Avenues of trees on urban streets can damage the health of pedestrians by trapping air pollutants beneath their canopy, research suggests.

Scientists say the effect could be so bad on streets with slow-moving traffic that councils should consider chopping down trees. Many have been planted in cities under schemes aimed in part at improving air quality.

Fine particles which cause respiratory illnesses are removed by trees by becoming trapped on their leaves. Trees also absorb some of the nitrogen dioxide and other harmful gases emitted by vehicles, but these beneficial effects may be more than cancelled out by what scientists describe as the “green roof” effect, in which dense canopies prevent pollutants from escaping, and block the inward flow of cleaner air.

John Gallagher, an