The Tear Gas grenade is a non-lethal chemical device used to temporarily hinder a person's ability to see, as well as causing massive pain to the target's eyes. While it does affect enemies for a while, it does not knock them out. This grenade type is ideal for the Panther playstyle in Splinter Cell: Blacklist.
Tear Gas grenades release a chemical agent formulated to severly irritate the corneal nerves in the eyes, causing tears, pain, and blindness in any person affected by it. Any person within the cloud radius will be knocked out. Tear Gas is commonly used as a form of riot control, or non-lethally subduing enemies.
In-game, the tear gas is somewhere between smoke and sleeping gas, it will provide a brief concealment and disorientate the enemies if caught in the gas, but will not kill/knock them out. This will cause the guards to be alerted to your presence, thus this gadget is not practical for Ghost and Panther styled players, sleeping gas is a more practical choice as it will knock out anyone without gas mask. Fisher himself will take damage if too close to the gas, so exercise caution if you do want to use it.