Fluorine

Fluorine is a ferocious yellow gas, which is a key building block for a string of other gases that pose a threat to mankind - from the ozone-depleting CFCs to potent greenhouse gases. Justin Rowlatt visits the electricity substation of University College London with Prof Andrea Sella to see one such gas in action. He speaks to Dr Stefan Reimann, who tracks their release into the atmosphere for the World Meteorological Organization. And he finds out from pioneering researcher Ian Shankland at chemicals giant Honeywell why an exploding Mercedes car caused a trans-Atlantic chemistry tiff.