The finding undermines the long held belief that the intensity of early life exposure to allergens directly influences the risk of developing childhood respiratory allergies and asthma, suggest the authors from the National Heart and Lung Institute, London.

They base their conclusions on 625 children in one town in southern England. They were monitored from birth until the age of 5 years, when 552 of them were tested for sensitisation to house dust mite, cat fur, and grass pollens.

Their mothers were also interviewed annually about whether their children had had any episodes of wheezing in the preceding 12 months. Allergen levels had been measured in house dust samples from the living room floor when the children were 8 weeks old.

One in 10 of the children was sensitised to house dust mite or cat fur by the age of 5 , and one in 14 had associated wheezing in the preceding year.

But no significant links were found between the levels of early life exposure to household allergens and either sensitisation or frequency of wheezing.

Both sensitisation and wheezing occurred in response to very low allergen levels, and they were significantly more likely if the father had an inherited susceptibility to allergy or the child was a firstborn.

"If exposure thresholds for sensitisation or asthma exist, then they appear to be exceedingly low," say the authors. "[Our findings] imply important and previously unrecognised gene-environment interactions in the development of atopy and associated asthma," they add.

The authors also suggest that their findings "make it improbable that reductions in domestic allergen exposures alone will have a major impact in reducing the incidence of these diseases in childhood." In fact, they suggest that so doing might actually increase rates of sensitisation.

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Asthma is a condition that affects the airways “ the small tubes that carry air in and out of the lungs. If you have asthma your airways are almost always sensitive and inflamed. When you come in to contact with something you are allergic to, or something that irritates your airways (a trigger), your airways will become narrower, making it harder to breathe. The muscles around the walls of your airways tighten. The lining of the airways becomes inflamed and starts to swell and often sticky mucus or phlegm is produced. This will lead to you experiencing asthma symptoms.

What are asthma symptoms?Asthma symptoms can vary. You may find that you start to cough or wheeze, get short of breath, or have a tight feeling in your chest. Despite what many people think, wheezing does not always occur. In fact, coughing is the most common asthma symptom.

Why did I get asthma?Asthma can start at any age. Some people get symptoms during childhood which then disappear in later life. Others develop 'late-onset' asthma in adulthood, without ever having had symptoms as a child. It is difficult to say for sure what causes asthma, but so far we know that:

asthma can be inherited (like the related allergic conditions eczema and hay fever). many aspects of modern lifestyles “ such as changes in housing and diet and a more hygienic environment “ may have contributed to the rise in asthma over the last few decades. smoking during pregnancy increases the chance of a child developing asthma. environmental pollution can make asthma symptoms worse but has not been proven to actually cause asthma. late-onset asthma may develop after a viral infection. irritants found in the workplace may lead to a person developing asthma.

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