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.
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.