In December 1984, a cloud of poisonous gas escaped from an American pesticide plant in the Indian city of Bhopal. It was the world's worst industrial accident, with half a million people exposed to lethal fumes from the Union Carbide factory. 25 years on, an estimated 120,000 people are still suffering ill-health. Satinath Sarangi tells Jonathan Charles why he is still fighting for justice for the survivors.