Young children who spend more than two hours glued to the TV every day double their subsequent risk of developing asthma, indicates research published ahead of print in Thorax.

The findings are based on more than 3,000 children whose respiratory health was tracked from birth to 11.5 years of age. The children were all participants in the Avon Longitudinal Study of Parents and Children (ALSPAC), which has been following the long term health of 14,000 children and their parents.

The parents were quizzed annually on symptoms of wheezing among their offspring and whether a doctor had diagnosed asthma in their child by the time s/he was 7.5 years old, but not before the age of 3.5 years.

Parents were also asked to assess their children's TV viewing habits from the age of 3.5 years, and these were subsequently compared with those of their symptomless peers.

The amount of time spent in front of the box was used as a proxy measure of sedentary behaviour, because personal computers and games consoles were not in widespread use at the time (mid 1990s).

The prevalence of asthma among children at the age of 11.5 years, who had no asthmatic symptoms when they were 3.5 years old was 6%. But 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 box less.

By the time the children were 11.5 years old, there was little difference in levels of sedentary behaviour between those with asthma and those without. The results were not confined to one gender, nor were they related to current weight.

The authors comment that the relationship between physical activity, sedentary behaviour and asthma is complex. But they point out that recent research has suggested that breathing patterns in children may be associated with sedentary behaviour, sparking developmental changes in the lungs and subsequent wheezing.