Deadly Gas

"Dim, through the misty panes and thick green light,

As under a green sea, I saw him drowning."

- Wilfred Owen, "Dulce et Decorum Est"

Lethal gas is found in many forms and in many environments. It is naturally found in many underground settings (in Real Life as well as in games, particularly near volcanoes) and is abundant in the atmosphere of many planets. Humans have used it as a weapon of war, a method of execution, and in certain historical mass death traps.

This gas often has a greenish colour, which is more a case of Technicolor Science than of Truth in Television. While chlorine gas is green, ever since World War 2, nerve gases have been the preferred choice for chemical warfare -- these are colourless and odourless, and can penetrate the body through exposed skin. A common mistake is to depict a gas mask as the full extent of the protective gear needed against nerve gas -- in reality, you would also have to prevent any of the gas coming into contact with the skin.

In video games, being surrounded by poisonous gas is likely to deplete your Oxygen Meter very rapidly.

Sister trope of Knockout Gas, the non-lethal alternative. Not to be confused with Fartillery.

Anime

 * Mazinger Z: When The Hero Kouji and a Mechanical Beast fought near the crater of a volcano, toxic volcanic fumes pervaded the atmosphere (and unlike other examples of this trope, those fumes had a greyish color). Given that the Home Base of the heroes was located on Mount Fuji, it happened more often than you would expect. One example happened in episode 19: Kouji is engaged in aerial battle with a Mechanical Beast -Debira X-1-. During the fight they fly over Mount Fuji, and Kouji's visibility gets hindered by the dense curtain of poisonous gas rises from the crater.
 * The Wolf's Rain episode "The Successors" has a flashback showing how Zali's wolfpack attempted to reach Paradise via a tunnel from their island city. The tunnel filled with toxic gases from the city's factories and many of the wolves died. In the present day the factories have closed, meaning that the tunnel is now safe for Kiba's pack to follow.
 * Magellan of One Piece, Head Chief of Impel Down. Thanks to his Doku-Doku Fruit, his body is made up of poison, and chlorine gas is his most-used type of deadly gas.
 * And waaay down the Sorting Algorithm of Evil there was Don Krieg, who had poison gas canisters in his repertoire.
 * The Punk Hazard arc features Caesar Clown, a chemical weapons expert who ate the Gasu Gasu no Mi which allows him to create, control and turn his body into gas.
 * The Titans in Mobile Suit Zeta Gundam used poison gas on space colonies in an attempt to prevent them supporting the AEUG.
 * The later episodes of Transformers Energon have glowing green "Energon gas".

