Can street trees make air dirtier? Sometimes, yes.
Trees usually help cities. Their leaves catch some pollution. Their shade also keeps streets cool. However, the result depends on the tree and its place. (fs.usda.gov)
On narrow roads, thick branches can slow the wind. Dirty air from cars may then stay near people. Scientists call this place a “street canyon.” It means a street with tall buildings on both sides. (doi.org)
Trees can also send a natural gas into the air. One gas is isoprene. It is a chemical made by many plants. In hot weather, it can mix with traffic pollution. This can help create ozone near the ground. Ozone is a gas that can hurt our breathing.
A study in New York City examined future tree planting. It found that many new oak trees could raise ozone. This may happen especially on hot, polluted days. The study suggests choosing trees that make little isoprene. (atmoscomp.ldeo.columbia.edu)
This does not mean cities should stop planting trees. Trees offer shade and homes for animals. The lesson is simple: choose the right tree and place. Leave enough space for wind to move clean air.










