I recent did this for my server to give "SWAT" what I refer to as a "gas mask" effect.
It's a two part code on both the server and client to stop full effects, but unfortunatly I don't have the EXACT code available to you right now but I can tell you how to do it.
Server:-
On player damage, check if the weapon is tear gas, is yes, check if the person is in the right team or whatever, if yes, cancel event. (Or just give them back the health they lost, I can't remember exactly)
Client:-
OnClientPlayerChoak, if weapon == tear gas && player meets the requirements, cancel.
https://wiki.multitheftauto.com/wiki/OnClientPlayerChoke