It was a long-held-belief that quality of life was better in the greenest cities. Now there is also a study - by the Barcelona Institute for Global Health (ISGlobal), the University of Colorado and the World Health Organization - to certify that the greater the urban vegetation is, the longer the life expectancy. The case study the researchers worked on concerns the US city of Philadelphia but, the study warns, this applies for all the metropolis in the world. Numbers are really impressive: increasing greenery by 30% in a city the size of Philadelphia (1.5 million inhabitants, the fifth largest in the USA) would prevent 400 premature deaths every year and the economic benefit would amount to approximately 4 billion dollars (considering resources saved in health-care and working days earned).

The study was published in "The Lancet Planetary Health" and analyzed in detail the impact of increased greenery on premature mortality within a city. The "Philadelphia model" was born from the analysis of 9 longitudinal studies involving 8 million people from seven different countries.  Thus, a significant association was identified between the increase in public greenery and the reduction of premature mortality. After that, in the specific study on the American metropolis, the scientists went deeper, trying to give an answer to how many lives are saved by the increase of green spaces. A 2025 scenario has been set, a deadline by which the city municipal administration's plan to increase the number of trees by 30% in each neighborhood could materialize. Therefore, the answer was 403 less premature deaths each year (about 3% of the city's annual death rate). If the ambitious goal of urban forestation fails, here are the numbers: with an increase in greenery of 5 or 10%, mortality would decrease by 271 and 376 cases respectively. Moreover, the poorer neighborhoods would benefit the most from the increase in vegetation, because in areas where the presence of plants is lower, even a small increase would give significant benefits to citizens’ health, both physical and mental.

The most beneficial effect on health would come from the planting of trees able to reduce heat. According to the study, the tree foliage, much more than grass or other types of vegetation, can reduce the temperature of both the soil and the air. Therefore, only trees would be truly effective in mitigating the effects of heat islands. More parks, gardens and trees would also mean a lower concentration of pollutants and a contribution to improving sociability, an incentive for physical activity and a way to reduce psychological stress.

"Although each city has its own features, this study provides an example for all cities in the world," states Mark Nieuwenhuijsen, ISGlobal's Urban Planning, Environment and Health Initiative director and coordinator of the study.