Some say children under three should not watch any TV
Young children who spend more than two hours a day watching TV double their risk of developing asthma, a UK study has found.
Rather than telly per se being to blame, experts believe the viewing is symptomatic of a sedentary lifestyle which may be the root cause.
Taking deep breaths, such as when exercising, may keep the lungs fit.
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.
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.
At the age of eleven-and-a-half, 185 (6%) 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, 2% did not watch TV, 20% watched TV daily for less than an hour, 34% watched 1-2 hours a day and 44% watched more than two hours daily.
Given that none of the children had wheeze at the age of three-and-a-half, it is unlikely that the children who went on to develop asthma had been forced to do less exercise from an early age because of asthma symptoms, say the researchers.
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 did not directly monitor the children's exercise levels during the study.
And at the end of the study, when the children were 11.5 years old, there was little difference in the exercise levels of those with asthma and those without.
Co-author Dr James Paton, from the University of Glasgow, said: "We think the problem is inactivity, not watching TV. TV is simply the best proxy marker for this.
"There may be a window in early in life when activity does something to protect the lungs.
"It may be that not sitting still makes you take deep breaths and that might be important in the long run."
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.
"We have one of the highest rates of childhood asthma in the world so 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. This includes children with asthma, who can also greatly benefit from regular exercise."
Some say children under three should not watch any TV in order to prevent health and learning problems. However, others argue that TV can be educational and may aid child speech.
The data was taken from a long-term study of children being carried out in Bristol.