Can trees cut air pollution?

The World Health Organization estimates that every year, outdoor air pollution is linked to more than three million premature deaths - making it the world's greatest environmental health risk.

A study by Dr David Nowak, senior scientist at the US Forest Service, has shown that trees only reduce urban air pollution by up to 1%. Yet even this small reduction can have huge health benefits.

Listen to CrowdScience: Can trees help us fight air pollution?

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A week of coverage by BBC News looking at ways to cut air pollution.