Fluorine is a ferocious yellow gas that is the key building block for a string of other gases that pose a threat to mankind if released into the atmosphere. From the ozone-depleting CFCs (Chlorofluorocarbons) to potent greenhouse gases such as sulphur hexafluoride, Justin Rowlatt gets the full rundown from professor Andrea Sella of University College London.

Justin travels to the source of fluorine in Britain, a fluorspar mine in Derbyshire, before following the ore to the giant acid works of Mexichem in Runcorn in the UK, where site director Ron Roscher explains the incredible array of uses for this chemical element.

And, he also hears from environmental scientist Stefan Reimann about the environmental legacy of CFCs and the threat posed by Chinese and Indian air conditioners.