When it’s hot, plants become a surprisingly large source of air pollution

Trees and shrubs emit more chemicals during heat waves that can react to form ozone

Trees in Berlin

FEELING THE HEAT  Tree-lined streets in Berlin, shown, offer a lot of benefits for city residents. But during heat waves, the trees, along with other plants, can also contribute to poor air quality, a new study suggests.

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Planting trees is often touted as a strategy to make cities greener, cleaner and healthier.