Young children who watch TV for more than 2 hours a day run the risk of developing asthma before their 12th birthday, according to a study of more than 3,000 kids whose health and habits were tracked from birth.

"In children who had no wheezing symptoms up to age 3.5 years, those who reported watching TV for more than 2 hours per day were almost twice as likely to have asthma by age 11.5 years as those watching TV for 1 to 2 hours per day," Dr. Andrea Sherriff told Reuters Health.

The amount of time spent in front of the TV was used as a measure for sedentary behavior because personal computers and video game consoles were not in widespread use at the time the study was conducted in the mid-1990s, explained Sherriff, who is at the University of Glasgow, UK.

According to a report in the journal Thorax, 6 percent of the children who had no asthma symptoms when they were 3.5 years old had asthma at the age of 11.5 years, with no marked differences between boys and girls.

As the length of time spent watching TV each day increased, the rate of asthma at age 11.5 years increased, the investigators report. Children who watched TV for more than 2 hours a day were 1.8 times more likely to have been diagnosed with asthma than those who watched less TV each day.

"We do not yet know why this is the case and therefore it is too early to make recommendations to parents based on this research," Sherriff said.

The relationship between physical activity, sedentary behavior and asthma is complex, the researchers note in their report. However, recent research has suggested that sedentary behavior affects the way children breathe, perhaps sparking developmental changes in the lungs and subsequent wheezing.

"Further research is required into, for example, the breathing patterns and lung development of children at early stages of childhood," Sherriff said.