Film

 * In the first Batman movie, this was the Joker's favorite method of killing people. He did it twice, once in the museum where Bruce was supposed to meet Vicki, and once in the big parade scene with big parade-float balloons full of the Deadly Gas. Batman took the balloons away in the Batwing and sent them well away from the city in order to stop him.
 * And of course in the comics, the Joker just loves his Smilex.
 * The Animated Series and all other DCAU series where the Joker features also have his signature toxin, though there it isn't so much "Deadly Gas" as it is "Insanity-Inducing Gas".
 * Has the "gas masks protect against nerve gas" mistake twice: first with Vicki Vale in the museum and again when Knox uses a filter mask during the balloon attack.
 * Might be a Justified Trope: the Smilex toxin affects the respiratory systems and makes you literally laugh yourself to death, so it might be more like a very potent allergenic agent than an actual neurotoxin.
 * In Star Wars: The Phantom Menace, the Jedi are trapped by the Trade Federation in a room being filled with nerve gas. Fortunately one of the Jedi's powers are to hold their breath for a very long time.
 * And were shown later in the movie to have been carrying breathing apparatuses the entire time. Wait, what?
 * Why use them until absolutely necessary? Those breathing apparatuses are for diving, not gas.
 * The James Bond film Goldfinger has the titular villain using Delta Nine nerve gas to kill his associates, and planning to use it in the Fort Knox guards. In a bit of realism, the gas is transparent (and in a bit of "cheap budget", you only see the gas is being used by the sound effect and affected people falling dead, which turns the Heel Face Turn even easier to figure out).
 * Has the "gas masks protect against nerve gas" mistake when Goldfinger's troops approach Fort Knox.
 * Not necessarily a mistake. With the exception of VX, most nerve agents are non-persistent and are primarily an inhalation hazard because they either dissipate quickly or break down rapidly in the environment. Not only that, but according to The Other Wiki, as long as you do not inhale the gas, you have at least an hour to seek treatment, which is often as simple as an injection of atropine or biperiden. Blistering agents like mustard gas, obviously, require full-body protection, though a gas mask would at least prevent you from having your lungs burned.
 * The James Bond film Moonraker centers around a villain's plan to build himself a space station, then drop satellites filled with enough nerve gas to kill everything on Earth. One scene involves James infiltrating the lab where the nerve gas is being synthesized, taking out a nerve gas canister and looking at it in puzzlement, then sloppily putting it down and hiding in the next room (which conveniently has glass windows and an airlock separating it from where he was) when the scientists come back. The scientists fail to notice the moved canister until one of them knocks it off the table, shattering it-giving Bond a demonstration of the canister's contents and purpose, as the scientists immediately die horribly.
 * The RDA in Avatar uses tear gas before they cut down the tree's pylons with missiles. Not strictly lethal, but very unpleasant, and very visible.
 * Contrariwise, Pandora's atmosphere is toxic to humans.
 * A simple breathing mask is all that humans need to survive the atmosphere for extended duration while leaving the rest of the body completely unprotected. Even getting a good lung full of the atmosphere seems to cause no lasting effects once a human gets back to breathing oxygen. Presumably, then, the Na'vi need oxygen just like we do; the main difference is that there's probably something in their atmosphere (maybe something inert, similar to carbon dioxide but not as greenhousey?) that is toxic to humans, and the filter masks just take it out.
 * This troper suspects hydrogen cyanide. The local life likely exhales it.
 * Supposedly, the atmosphere of Pandora is extremely similar to Earth's save for a few key differences: it's 20% denser, contains 18% carbon dioxide, and 1% hydrogen sulfide. Breathing it in is more or less a lot like suffocation, meaning you'll pass out in 20 seconds or so and die in a couple minutes. Recovering from exposure shouldn't have any sort of effects other than the usual, and I guess prolonged exposure, like more than two minutes, might result in brain or lung damage. So really, all those masks do is scrub out the extra unwanted crap from the atmosphere.
 * The Rock uses a real-life gas, VX (and apparently the symptoms, aside from "your skin melts off", are as well).
 * Although, the 'your skin melts off' aspect was extremely played up-witness the dramatic death of the Marine-turned-Terrorist that gets locked in with the spilling VX during the initial theft of the missiles. Naturally, Nicholas Cage does not suffer this before managing to inject himself with antidote.
 * In 9|Nine, gas bombs were the main method used by the machines to kill all the humans.
 * In The Assassination Bureau, is asphyxiated when his saber pierces one of the hydrogen balloons in the airship.

