Poison gas

&nbsp Nerve gas is a type of toxic gas that is often used as a security measure against intruders. It is named as such because it directly infiltrates the person's nervous system and attacks it before the person succumbs to the poisoning.

Big Boss/Outer Heaven/Zanzibar Land
During the 1995 Outer Heaven Incident, Outer Heaven's second building basement floor and the third floor of Outer Heaven's first building contained nerve gas to prevent intruders. In addition, several areas of Outer Heaven's third building's 100th floor basement were gassed as a security measure against infiltrators from reaching the TX-55 Metal Gear. Solid Snake, during Operation Intrude N313, utilized a gas mask to bypass these areas.

The Zanzibar Fortress's main building's third floor was frequently gassed, as various research was being conducted there. In addition, the swamp armory as well as the biological research lab also released nerve gas if an intruder was to break in and set off the alarm. Because of this, the Zanzibar Land researchers and personnel were strongly advised to don gas masks when working/guarding these areas. Similar to Operation Intrude N313, Solid Snake, during Operation Intrude F014, managed to evade these areas by procuring a gas mask.

Shadow Moses
During the 2005 Shadow Moses Incident, FOXHOUND and the Next-Generation Special Forces, upon taking over control of Shadow Moses Island as well as the in-development Metal Gear REX, released organophosphates (such as Soman) into the western labs in order to prevent Dr. Hal Emmerich from escaping. In addition, the tank hangar building of Shadow Moses's garage was designed to release organophosphates into the garage and seal up the garage if an intruder managed to trip one of the infrared sensors, or if the intruder was spotted. The control room in the underground maintenance base was also equipped with nerve gas release valves if the intruder was detected as having infiltrated by one of the cameras. These gas releases could also be activated remotely, as evidenced by their activation shortly after Liquid revealed his ruse as Miller when REX was unwittingly activated by Solid Snake. The nuclear warhead storage buildings would also release gas when an intruder was detected, and it was for this reason why the NBC unit of the Next-Generation Special Forces often donned gas masks, as they can't risk using their heavy weapons in the area. Solid Snake managed to evade them by avoiding tripping the infrared sensors in the tank hangar as well as being spotted, and also donned a gas mask in the event that he did have to go through them. In addition Otacon unlocked the door to release Snake after Liquid activated the gas, also resulting in the gas dissipating. Because of the Soman being naturally colorless in composition while in its pure form, the Sons of Big Boss, and presumably the original owners of Shadow Moses, had colored the gas yellow to make detecting a leak easier.

Unconfirmed usage
Miller's assassin managed to leak nerve gas into Kazuhira Miller's house, which Miller attempted to escape from by donning gear in his ice cave, but ultimately didn't make it before he succumbed to the nerve gas poisoning.

Behind the scenes
Nerve gas, otherwise known as poison gas, was a recurring element in the Metal Gear series. In the original Metal Gear, gassed rooms were depicted as having puffs of smoke emerging from the ground. Metal Gear 2: Solid Snake and Metal Gear Solid depicted the gas as having flashes of color (red and yellow, respectively), as well as a hissing sound.