Petrichor: where does the smell of rain come from?

Molecules from soil and plants contribute to the smell of rain. Experiments filmed at very high speed shed light on how raindrops set them circulating again.

After a shower, the air can take on the smell of damp earth, sometimes fresh, sometimes almost vegetal. This scent does not come from rainwater alone: it arises from molecules present in the soil and their release into the air. Petrichor is the name for this characteristic smell, but it does not correspond to a single substance — and the fact that it is pleasant does not show that it has any beneficial effect on health.

One smell, several components

The word “petrichor” was introduced in 1964 by Australian researchers Isabel Joy Bear and Richard G. Thomas. It combines the Greek words for stone and ichor, the mythical fluid of the gods. The term names a recognisable smell, not a specific molecule.

One of its components is geosmin, a compound produced by certain soil microorganisms, including bacteria of the genus “Streptomyces”. It contributes to the earthy note and can be detected by humans at very low concentrations. Other molecules, including compounds derived from plants, can also contribute to the mixture. The sensation therefore varies according to place and conditions: “the smell of rain” is not always exactly the same.

This distinction helps explain a common confusion. Geosmin is one possible ingredient of the smell, whereas petrichor refers to a combination of scents. Ozone, sometimes associated with the smell of a storm, is yet another phenomenon: it can contribute a sharper note, but it does not sum up the smell of wet earth.

How raindrops bring smells to the surface

Rain starting to fall does more than simply wet the ground. When a raindrop hits a porous surface, small air bubbles can form in the liquid. As they rise and then burst, they eject tiny particles into the air: aerosols, meaning droplets or particles fine enough to remain suspended. These can carry fragrant compounds from the ground to our noses.

A study published in 2015 by MIT researchers examined this mechanism using high-speed cameras. The team conducted about six hundred experiments on twenty-eight types of surface, including soils. They varied the speed of the droplets and the properties of the surface to observe the formation and then bursting of the bubbles.

In this experimental setup, light and moderate rain produced more aerosols than the fastest impacts associated with heavy rain. The smell therefore does not depend solely on the amount of water: the speed of the raindrop and the soil’s ability to become wet also change what is released. The researchers proposed this mechanism as a possible physical explanation for petrichor, not as a measurement of every smell from every shower.

Rain itself is therefore less a “fragrance” than a trigger and a carrier. It sets compounds that have accumulated or been produced at the surface circulating again, and the air then disperses them. The image of a scent hidden in every raindrop would be misleading: the phenomenon depends on the interaction between water, soil and the substances present.

Why can the smell seem comforting?

Recognising a smell and enjoying it are two different things. A smell can evoke a place, a season or a personal experience; these associations can contribute to how it is felt. But this does not allow us to infer a general psychological or medical benefit.

The pleasure of a smell can be real without constituting proof of a treatment. Laboratory findings on the transport of aerosols answer a question in physics; they do not measure an improvement in human health. This distinction allows us to take an interest in the phenomenon without turning a familiar sensation into a remedy.

A natural smell is no promise in a bottle

Candles, home fragrances and skincare products try to evoke damp earth. But reproducing one fragrant component is not the same as recreating the whole phenomenon: the mixture of soil, plants, moisture and air makes for an experience more complex than any single molecule.

It is also important to distinguish the smell of rain from products designed to copy it. A health claim about a fragrance or candle does not follow simply from the fact that some people enjoy the natural smell. Franceinfo’s report also recalls concerns about indoor air quality and fragranced products. The appeal of a scent is therefore not enough to establish that a product is harmless or has a beneficial effect.

Petrichor thus remains a scientifically interesting phenomenon because of its mechanism: compounds in the soil, including geosmin, are mobilised and carried through the air when raindrops hit certain surfaces. As for the comfort a person may find in it, that belongs to their own experience — not, on its own, to a medical conclusion.