Just five minutes of exercise a day in the great outdoors can improve mental health, according to a study, and policymakers should encourage more people to spend time in parks and gardens.
Researchers from the University of Essex found that as little as five minutes of a "green activity" such as walking, gardening, cycling or farming can boost mood and self esteem.
"We believe that there would be a large potential benefit to individuals, society and to the costs of the health service if all groups of people were to self-medicate more with green exercise," says co-author Dr Barton.
The study appear in the journal Environmental Science & Technology.
Many studies have shown that outdoor exercise can reduce the risk of mental illness and improve a sense of well-being. But Barton and colleague Professor Jules Pretty, say that until now no one knew how much time needed to be spent on green exercise for the benefits to show.
Barton and Pretty looked at data from 1252 people of different ages, genders and mental health status taken from 10 existing studies in Britain.
They analysed activities such as walking, gardening, cycling, fishing, boating, horse-riding and farming.
They found that the greatest health changes occurred in the young and the mentally ill, although people of all ages and social groups benefited. The largest positive effect on self-esteem came from a five-minute dose of "green exercise".
All natural environments were beneficial, including parks in towns or cities, they say, but green areas with water appeared to have a more positive effect.
In a commentary appearing in the same issue of the journal, researchers point out that none of the studies in the analysis were randomised trials.
But William Sullivan of the University of Illinois at Urbana-Champaign says it will be valuable to those advocating the building of green spaces.
"Planners and consultants can put this in front of policy makers and say this is serious research that's been published in the scientific literature."