By DAVID DERBYSHIRE

Last updated at 01:03 15 June 2007

When George Thomas was eight he walked everywhere.

It was 1926 and his parents were unable to afford the fare for a tram, let alone the cost of a bike and he regularly walked six miles to his favourite fishing haunt without adult supervision.

Fast forward to 2007 and Mr Thomas's eight-year-old great-grandson Edward enjoys none of that freedom.

He is driven the few minutes to school, is taken by car to a safe place to ride his bike and can roam no more than 300 yards from home.

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Even if he wanted to play outdoors, none of his friends strays from their home or garden unsupervised.

The contrast between Edward and George's childhoods is highlighted in a report which warns that the mental health of 21st-century children is at risk because they are missing out on the exposure to the natural world enjoyed by past generations.

The report says the change in attitudes is reflected in four generations of the Thomas family in Sheffield.

The oldest member, George, was allowed to roam for six miles from home unaccompanied when he was eight.

His home was tiny and crowded and he spent most of his time outside, playing games and making dens.

Mr Thomas, who went on to become a carpenter, has never lost some of the habits picked up as a child and, aged 88, is still a keen walker.

His son-in-law, Jack Hattersley, 63, was also given freedom to roam.

He was aged eight in 1950, and was allowed to walk for about one mile on his own to the local woods. Again, he walked to school and never travelled by car.

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By 1979, when his daughter Vicky Grant was eight, there were signs that children's independence was being eroded.

"I was able to go out quite freely - I'd ride my bike around the estate, play with friends in the park and walk to the swimming pool and to school," said Mrs Grant, 36.

"There was a lot less traffic then - and families had only one car. People didn't make all these short journeys."

Today, her son Edward spends little time on his own outside his garden in their quiet suburban street. She takes him by car to school to ensure she gets to her part-time job as a medical librarian on time.

While he enjoys piano lessons, cubs, skiing lessons, regular holidays and the trampoline, slide and climbing frame in the garden, his mother is concerned he may be missing out.

She said: "He can go out in the crescent but he doesn't tend to go out because the other children don't. We put a bike in the car and go off to the country where we can all cycle together.

"It's not just about time. Traffic is an important consideration, as is the fear of abduction, but I'm not sure whether that's real or perceived."

She added: "Over four generations our family is poles apart in terms of affluence. But I'm not sure our lives are any richer."

The report's author, Dr William Bird, the health adviser to Natural England and the organiser of a conference on nature and health on Monday, believes children's long-term mental health is at risk.

He has compiled evidence that people are healthier and better adjusted if they get out into the countryside, parks or gardens.

Stress levels fall within minutes of seeing green spaces, he says. Even filling a home with flowers and plants can improve concentration and lower stress.

"If children haven't had contact with nature, they never develop a relationship with natural environment and they are unable to use it to cope with stress," he said.

"Studies have shown that people deprived of contact with nature were at greater risk of depression and anxiety. Children are getting less and less unsupervised time in the natural environment.

"They need time playing in the countryside, in parks and in gardens where they can explore, dig up the ground and build dens."

The report, published by Natural England and the Royal Society for the Protection of Birds, also found that children's behaviour and school work improve if their playground has grassy areas, ponds and trees.

It also found evidence that hospital patients need fewer painkillers after surgery if they have views of nature from their bed.