Neurotoxin is not an environmental hazard in the traditional sense of gameplay, as it only appears during specific, scripted events in the Portal series, mostly at the game's climax. In the Lab Rat comic, it is confirmed that GLaDOS ventilated neurotoxin as a means of killing all Aperture Science personnel prior to Chell's awakening in Portal, and is frequently used as an attempt to kill her. Later on in the single-player campaign of Portal 2, Wheatley also attempts to kill Chell with it.
The entirety of these gases are stored and manufactured in a reinforced tank, located in the bowels of the Enrichment Center. Eight main hoses allow the gas to be transferred to any location within the facility. During the chapter The Escape, Chell sabotages the system by cutting the eight primary hoses, thereby destroying the generator.
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.
