(CNN)Many cities around the world are planting trees as a way to fight climate change.
But they might also reduce our risks of dying early.
Scientists say these trees, and other ways to green urban areas, could be just as beneficial to our mental and physical health and reduce the risk of premature death.
New research has put a number on just how many premature deaths could be prevented in one US city if it were to increase tree cover from 20% to 30% within five years.
Philadelphia, America's fifth-largest city, could help as many as 403 adults a year live longer if it meets its existing target, according to a study published Monday in the journal Lancet Planetary Health. The city's efforts could also yield a nearly $4 billion estimated annual economic benefit.
The authors said there's no reason that other cities, particularly ones in climates similar to Philadelphia, shouldn't benefit to the same extent.
"Although every city has its own characteristics, this study provides an example for all the cities in the world: Many lives can be saved by increasing trees and greening urban environments, even at modest levels," said Mark Nieuwenhuijsen, the study coordinator and director of the environment and health initiative at the Barcelona Institute for Global Health (ISGlobal).
"What's more, green spaces increase biodiversity and reduce the impact of climate change, making our cities more sustainable and more liveable."
ISGlobal took part in a systematic review published last year of research involving 8 million people in seven countries that found that urban dwellers who live in close proximity to green space are less likely to die prematurely.
"The major ways that nature or green space can improve health include improved social contact and cohesion -- how we relate to each other other," said Michelle Kondo, a research social scientist with the US Forest Service and the first author of the study.
"It can also improve stress levels and increase opportunity for physical activities," she added.
In this latest research, using the review's findings that the more green space the bigger the reduction in premature mortality -- they studied three different scenarios in Philadelphia.
Data on the city's existing canopy was obtained from aerial and satellite imagery, which allowed the researchers to measure the tree coverage by viewing the crown, leaves, branches and stems of trees from above.
If the city meets the ambitious 30% goal set by the Philadelphia City Council to increase tree coverage, the study estimated there would be 403 fewer early deaths across the city per year, representing 3% of the mortality rate.
Even if that goal is missed, their analysis showed that a 5% or 10% increase in tree coverage would result in a reduction of 271 or 376 deaths a year, respectively.
While other types of greenery are thought to have health benefits, the study said trees are thought to be particularly beneficial. The study said tree canopy is more effective than other greenery at reducing air temperatures, and one study from 2017 found that trees were associated with better health compared with grass.
"Achieving this goal does not come without challenges. Large tree planting initiatives are faced with many problems, including losses from climate change, tree pests and invasive species, and urban development," Kondo said.
Trees can benefit our health in several different ways, the study said, citing earlier research. At the individual level, green spaces can make it easier to do physical exercise, and soaking up nature can help reduce stress and improve mental well-being. They can also reduce heat island effects that lead to higher temperatures in built-up areas.
The authors said that experimental work in Philadelphia had shown that "greening interventions" throughout the city led to improved mental health, reduced crime and gun violence.
"One thing we do know: In areas where neighbors are coming together to do gardening, there is less gun violence -- as opposed to hiring contractors to come in and do the work. There's more social cohesion, and people activate [public] spaces," Kondo said.
"There's also the broken windows theory -- when you see a space that's uncared for," she said it may encourage anti-social behavior.
The study added that poorer neighborhoods in the city, where there are fewer trees, would particularly benefit from any increase in green spaces.