At the limits of the sense of smell

We have all experienced at some time, shortly after the first raindrops fall, that characteristic and evocative smell known technically as petrichor; a combination of words from Greek mythology, ‘petri’ (rock) and ‘ichor’ (the ethereal blood of the gods). Petrichor is often described as the smell of wet stone, but the odour does not come from the stone minerals themselves. Layers of volatiles can accumulate on the stone surfaces, for example those emitted by microbes and plants, and are modified by the atmosphere. These volatiles are often not well perceivable, unless we put our noses up close. Raindrops can drive them out of their stone surfaces and make them better noticeable. One of the organic molecules enhanced by rain, whose smell we colloquially describe as ‘wet earth’, is the so called 4,8a-dimethyl-decahydronaphthalene4a-ol. This odorant also known as geosmin comes from the Greek, meaning ‘aroma of the earth’ because of its smell. It is a hydrocarbon belonging to the terpene family produced by some soil bacteria and fungi, mainly belonging to the genera Streptomyces and Penicillium which, among many other chemical compounds, also synthesise antibiotics as fundamental to mankind as streptomycin and penicillin.

The ability of our olfactory system to perceive geosmin is surprising. Some studies indicate that we can detect it when it is present at a concentration of 0.0000065 parts per million, in terms of volume of air (ppm v/v). To give an idea, this is equivalent to diluting the contents of a teaspoon of coffee in about 300 Olympic-size swimming pools. Some researchers have suggested that this heightened sensitivity and fascination with its scent is the result of our evolution, when our ancestors lived on the African savannah and the smell of geosmin served to stimulate the search for water.

This olfactory acuity is far superior to the ability to detect geosmin by analytical techniques in the laboratory, and makes its unwanted presence problematic. In drinking water, it confers an unpleasant musty odour, and can be responsible for some undesirable aromas in wine when grapes have been attacked by certain fungi. But the ‘earthy’ note provided by geosmin has also been used in many perfumes, such as Me Paraissait Une Ombre (Etat Libre d’Orange), La Vamp (Bouge), or Labaie 19 (Le Labo).