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?

Find out more with So I Can Breathe on BBC World Service

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