Sulphur dioxide

Sulphur dioxide was a highly toxic gas which was produced by burning sulphur. When the First Doctor and Ian Chesterton offered to make smoke bombs for Lord Takagi Mamoru, and showed one off to him, Lord Mamoru requested, on Casper Knox's suggestion, that they include sulphur. This would transform their mere distraction, intended to buy Mamoru more time, into a hazardous weapon against Hiroshi's men. (AUDIO: The Barbarians and the Samurai)