In the Kenyan capital Nairobi, one in every five cars on the road emits black soot. Vehicles are normally fitted with a device that reduces toxic emissions. Youths comb mechanical shops on the hunt for these devices that contain rare metals and sell them for export.

If dumped in landfills, these metals would be toxic to the environment. But removing them is contributing to air pollution as the majority of second-hand cars in Nairobi continue to be used without any emission control device.

Click on "Play" above to hear Sophie Mbugua's report from Nairobi.