back before source there was a smoke grenade plug-in for adminmod that changed the smoke to tear gas. It as a bit more advanced than the one I found for sourcemod. The one for source mod just continually gives damage to the person(s) inside the gas cloud.
The one I'm referring to would disorient the player screen, activate and play a cough sound file and then only give damage as the player heard the cough sound as per the amount in the specified cvar per cvar set intervals.