperature and even to the sound of rain; without those, the fragrance can only do half the work.