Send Doris a message she cannot ignore: John Emsley sniffs out the secrets of androstenone, and the fragrant attractions on which we spend pounds 400m a year

Christmas is traditionally the time of tantalising aromas: roasting turkey, warm mince pies, smouldering Chanel No 5 and hot Jazz. These two were the best selling brands of perfume for women and men last Christmas. Among the 500 or so perfumes for women, Chanel, Anas Anas and Opium are the top three. Men's leading brands are Jazz, Kouros and Aramis, but they also have more than 300 to choose from.

Smell is the human sense we use least. It relies on our detecting individual molecules as we breathe, which we do with receptors on millions of fine hairs, called cilia, in the upper nasal cavity. Perfume molecules fit the receptors on the cilia and trigger pleasurable responses in our brain. To achieve this effect, the British spend pounds 400m a year, mostly at Christmas.

The profits on perfumes are high because the content of the product is more than 80 per cent alcohol, a chemical that industry makes very cheaply. Aftershaves and colognes may be 95 per cent alcohol and water.

Human skin produces its own smells, but we may find them offensive and some are more powerful than we imagine. When a 15-stone pig living in Gloucestershire took a fancy to a newspaper boy last month and cornered him in a telephone box, it was only responding to molecules he was giving off.

Little did he realise it, but he was sending Doris a message she could not ignore. The auxiliary glands in his armpits were releasing androstenone, which is a sex-attractant for the wild boar. In his book The Scented Ape, Michael Stoddart, of the University of Tasmania, discusses whether it could also be a human sex-attractant. Men give off much more of this than women and, curiously, the amount a man gives off reaches a maximum at this time of year. However, Professor Stoddart is doubtful of its potency because it has a weak smell, and in a concentrated form some women even find its aroma off-putting. Most of us use perfumes to make ourselves smell better.

Perfumers can be seen at work on the wall paintings of Egyptian tombs of 1,400 BC, but the perfume industry as we know it began in the Middle Ages at Grasse in Provence, southern France, where the climate and soil were ideal for growing plants from which the fragrant essences were extracted.

The traditional Grasse way of extracting the essential oils from flowers was to place them between layers of pork or beef fat, which absorbed the fragrant oil - a process known as enfleurage.

This method was superseded by steam distillation or solvent extraction, but nowadays chemical manufacture is more important. The synthesis of fragrance and flavour molecules began in the 1840s, when aromas such as cinnamon were first extracted and manufactured. However, it was the success of Chanel No 5, introduced in 1921, that gave the synthetic materials their big breakthrough. This was the first perfume to use both natural and man-made components.

Chemists can make exact copies of natural aroma molecules or modify them to produce other fragrances. Not only that, but synthetic fragrances are better than natural ones because they are chemically more stable.

'A considerable number of fragrance ingredients are man- made,' says Dr Charles Sell, head of organic chemistry at Quest International of Ashford, Kent, one of the world's leading fragrance manufacturers. 'The new products may have odours not found in nature, although they are often described by perfumers in terms of known scents.'

People put on scent to send a message. The warmth of our bodies causes the perfume to evaporate and its molecules register on sensors in the noses of those around. A perfume can communicate a message we may be afraid to put into words: at its simplest it says 'come closer', but it may be saying something much more suggestive.

There are hundreds of molecules in natural fragrances. Some are very volatile, the so-called top notes, and others less so, the middle and base notes. The deeper we get, the more subtle and suggestive is the message. A typical top-note will be instantly recognised as fresh and pleasant, such as citrus fruit or crushed leaves, while middle notes are more floral, including the heavy scents of jasmine, lilies and orchids.

The base notes hint of leather, resin, moss, earth, fragrant woods such as cedar or sandal, and even smells that we associate with intimacy. The deepest notes we may not even be aware of, but they are there and carry the most primitive messages of all. These chemicals hint of urine, blood, semen, and even excrement.

The perfume I buy my wife is Chloe, by the perfume house of Lagerfeld. The trade manual Fragrance Guide to Feminine Notes does not reveal the exact recipes of this perfume, which of course is a trade secret, but it does reveal the components. Chloe has a 'fruity green' top note of coconut, bergamot and peach, 'exotic floral' middle note of tuberose, jasmine and hyacinth, and 'feminine sensual' base note of musk, moss, sandal and cedar. All is revealed.

So where do the deepest notes of all come from? Not surprisingly they originate from the sex glands and excretory organs of animals. The chief ones are musk, civet and ambergris. Musk comes from the Himalayan musk deer, civet from the Ethiopian cat of that name, and ambergris is an intestinal secretion that is vomited up by whales and is found on beaches in lumps the size of footballs.

The most potent is musk. In its raw state this smells like a mixture of urine and animal dung; collectors used to plug their noses as they gathered it. Musk is a powerful sex-attractant for female deer, produced by the male in a pouch the size of a walnut at the base of the penis. It also contains androstenone. Humans find musk alluring when it is diluted and put into perfumes. Chemists have identified several components of musk. One, muscone, is now made in the laboratory and is the synthetic material used in modern perfumes.

The civet cat of North Africa and Ethiopia also secretes a fluid that humans find sensuous to smell. Both male and female civets excrete a white fluid into an anal sac, and they used to be kept in captivity just for this reason. The chief aroma constituent is civetone, which consists of 17 carbon atoms in a ring, to one of which an oxygen atom is attached. This substance has a floral and musky odour and has been used in perfumes for more than 2,000 years - Cleopatra was fond of it. Artificial civet is now made in the laboratory.

Perfumers have about 4,000 molecules to choose from when they design a perfume. The composition of a new scent is still more an art than a science, and is the work of a team referred to as les grands nez. It is these clever fragrance designers who make the modern Christmas morning such a celebration of the chemist's art.

Dr John Emsley is science writer in residence, department of chemistry, Imperial College, London.

(Photograph omitted)