Children under three should be kept away from television altogether, according to the review of a study which  looks at the amount of "screen time" toddlers endure each day.

A review of the evidence published this week in the British Medical Journal's archives of disease in childhood said the sheer volume of screen time experienced by children in early years of development could cause serious cognitive issues as well as long-term physical harm.

The original study, published in the Archives of Pediatrics & Adolescent Medicine in 2010, followed 1314 children born in Quebec in 1997 and 1998.

It came after New Zealand research carried out by Otago University researchers, which found links between watching too much TV in childhood and developing problems later in life, including poor concentration, shorter attention span, smoking, high cholesterol and obesity.

Written by psychologist Dr Aric Sigman, the latest review of the Canadian study estimates children by the age of 10 have regular access to an average of five different screens at home.

"In addition to the main family television, for example, many very young children have their own bedroom TV along with portable handheld computer game consoles, smartphone with games, internet and video, a family computer and a laptop and/or a tablet computer."

He said children routinely engaged in two or more forms of screen viewing at the same time, such as watching the TV and using a laptop.

Screen viewing was also believed to be starting much earlier in life.

Nearly one in three American infants were estimated to have a TV in their bedroom, and almost half of all infants watched TV or DVDs for nearly two hours a day, the study found.

Sigman told British paper The Guardian children watched the equivalent of nearly a full working day of television outside school hours.

"On average a British teenager spends six hours a day looking at screens at home - not including any time at school. In North America, it is nearer eight hours."

But he said that after about two hours of sitting still, negative effects on health kicked in, which included increased long-term risks of obesity and heart problems.

The review said the critical time for brain growth was the first three years of life.

"That is when babies and small children need to interact with their parents, eye to eye, and not with a screen.

"Irrespective of the content or educational value of what is being viewed, the sheer amount of average daily screen time during discretionary hours after school is increasingly being considered an independent risk factor for disease."

- © Fairfax NZ News