Clean children link to asthma

The finding, from scientists in Bristol, supports the theory that the developing immune system may actually benefit from some contact with dirt.

However, doctors are worried this message could prove counterproductive and place infants at risk of infection.

The origin of allergic disease is still far from fully understood.

Although a genetic root has been blamed for many cases of asthma and eczema, environmental factors may also play a role.

Other studies have suggested that asthma is less likely if the child mixes with others at day nursery, owns a pet or lives on a farm.

The researchers in this case, from the University of Bristol Department of Child Health, examined questionnaires from thousands of parents.

They had been quizzed on the washing habits of their 15-month-old children, in particular how often their hands were washed before meals, and how often they were bathed.

Wheeze

Parents scoring more highly for hygiene were slightly more likely to have a child who developed wheeze - or an eczema-like rash.

The effect could not be entirely linked to heavy use of household cleaners.

The researchers wrote: "The importance of hygiene in public health should not be dismissed.

"However, the creation of a sterile environment through excessive cleanliness may potentially be harmful to the immune system."

Allergies develop during childhood as the infant builds up an immune system.

Overactive response

If the child does not come into too many potential allergens early in life, some scientists believe that when they do, they trigger an alternative immune response which makes them over-sensitive.

Professor John Foreman, from the Centre for Allergy Research at University College London, said the contribution of cleanliness to asthma as likely to be small.

He said: "If you run around telling parents that their kids aren't going to get asthma if they roll around in the dirt, you'll find plenty who said they did this and their child still got asthma.

"It may be a factor, but if there is an effect, it will only be a small one."