Like many of the phytoncides, it is a turpene, a family of aromatic
hydrocarbons and a major component of natural resin (incidentally,
turpenes are also a big ingredient of hops, giving dark beer its rich
flavor and aroma).
Geosmin comes from soil organisms, particularly the
streptomyces bacteria that are key to so many antibiotics. According
to the Royal Society of Chemistry, we are alert to this rich smell in
incredibly small quantities. We can detect the equivalent of seven
drops of geosmin in a swimming pool. This sensitivity likely reflects
an important evolutionary adaptation because it tipped our thirsty
ancestors off to sources of water. That may also explain why its
presence helps put us at ease. Camels probably get off on it even more
than we do. Keith Chater, the Norwich scientist who sequenced the
genome of Streptomyces coelicolor in 2007, believes camels can
smell geosmin in oases miles away. In return for their helpful homing
service, some spores of the bacterium then hitch a camel ride to the
next watering hole. Geosmin is the smell of survival.
It’s no surprise by now that Korea and Japan lead the world in the
science of forest smells. There’s the Natural Killer cell work of
Japan’s Qing Li, and also that of a young psychologist there named
Yuko Tsunetsugo. A senior researcher with the Department of Wood
Engineering at the Forestry and Forest Products Research Institute,
Tsunetsugo misted fifty-two infants with the major components of
hinoki: pinene and limonene. The pinene instantly lowered their heart
rates four points, while the limonene and the control did not make a
difference.
When I’d been in Japan at the Nippon Medical School lab of Qinq
Li (the man who put subjects in hotel rooms for three nights with
hinoki oil misting around them), he’d given me a demonstration of
the immediate effects of the stuff. I’d put my arm in a blood-pressure
cuff. Then he unscrewed the cap off the forest elixir. “This is very
toxic!” he’d giggled. “It’s very good but very toxic.” When I inhaled,