Need for drowsiness causing gas
A family friend works for the special cell of Police in India, involved in fighting with terrorists and underworld.
I talked to him about the Taj Hotel terrorist attach incident in Mumbai, 6 years ago on 26/11/2008 and asked him if they could have used some gas to make all people inside the hotel unconscious, thus making it much faster to catch the terrorists and rescue the hostages.
He conveyed that the police does not use any such gases; though there are chemical gases like Chloroform, Carbon monoxide, Methane which can make people unconscious, these gases can kill people with their overdose.
A gas which would instantly knock-down people by making them drowsy/unconscious, in a medically safe manner could be useful in such operations.