For the modern, middle-class North American, “clean” means that you shower and apply deodorant each and every day without fail. For the aristocratic 17th-century Frenchman, it meant that he changed his linen shirt daily and dabbled his hands in water, but never touched the rest of his body with water or soap. For the Roman in the first century, it involved two or more hours of splashing, soaking and steaming the body in water of various temperatures, raking off sweat and oil with a metal scraper, and giving himself a final oiling - all done daily, in company and without soap.

Even more than in the eye or the nose, cleanliness exists in the mind of the beholder. Every culture defines it for itself, choosing what it sees as the perfect point between squalid and over-fastidious.

It follows that hygiene has always been a convenient stick with which to beat other peoples, who never seem to get it right. The outsiders usually err on the side of dirtiness. The ancient Egyptians thought that sitting a dusty body in still water, as the Greeks did, was a foul idea. Late 19th-century Americans were scandalised by the dirtiness of Europeans; the Nazis promoted the idea of Jewish uncleanliness. At least since the Middle Ages, European travellers have enjoyed nominating the continent's grubbiest country - the laurels usually went to France or Spain. Sometimes the other is, suspiciously, too clean, which is how the Muslims, who scoured their bodies and washed their genitals, struck Europeans for centuries. The Muslims returned the compliment, regarding Europeans as downright filthy.

Most modern people have a sense that not much washing was done until the 20th century, and the question I was asked most often while writing this book always came with a look of barely contained disgust: “But didn't they smell?” As St Bernard said, where all stink, no one smells. The scent of one another's bodies was the ocean our ancestors swam in, and they were used to the everyday odour of dried sweat. It was part of their world, along with the smells of cooking, roses, garbage, pine forests and manure. Twenty years ago, aircraft, restaurants, hotel rooms and most other public indoor spaces were thick with cigarette smoke. Most of us never noticed it. Now that these places are usually smoke-free, we shrink back affronted when we enter a room where someone has been smoking. The nose is adaptable, and teachable.

To modern Westerners, our definition of cleanliness seems inevitable, universal and timeless. It is none of these things, being a complicated cultural creation and a constant work in progress.

The most menacing aspect of the smells that came with poor-to-middling hygiene was that, as we were constantly warned, we could be guilty of them without even knowing it. There was no way we could ever rest assured that we were clean enough. For me, the epitome of feminine daintiness was the model who posed on the cover of a Kotex pamphlet about menstruation, titled: You're a Young Lady Now. This paragon, a blue-eyed blonde wearing a pageboy hairdo and a pale blue shirtwaist dress, had clearly never had a single extraneous hair on her body and smelled permanently of baby powder. I knew I could never live up to her immaculate blondness, but much of my world was telling me I had to try.

While ads for men told them they would not advance at the office without soap and deodorant, women fretted that no one would want to have sex with them unless their bodies were impeccably clean. No doubt that's why the second most frequent question I heard during the writing of this book - almost always from women - was a rhetorical: “How could they bear to have sex with each other?”

In fact, there's no evidence that the birth rate ever fell because people were too smelly for copulation. And, although modern people have a hard time accepting it, the relationship between sex and odourless cleanliness is neither constant nor predictable. The ancient Egyptians went to great lengths to be clean, but both sexes anointed their genitals with perfumes designed to deepen and exaggerate their natural aroma.

Most ancient civilisations matter-of-factly acknowledged that, in the right circumstances, a gamey, earthy body odour can be a powerful aphrodisiac. Napoleon and Josephine were fastidious for their time in that they both took a long, hot, daily bath. But Napoleon wrote to Josephine from a campaign: “I will return to Paris tomorrow evening. Don't wash.” As I read about cleanliness, people began taking me aside and confessing things: several didn't use deodorant, just washed with soap and water; some didn't shower or bathe daily. Two writers told me separately that they had a washing superstition: as the end of a long project neared, they stopped washing their hair and didn't shampoo until it was finished. One woman confided that her husband of some 20 years takes long showers at least three times a day: she would love, she said wistfully, to know what he “really” smells like.

The surreptitious way people revealed their deviations to me indicates how thoroughly we have been conditioned: to risk smelling like a human is a misdemeanour, and the goal is to smell like an exotic fruit or a cookie. The standard we read about in magazines and see on television is a sterilised and synthetic one.

What could be more routine and apparently banal than taking up soap and water and washing yourself? Yet it echoes, and links us to, some of the most profound feelings and impulses we know. In almost every religion, water and cleansing are resonant symbols - of grace, of forgiveness, of regeneration. Worshippers around the world wash themselves before prayer, whether literally, as the Muslims do, or more metaphorically, as when Catholics dip their fingers in holy-water fonts at the entrance to the church.

The archetypal link between dirt and guilt, and cleanliness and innocence, is built into our language - perhaps into our psyches. We talk about dirty jokes and laundering money. When we step too close to something morally unsavoury at a business meeting or a party, we say: “I wanted to take a shower.” Pontius Pilate washed his hands after condemning Jesus to death, and Lady Macbeth claims, unconvincingly: “A little water clears us of this deed,” after persuading her husband to kill Duncan. Baths and immersions also have a natural kinship with rites of passage, the ceremonies that mark the transition from one stage of life to the next - from being an anonymous infant to a named member of the community, from singlehood to marriage, from life to death.

One of the most widespread rites of passage involves bathing the dead, an action that serves no practical purpose but meets deep, symbolic ones. The final washing given to Jewish corpses is a solemn ceremony performed by the burial society, in which the body is held upright while 24 quarts of water are poured over it. Other groups - the Japanese, the Irish, the Javanese - enlist the family and close neighbours to wash the dead. All have a sense that respect for the dead means that he or she must be clean for the last journey, to the last resting place. Climate, religion and attitudes to privacy and individuality also affect the way we clean ourselves. For many in the modern West, few activities demand more solitude than washing our naked bodies. But for the ancient Romans, getting clean was a social occasion, as it can still be for modern Japanese, Turks and Finns.

In cultures where group solidarity is more important than individual- ity, nudity is less problematic and scrubbed, odourless bodies are less necessary. As these values shift, so does the definition of “clean”.

©Katherine Ashenburg 2008

Extracted from Clean: An Unsanitised History of Washing, by Katherine Ashenburg, to be published on March 20 by Profile Books, £12.99.

Available for £11.69 from Times BooksFirst: 0870 1608080; timesonline.co.uk/booksfirst

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