The World Health Organisation say that 95% of people who live in cities breathe unsafe air. But what do they mean by ‘unsafe’? And how do they calculate the levels or air pollution for every city in the world? Plus Mt Etna in Italy has reportedly moved by 14mm, but who is calculating this? And how do they know the answer with such accuracy? (Photo: People wear masks as smoke billows from a coal fired power plant, Shanxi, China. Credit: Kevin Frayer/Getty Images)