Look out your window. Do you see three trees? Is there 30% vegetation cover in your neighborhood? Do you live less than 300 meters from a park? These questions pertain to the 3-30-300 rule proposed by the urban forester Cecil Konijnendijk to create healthier cities. He argued that trees, plants and rooftop vegetation help mitigate high temperatures, prevent flooding and improve the health of the population. And now, a new study by Spain’s ISGlobal Institute for Global Health, carried out in Barcelona, shows that living near trees and green spaces is linked to better mental health and less need for medication. The same scientific entity co-directed another investigation with the United States Forest Service that shows that planting trees in the street can save lives; in this case, the study was carried out in Portland, Oregon.
The researchers from ISGlobal explain that the Barcelona study on the 3-30-300 rule is the first to measure it in a city. The good news is that it confirms that it works; the bad news is that the city got a terrible score. Only 4.7% of the population met all three criteria of the green space rule. More than 62% of residents have a “major” green space less than 300 meters away, 43% do have at least three trees 15 meters away from home and only 8.7% live in an area with sufficient surrounding greenness. Then there is the worst figure: almost 23% of residents do not comply with any of the three provisions. “There is an urgent need to provide citizens with more green space. We may need to tear up asphalt and plant more trees, which would not only improve health, but also reduce heat island effects and contribute to carbon capture,” says Mark Nieuwenhuijsen, lead author of the study.
The report took data from the 2016 Health Survey of the Barcelona Public Health Agency, which evaluated the mental health of residents between the ages of 15 and 97. On average, 18% claimed to have poor mental health, 8.3% said that they had visited a psychologist in the last year and between 8.1% and 9.4% admitted to having taken tranquilizers or antidepressants in the last two days. The researchers cross-referenced the data from the health survey with indicators of green spaces, sensors and land cover maps. The sample is highly representative. The investigation revealed that meeting the 3-30-300 rule was “clearly associated with better mental health, less medication use and fewer visits to a psychologist.” Of the three variables, the one with the biggest impact is tree cover
As for the study carried out in Portland, the researchers had precise data from the Friends of Trees NGO, which has been planting trees in the city for three decades. The researchers concluded that the number of trees is associated with a significant reduction in non-accidental mortality (20%) and cardiovascular mortality (6%) when the trees are 15 years or older. The older and larger the trees are, the greater their beneficial effect. Thus, preserving mature trees can be particularly significant for public health.
Unlike other studies that use satellite images that make no distinction between the different types of vegetation, in this case the NGO recorded when and where it planted almost 50,000 trees between 1990 and 2019. The researchers analyzed the trees in a census area with 4,000 inhabitants and cross-referenced the information with mortality from cardiovascular, respiratory or non-accidental causes, with data from the Oregon Health Authority. In the neighborhoods where more trees had been planted, the mortality rates were found to be lower, especially in men and people over 65 years of age.
The study does not provide direct evidence regarding the mechanisms by which trees improve health, but the fact that older specimens have a greater positive effect is revealing: they have a greater capacity to absorb pollution, moderate heat and reduce noise, three factors related to the increase in mortality.
Payam Dadvand, ISGlobal researcher and lead author of the study, explains that the results provide a solid scientific basis to guide tangible interventions, such as planting trees, aimed at increasing the longevity of urban residents.
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