Live Action TV

 * In the 2008 Doctor Who two-part story "The Sontaran Stratagem" and "The Poison Sky", the Sontarans use atmospheric converters fitted to cars in an attempt to replace the Earth's atmosphere with a grey, smog-like vapour that is deadly to humans.
 * See also: "The Sound of Drums": Harold Saxon kills his cabinet by Deadly Gas. Hilarity Ensues.
 * Shows up fairly frequently in the Classic Series as well; Underworld and The Caves of Androzani are two examples. The Fifth Doctor serial Planet of Fire" inverts the trope by focusing on a gas with particularly healthful properties.
 * In Torchwood: Children of Earth, the 456 live in an opaque, poisonous gas. It doesn't kill anyone though, merely adds to the sense of mystery/horror.
 * Twenty Four had a number of canisters of this as a MacGuffin.
 * One That Mitchell and Webb Look sketch had Numberwang going to Sudden Death, where the winner was the first contestant to inhale enough of the "poisonous number gas" to kill them.
 * In a second season episode of Fringe, we have a creepy Nazi who's genetically tailoring a design toxin that will cause fatal asphyxiation to anyone who isn't blond and blue-eyed. In a crowning moment of badassery, Walter displays his hatred of Nazis by coldly re-designing the same toxic gas to only kill the creepy Nazi guy. At a crowded peace summit no less.
 * Instead of deadly gas, the Scarrans in The Peacekeeper Wars use a paralyzing gas. Scorpius notes that they use this on subjects they wish to dissect...while they're still alive.
 * Immortal Xavier St. Cloud on Highlander liked to kill rich people with poison gas and then steal stuff from them. Or as in the actual episode, rob a jewelry store with that method. He also uses it on a WW 1 battlefield, killing everyone except immortal Duncan-but can't behead him before immortal Darius shows up and he scurries off.
 * Giles, Buffy and Willow almost get gassed on Buffy the Vampire Slayer in "Out of Sight, Out of Mind", when an invisible girl locks them in the school boiler room and turns on the gas. Angel fortunately arrives in time and shuts it off. They're almost unconscious, but Angel, being a vampire, doesn't have to breathe.

Literature

 * Various kinds of deadly gas show up in the Star Wars Expanded Universe.
 * Wedge's Gamble had some Rogues being balked by a room flooded with Fex-M3d, which would bind to neuroreceptors and make them suffocate. Fortunately, it turned out that one of them didn't breathe.
 * Korfaise gas was a coolant used on YT-1000 ships like the Millennium Falcon. It was also dangerous to breathe, and in The Thrawn Trilogy Han Solo cut the coolant lines on a YT-1000 to incapacitate the people inside.
 * In Allegiance, the heroes find a wall with embedded canisters of acidic, corrosive cryseefa gas, and have to go through the wall without touching the canisters or setting off the alarms that would make the canisters release.
 * In Z for Zachariah, nerve gas is used alongside a lot of depopulation bombs, causing a Class 2, possibly borderline Class 3 Apocalypse How.
 * In Michael Slade's Ghoul, a paranoid bomber in London attempts to kill everyone in a theater with this trope, which he planned to generate by incinerating a whole lot of PFOA-laden Teflon.
 * The Martians in The War of the Worlds use the "Black Smoke", a heavy, ink-dark poison gas compound that kills instantly on being breathed. They stay above the dense clouds in their tripods. The toxin is also designed to be disabled by contact with water, which means that it would pose no long-term danger to them in rainy Britain.
 * The Lensmen universe has Vee-Two, a paralysant which is lethal if not reversed within a short space of time. The Triplanetary Patrol carries the antidote at all times because space pirates are always using it, a fact used to effect by Conway Costigan on two occasions. Although it's clearly a neuroactive gas, it seems to act by inhalation only as a mask is protective.
 * Animorphs :all Gleet Bio-Filters at Yeerk Pool enterances are equipped with poison gas in case of entry attempts by unauthorized life forms. The main characters barely make it out alive when they trigger one.

Tabletop Games

 * Dungeons and Dragons includes numerous poison gases, ranging from the merely annoying Stinking Cloud to the extremely lethal Cloudkill.
 * Merely anoying? Stinking Cloud is one of the most powerful spells you can get at the level. It covers a large area and every creature that gets affected can only stagger around slowly and needs about half a minute before being back into the action. It doesn't kill anything, but makes killing them so much easier.
 * Apart from spells that create clouds of poisonous gas, like those mentioned above, there are also mundane poison gases, such as Burnt Othur Fumes and Insanity Mist.
 * The 1st Edition adventure Hidden Shrine of Tamoachan is set in an abandoned temple which is filled with a mild toxic gas. It causes gradual hp loss every few minutes until the heroes reach a more-ventilated level of the complex.
 * GURPS: Ultra-Tech and High-Tech both have generic poison gases. Ultra-Tech also has a clever variant on it, a cloud of microbots that inject the poison into people.
 * Shadowrun has two types of nerve gas: Green Ring 3 (from Real Life) and Seven-7, a fictional gas that can penetrate normal chemsuit protection.
 * Classic Traveller. Several adventures in Supplement 6 76 Patrons involve PC attempts to acquire nerve gas produced by the Mega Corp Schunamann und Sohn, AG.
 * The Imperial Guard in Warhammer 40000 have access to a tank variant known as the Bane Wolf - it's only short ranged, but fires a cloud of toxic chemicals that kills off almost all infantry in one blast.

