Urban greenery is important but it is necessary to plant the right trees, some can worsen the air quality: trees such as oaks produce some substances that react with the nitrogen produced by cars and can transform into ozone, a molecule that facilitates bronchitis and asthma.
This is according to a study by researchers at the Columbia Climate School, led by Dandan Wei, who analyzed the situation in New York City in particular.
“We are all in favor of planting more trees because they bring so many benefits, but if we are not careful we risk worsening air quality,” said Róisín Commane, one of the study’s authors. A surprise that comes from an in-depth study of the possible interactions between chemicals emitted by tree leaves and those released into the air by human activity, such as smog from cars and home heating systems.
It has been known for some time that some plants, especially oaks, release a molecule called isoprene, a waste product of photosynthesis that in itself is not a problem but that if it combines with nitrogen oxides, the so-called Nox, can generate ozone. A molecule that is considered one of the triggers of some pathologies such as chronic bronchitis or asthma. Analyzing in particular the situation in New York, researchers have observed that oaks represent 18% of all trees and, in light of climate change, they estimate that the presence of ozone produced by a mix of Nox and isoprene could increase by up to 140% in the coming decades. For this reason, researchers say, it would be important to carefully evaluate which new trees to plant in the coming years and first check for any adverse effects such as these.
“But we don’t want to convey the idea that it’s the trees that are polluting the air. It’s the cars,” Wei said. “If we significantly lowered the NOx, trees wouldn’t be a problem.”
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