Neurotoxin is not an environmental hazard in the traditional sense, as it only appears during specific, scripted moments in the Portal Series, usually at the game's climax. Neurotoxin was used by GLaDOS in order to kill all Aperture Science Personnel prior to the commencement of Portal, and is also used by GLaDOS and Wheatley in an attempt to kill Chell.
The Neurotoxin is stored in a reinforced tank (also known as the Neurotoxin Generator) which is located in the bowels of the Aperture Science Facility. Eight main hoses allow the gas to be transferred to any location within the Facility. During Chaper 5 in Portal 2, Chell sabotages the system by cutting the eight primary hoses, thereby destroying the Neurotoxin storage tank.
The variety of Neurotoxin used within the Aperture Science Facilty is unusual in that a certain concentration is required before it becomes lethal. Until the saturation level has been reached, the gas is not lethal and is presumed to be merely toxic. It is this delay that provides Chell with the opportunity to defeat GLaDOS in Portal and, later, Wheatley in Portal 2.
