Children who spend more than two hours a day watching television double the risk of developing asthma, according to a new study in the UK. However, the researchers believe that excessive TV watching is symptomatic of a sedentary lifestyle, which is likely to be the root cause of the problem, rather than TV-watching itself.

Led by Dr Andrea Sherriff of Glasgow University, the researchers established a link between large amounts of TV watching and the respiratory condition. The study, published in Thorax journal, tracked the health of over 3,000 UK children from birth to 11.

The parents were quizzed annually on symptoms of wheezing among their children and whether a doctor had diagnosed asthma. There may be a window in early in life when activity does something to protect the lungs, co-author James Paton said.

Parents were also asked to assess their children's TV viewing habits from the age of three-and-a-half years. All of the children were free of wheeze as babies and toddlers. But at the age of eleven-and-a-half, 6 per cent of the children had developed asthma.

And children who watched TV for more than two hours a day were almost twice as likely to have been diagnosed with asthma as those who watched the telly less. However, the odds were still small - about two in 100.

Of the children with asthma, two per cent  did not watch TV, 20 per cent watched TV daily for less than an hour, 34 per cent watched 1-2 hours a day, and 44 per cent watched more than two hours daily.

Although they did not directly monitor the children's exercise levels during the study, they speculate that inactivity is the underlying explanation for their findings, if you assume that children who watch more TV lead less active lives.

They said it is especially important that parents in the UK try to prise their kids away from the TV and encourage them to lead an active lifestyle. They added that taking deep breaths, such as when exercising, may keep the lungs fit.

There is some evidence that breathing patterns may be important in regulating airway smooth muscle tone and how responsive this muscle is. Failure to stretch airway smooth muscle by taking regular deep breaths could lead to increased airway responsiveness, which is the problem in asthma.

Dr Elaine Vickers of Asthma UK said, "The findings add to a wealth of evidence linking a lack of exercise and being overweight with an increased risk of asthma, but this study is the first to directly link sedentary behaviour at a very young age to a higher risk of asthma later in childhood.''