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Do we all feel the same way about dirt?

Soap bar

Does dirt have different meanings for people from different cultures?

In the West many of us are shunning the traditional bar of soap for sophisticated antibacterial handwashing and fragrant shower gels. The sophistication goes beyond the physical. Lady Macbeth's exhortation to that "damned spot" encapsulated the association of physical cleanliness and moral 'purity' four centuries ago.

Recently, researchers at the University of Michigan have shown links between handwashing and the more subtle psychological mechanisms involved in modern consumerism. They found that handwashing with soap eliminated post-decisional dissonance (the mild discomfort people feel after making a choice between two CDs or types of jam).

In other parts of the globe, soap doesn't even feature in hygiene practices. Yet Dr Val Curtis says handwashing with soap could save over a million deaths from diarrhoeal disease per year in those regions. She is currently evaluating an intervention to promote the behaviour in rural Indian households, working with industry to make soap available at suitable prices.

In the West, we wouldn't dream of going out without shoes on, and are mostly lucky enough to be able to afford them. But in parts of Africa people go barefoot and risk picking up parasitic worms or absorbing silicate particles from the soil through their feet. In some people the silica particles cause podoconiosis, a disabling swelling in the lower legs. Since only some people react to the silicates in this way, a genetic cause is likely, and Gail Davey and colleagues are seeking to identify the gene involved.

Despite the disease-causing agents it harbours, soil is valued in many cultures. Paul Wenzel Geissler noted that geophagy - the practice of eating soil - is particularly common in pregnant women and primary-schoolchildren who have depleted levels of iron in their blood. The iron in the soil they ingest may be compensating for the deficiency.

He also described how earth-eating is an embedded cultural practice. After childhood it is almost exclusively women of reproductive age who eat earth. Earth symbolises female, life-giving forces, fertility and the continuity of the lineage, and the termite hills from which most earth eaten is taken represent the home and the graves of ancestors.

Values and beliefs about dirt vary widely across cultures. However, Val Curtis notes that the emotion of disgust is common to nearly all of them, and is elicited by similar groups of stimulus, including bodily excretions and waste, living creatures such as lice and insects, decaying or spoilt food, and other people in poor health. (See 'Dirt, Disgust and Disease' by Valerie Curtis and Adam Biran [PDF].) She suggests this aversion evolved to protect us from potential sources of infection. Interestingly, disgust of other people extends to those who commit crimes or violate social 'norms', and may serve to protect individuals and society by ostracising and punishing 'social parasites'.

Val talks to Wellcome Collection's Lisa Jamieson about the protective power of disgust (Packed Lunch event, recorded live on 23 October 2009).

The spaces differ, but infections that thrive in closed environments - such as hospitals, cruise ships, military camps and slums - are common across the globe. Wellcome-Trust funded researchers are using genetic approaches to tackle two of these. Ian Goodfellow is studying the molecular mechanisms of the notorious norovirus to identify new methods of controlling and preventing outbreaks, while researchers at the Wellcome Trust Sanger Institute have developed a new method that can detect single-letter changes in the genome of the superbug MRSA, which will make it possible to precisely track its transmission from person to person in hospitals and develop targeted infection control strategies.

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