A study of more than 7,000 children shows that children born into households which use them most are twice as likely to suffer persistent wheezing, often a precursor to asthma.

Incidence of the disease has tripled since the 1970s and the total number in the country who suffer is estimated to have reached 1.4 million. Britain has one of the highest rates of wheezing children in the world.

The study shows a clear connection between persistent wheezing and use of a range of domestic chemicals, such as bleach, paint stripper, carpet cleaner and air freshener. The use of household cleaning products has soared in the past two decades: the market has grown by 60 per cent since 1994.

The researchers are not claiming that these chemicals cause asthma but that there is a strong link. Their results back up an Australian study published in August.

The data comes from Bristol University’s Children of the 90s project, which has been following a group of children born in the Avon area in the early 1990s. This study, published in Thorax, correlates health with information about their homes and lifestyle.

“We are seeing what appear to be effects on lung function, either while the baby is still in the womb or after birth,” Dr Andrea Sherriff, of the university, said. “We cannot say exactly what chemicals are involved but our results are highly validated. We know the participants in the study well and can rely on the information they give us.”

Before they gave birth, mothers were asked how often they used certain chemical-based products. From these questions, their households were divided into categories based on “total chemical burden”.

The team compared this with the incidence of wheezing in children up to the age of 3½. Those in the top 10 per cent were more than twice as likely to suffer persistent wheezing as those in the lowest 10 per cent.

“We have since followed children to the age of 8,” Dr Sherriff said. “The effects seem to persist.” The team concludes: “These findings suggest that children whose mothers made frequent use of chemical-based domestic products during pregnancy were more likely to wheeze persistently throughout early childhood, independent of many other factors.”

The Australian study, based on a smaller sample, linked volatile compounds in household chemicals with asthma. The Bristol team suggests that the chemical formaldehyde could be a common factor.

Another possible explanation is that cleanliness itself may cause asthma. This theory suggests that the immune systems of children raised in over-clean environments do not develop properly. As a result they turn against the body and trigger allergies, asthma or eczema.

Professor Andrew Peacock, of the British Thoracic Society, said: “More long-term studies are needed before we advise pregnant women to throw out all their air fresheners. But there are measures that can be taken to protect yourself and your baby, such as reducing the number of household products that you use and by wearing gloves and keeping windows open when cleaning.”

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