Sodium: Soap, paper, heart disease, murder
On the face of it, sodium seems a rather dull element - used by industry to manufacture such staples as paper and soap. But it also has a darker side.
Justin Rowlatt hears from chemistry regular Prof Andrea Sella about sodium's caustic reputation, and what exactly makes soap soapy.
We travel to the Munksjo paper company's pulp mill in Sweden to discover how the "Kraft process" exploits sodium's tenacious ability to digest organic matter.
We hear the confessions of Leonarda Cianciulli, an Italian housewife dubbed the "Soap-maker of Correggio", who once put that same tenacious property to a far more disturbing use.
And we ask whether sodium is the biggest killer in our diet, pitting Morton Satin, the self-styled "Salt Guru" and spokesman for the US salt industry, against arch nemesis Prof Graham MacGregor.