Balerium gas

Balerium gas was a substance that, if inhaled, rendered a person unconscious. It affected the throat, making speech difficult

The Company used the gas in its Consum Banks to target those guilty of fraud. When the Fourth Doctor went to a Consum Bank on subway 37, his consumcard was detected as a fake and the gas was released. A stretcher team took him away once the gas had dispersed. (DW: The Sun Makers)