Should a scientist who believes that he has evidence in his hands of dangerous pollution speak out before the proof is conclusive and risk alarming the public? Or should he remain silent? Three metals - mercury, lead and cadmium-that have no place in the human body are threatening the existence of human beings in several parts of the world. Fishermen in Japan and children in Missouri have suffered physical and mental illnesses attributed to metallic poisoning. On the other side of the coin the resulting scare has put hundreds of fishermen out of work in Canada and America. There is now evidence that we on this island might be running risks of similar poisoning. What would happen if we avoided the evidence and how big would be the price to pay if we heeded it too soon? Tonight's programme examines the consequences of man's interference with his environment. Narrated by IAN HOLM Written by JOHN LLOYD Producer MICHAEL BLAKSTAD