Despite all the advances in domestic hygiene, dirt is a fact of life, especially in our big, polluted cities. A new exhibition at the Wellcome Trust, Dirt: The Filthy Reality of Everyday Life looks at how dirt has affected — and continues to affect — our daily lives both physically and psychologically.
Cleaning, it seems, is rarely organised in response to scientific evidence. What is "clean" and what is "dirty" is frequently defined by historical and social context, rather than what has been proven in laboratory tests.
Approaches to cleaning are emotional as well as logical: witness the power of cleaning-product advertisements.
In the 17th century the English were praised for their dirt-removing energies, even if cleaning was often just for show; spotless reception rooms seen by visitors contrasted with squalid kitchens and filthy bedrooms. Cleanliness was related to social status and the keeping up of appearances.
The exhibition uses the examples of 17th-century Delft blue-and-white tiles. Used as skirting to protect walls when floors were scrubbed, these tiles were practical and decorative and became important status symbols.
Dutch paintings of the period often show scenes of assiduous cleaning; here the moral virtues of cleanliness are emphasised, leaving dirt to be associated with sloth and depravity. The English concurred: think of John Wesley's declaration that "cleanliness is next to godliness".
The Industrial Revolution transformed the way cleaning was done. Urbanisation and advances in sanitation provided clean water and sewers. The introduction of mains gas for lighting and then electricity changed the nature of domestic chores and powered an evergrowing range of appliances.
The electric iron was the first popular domestic appliance, patented in 1882. By 1926, 59 per cent of American households had one. The Hoover was patented in 1908 and by the Twenties was in widespread use across America.
These new technologies revolutionised the work of dealing with dirt, and as domestic cleaning became more dependent on bought-in machines and products, the role of women within the household changed from workhorse to consumer.
Where is dirt going?
It seems in two distinct directions. First, there is a noticeable shift back to old-fashioned methods. These include re-learning old skills and using traditional household products.
John Lewis has seen a 50 per cent increase in sales of its wooden airer in the past year and a rise of more than 30 per cent in sales of outdoor rotary airers and washing lines. "It is about saving energy and becoming eco friendly," says assistant buyer Gillian O'Hara.
Conversely, there has also been an increase in higher-tech appliances. All manufacturers followed Dyson's "cyclone technology" for vacuum cleaners, invented 30 years ago. Dyson is now working on lighter, smaller machines for an ageing population.
Sir James, who has spent years proving that where there's muck there is an enormous quantity of money, says: "Our future attitude lies in hi-tech engineering. Time is precious. We'll want homes that have self-cleaning properties built in."
Zoe Laughlin, curator of the Materials Library at King's College, agrees: "The number of self-cleaning materials is on the increase. Most are based on the mimetic principle, copying processes within nature to engineer products that do the same. The lotus leaf, for example, held the key to developing self-cleaning glass." Pilkington has developed glass where organic dirt on the surface is first broken down by sunlight and then washed away by rain.
"Advancements in nanotechnologies are also being made in an attempt to formulate coatings for other materials that will decrease the need to clean things," says Laughlin. "Basically, things can be self-cleaning by virtue of being difficult for dirt to stick to, or through a propensity to shed dirt due to a reaction of some kind."
Scientists in Australia have developed an eco-coating nanotechnology that may in future do the job of cleaning and disinfecting without the need for human input.
"Furniture and fittings will also have to adapt to become easier to clean, with minimal joins and fewer nooks," adds Dyson. "Buckminster Fuller's Dymaxion House design in the late Twenties was circular, with a moulded bathroom and an ingenious method of cleaning dust out."
But don't worry too much about a bit of dirt. In 1989, after examining the medical records of 17,000 British children, researcher David Strachan came up with his "hygiene hypothesis". This suggests the fewer germs children are exposed to, the more likely they are to get ill as grown-ups. Research since then tends to show he was right.
* Dirt: The Filthy Reality of Everyday Life runs until August 31 at Wellcome Trust, 215 Euston Road, NW1. Call 020 7611 8888, or visit wellcome.ac. uk for more details.
* Dyson products can be found at John Lewis shops (0845 604 9049; johnlewis.com) and at Selfridges.