Toddlers who are washed frequently are more likely to develop asthma and eczema, a scientific study reveals today.

Toddlers who are washed frequently are more likely to develop asthma and eczema, a scientific study reveals today.

The results support the so-called hygiene hypothesis, which says that excessive washing and cleanliness are responsible for a rapid increase in the two disorders. Researchers investigated how frequently 11,000 small children in the Bristol area were bathed, showered or had their hands washed every day. Parents then said whether their children suffered the symptoms of asthma or eczema before they were six months old and when they were between two and four.

The survey found that "increased levels of hygiene" were linked to higher rates of asthma and eczema when children were 30 to 42 months old, but not in the first six months. In The Archives of Disease of Childhood, the researchers suggest children who become dirty are exposed to more bacteria and infections that prime their immune system and protect them from disease.

They add that parents who are obsessed with cleanliness may be reluctant to let their children play outdoors or have contact with others, so they have less contact with bugs.

The authors, led by Dr Andrea Sherriff of Bristol University, conclude: "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."

The research is the first epidemiological study to investigate whether infant hygiene is related to allergic disorders.

The results show that for every increased unit in the hygiene score the likelihood of a child wheezing between the age of 30 and 42 months increased by 4 per cent. In children under the age of six months, wheezing was partly explained by high levels of chemical products used to clean the home, which can irritate the airways. Eczema was significantly associated with high hygiene scores, irrespective of the amount of chemicals used. The link was even stronger for those children with severe skin complaints.

Dr Sherriff said: "A possible explanation for the positive association between hygiene and wheezing and atopic eczema is that children with high hygiene scores would have less contact with infectious agents. Alternatively, high hygiene scores may reflect a difference in behaviour that would reduce this exposure to infectious agents, such as less outdoor play or reduced contact with other children. This increased risk of atopy in the hygienic children is in accord with the hypothesis that early exposure to infections somehow protects against allergies in later life."

Last month, some of the world's top scientists gathered to discuss the dramatic rise in asthma and allergies.

Professor David Strachan, from St George's Hospital Medical School in London, who was present, said that since the 1960s there had been a 6 per cent rise in adult and a 10 per cent rise in children of diagnosed cases of asthma. He said there had been a five to ten-fold increase in hospital admission rates because of atopy, an allergic response to pollen, dust mites or pet fur that could bring on conditions such as hay fever, eczema and asthma.

In the 1950s, when the parents' favourite for washing their children was a bar of soap, less than 5 per cent of children had eczema. Now 20 per cent of children suffer at some time, according to separate research by Sheffield University last month.