Chromium: Justin Rowlatt visits the Warrs Harley dealership to find out from Professor Andrea Sella why this metallic element links the motorbikes on show, with the leather jackets and flick-knives of the archetypal biker gang. He hears from Erin Brockovich about the insidious role hexavalent chromium has played in drinking water and human health. And he travels to the luxury Savoy hotel in London, and the Harry Brearley memorial on a dingy post-industrial corner of Sheffield, to discover crucial role chromium plays in stainless steel.