The world's biggest problem right now is air pollution. It's the leading cause of death worldwide, way ahead of things like obesity, HIV and terrorism. For our Vital Signs month on How We Get To Next, I wrote a piece arguing that we need to take action now.
As part of it, I put together this graphic, showing which cities in the world have air that's unsafe for humans to breathe.
It's worth noting, in the wake of Britain's regrettable vote to exit the European Union, that the EU has been one of the strongest voices in getting cities to clean up their air (and their living environment in general). I hope that the UK will continue to maintain high environmental standards in the coming decades.