"Posion Smoke"
Did you mean "potion" xD ?
Also... you might want to take a look at the plugin :
http://forums.alliedmods.net/showthread.php?p=1280363
Maybe you can try replacing the entity smoke grenade created by normal smokes by his... (for green gas)
Also a "coughing" effect would be cool (and easier to understand; so you don't think you're being shot).