Smelly chemicals can be quite a mouthful

Putrescine and cadaverine, the two most frighteningly named of all chemicals, lurk in our mouths all day, every day. This simple fact emerged in 2003 when Professor Michael Cooke, of the centre for chemical sciences, Royal Holloway, University of London, and two colleagues published a delectable horror story of a study called Time Profile of Putrescine, Cadaverine, Indole and Skatole in Human Saliva. It appeared in the Archives of Oral Biology. That journal - surprisingly, given its content - is a regular haunt of only a tiny fraction of the world's horror-fiction enthusiasts.

Cooke and his companions imply that other chemists had grown discouraged at the prospect of doing a time profile of putrescine, cadaverine, indole and skatole in human saliva. The odour of saliva is intensely bland, compared to that of its most appallingly stenched components, and in a certain chemical sense, stable. An American group, they say, "reported their inability to increase the odour of saliva". But Cooke and his team gave it a go, and succeeded.

In isolation, putrescine and cadaverine are anything but bland. They smell even worse than their names suggest. The one was so named because it evokes, and is involved in, the putrefaction of flesh. The other's name suggests, deliberately and accurately, the stench of rotting corpses.

Together with their mundane-sounding (yet also pungent) companions indole and skatole, putrescine and cadaverine are formed by "bacterial putrefaction of saliva in the oral cavity", explains the study.

Here's how Cooke and his companions monitored their presence.

Twelve dentally healthy volunteers, three women and nine men, supplied the spit. The scientists took pains to collect and handle it properly. They explain that "the unstimulated saliva was expectorated into a glass vial coated with 5 milligrams of sodium fluoride to inhibit further [chemical reactions]".

The results appear in a simple graph. Its lines and data points tell an engrossing tale: the "mean concentration of cadaverine, putrescine and indole in saliva throughout the day".

Cadaverine, putrescine and indole levels, having built up overnight, are high when we awaken. But "they are rapidly reduced by the combined action of eating breakfast and oral cleaning". Then until around 9am the levels remain steady.

The rest of the day has its ups and downs. Cadaverine, putrescine and indole levels rise until mid-morning, then slowly decline. Lunchtime brings dramatic change - the amounts are "reduced by the mechanical action of chewing involved in the ingestion of lunch". After lunch, the levels rise pretty steadily until it's time to knock off work.

The researchers stopped collecting data every day at 5pm. What happens between then and dawn remains a mystery.

Marc Abrahams is editor of the bimonthly Annals of Improbable Research and organiser of the Ig Nobel prize. The Ig Nobel show is touring the UK until Thursday. For more information, go to