Look out your window. Do you see three trees? Is there 30% tree cover in your neighborhood? Do you live less than 300 meters from a park? These questions are about the 3-30-300 rule proposed by urban forester Cecil Konijnendijk to create healthier cities, because green (whether it’s trees, plants or rooftop vegetation) helps to mitigate high temperatures, prevent flooding and improve people’s health. The ISGlobal Institute for Global Health recently published a study, carried out in Barcelona, showing 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 survey with the United States Forest Service which shows that planting trees in the street can save lives; in this case, the study was conducted in Portland, Oregon.
ISGlobal researchers 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 the improvement it produces; 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. 62.1% have a “major” green space within 300 meters, 43% have at least three trees within 15 meters of their home and only 8.7% live in an area with sufficient greenery. Then there is the worst figure: almost 23% of the inhabitants do not respect any of the three provisions. “There is an urgent need to offer citizens more green spaces. We may need to rip asphalt and plant more trees, which would not only improve health, but also reduce heat island effects and aid in carbon sequestration,” says Mark Nieuwenhuijsen, lead author of the study.
The report took data from the Barcelona Public Health Agency’s 2016 health survey, which assessed the mental health of residents aged 15 to 97. On average, 18% said they had poor mental health, 8.3% said they had visited a psychologist in the past year, and between 8.1% and 9.4% admitted to having taken tranquilizers or antidepressants during course of the last two days. The researchers crossed the data from the health survey with indicators of green spaces, sensors and land use maps. The sample is very representative. The survey found that following the full rule was “clearly associated with better mental health, less medication use and fewer visits to a psychologist”. Of the three variables, the one that weighs the most is tree cover
As for the study conducted in Portland, the particularity is that the researchers had precise data from the NGO Friends of Trees, which has been planting trees in the city for three decades. The conclusion is that the number of trees is associated with a significant reduction in non-accidental mortality (20%) and cardiovascular mortality (6%) when trees are 15 years or older. The older and larger the specimens, the greater their beneficial effect. Thus, the preservation of mature trees may be particularly important for public health.
Unlike other studies that use satellite images that do not distinguish between different types of vegetation, in this case the NGO recorded when and where it planted nearly 50,000 trees between 1990 and 2019. Researchers analyzed trees in a census area of 4,000 people and matched information on mortality from cardiovascular, respiratory, or non-accidental causes with data from the Oregon Health Authority. In neighborhoods where more trees had been planted, death rates were found to be lower, especially among men and people over 65.
The study does not provide direct evidence on the mechanisms by which trees improve health, but the fact that older specimens have a greater positive effect is telling: they have a greater ability to absorb pollution, moderate heat and reduce noise, three factors linked to increased mortality.
Payam Dadvand, ISGlobal researcher and lead author of the study, says the findings provide a strong scientific basis to guide tangible interventions, such as tree planting, aimed at increasing the longevity of urban residents.
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