Overall mortality was 12 per cent less for people who had the most greenery within 250 metres of their homes during the eight year follow-up period.

It is thought that being surrounded by vegetation improves mental health and lowers depression. It also allows people to get out and about more, giving more opportunities for exercise and social engagement, both of which are known to be protective against disease. The lack of air pollution in green areas also plays an important role.

Nevertheless, the researchers said they were not expecting the effect size to be so startling.

"We were surprised to observe such strong associations between increased exposure to greenness and lower mortality rates," said Peter James, research associate in the Harvard Chan School Department of Epidemiology.

"We were even more surprised to find evidence that a large proportion of the benefit from high levels of vegetation seems to be connected with improved mental health.