Video Games
"Gruntilda: Mmmm, cyanide and mustard gas flavor. My favorite!"
 * The gas you can throw in MAG is substantially less dangerous than the other examples on this page.
 * This is somewhat common in WoW. You would be surprised how many people don't understand how to not stand in it.
 * In Final Fantasy Crystal Chronicles, the plot is about removing miasma (deadly purple gas) from the world.
 * In Banjo-Tooie, Deadly Gas can be found in many places, including the part of Glitter Gulch Mine where you rescue Canary Mary, packed into barrels in Grunty Industries (many of these barrels are trying to kill you, of course), and released by Gruntilda as part of the Final Boss battle.

"Scorch: That's the last one! I guess we know how long Trandoshans can hold their breath."
 * Super Mario 64 has the Hazy Maze Cave, which has the aptly-named Hazy Maze complete with poison yellow gas. Unless Mario is wearing the Metal Cap, prolonged stays in the Hazy Maze prove to be fatal.
 * In the DS remake, the gas is purple, and it's Metal Wario that can survive in it. If any character gets a hidden Mega Mushroom, they can stand in it since their heads are physically above the gas... until the mushroom wears off. Also, any character in the gas actually coughs visibly if you don't move them while losing health.
 * Don't forget the unique death animation for this. Your character actually chokes and falls over, dead.
 * Greenish poison gas -- and it's always greenish poison gas -- is quite common in Deus Ex. It's available in movable/puncturable barrels, for a little variety from the usual kind, as well as gas grenades. At one point you, can play with some in the ventilation system and gas a whole facility of people.
 * The green poison gas returns in Deus Ex Human Revolution, in the form of occasional environmental hazards and gas grenades. Jensen can get an augmentation that negates the effects of gas, allowing him to use grenades like smoke bombs when mobbed. Also, toxic gas is specifically not deadly (for the enemies) in both games, and a gas grenade is easily the best nonlethal option for groups of enemies (important in Human Revolution if you're chasing after the Pacifist achievement).
 * In King of Fighters '95, poisonous gas makes an appaearance... but only in cut scenes, as Rugal uses sleeping gas to kidnap the Ko F champions and bring them to his presence.
 * The Legend of Zelda: Twilight Princess has one area with a deadly purple gas/mist that can be temporarily locally dispelled by waving a lantern about.
 * Although not itself deadly, there is a purple poison gas in Wind Waker that temporarily disables your ability to attack or use items, and there are usually enemies in the gas. Later in the same dungeon there are purple flying skulls with the same ability.
 * Sonic & Knuckles features Toxomister badniks in the Lava Reef Zone that spew clouds of deadly gas. If you get caught in one, it slows your speed to a crawl and drains your rings until you die. The only way to dispel the cloud is to destroy the Toxomister that made it.
 * In Portal, in the final level, after you, she starts releasing a deadly neurotoxin into the room; you have only six minutes to finish the job before the gas kills you. How did she get the deadly neurotoxin? It appears that the researchers gave it to her (presumably related to Schrodingers Cat). And then she killed them with it.
 * Portal 2 brings the deadly neurotoxin back and makes it a greater part of the plot. Part of Chapter 4 involves shutting down the production facility so GLaDOS will be deprived of it as a weapon. It then makes a reappearance, albeit briefly, in the Final Boss fight.
 * The second game, and all the side media like the Ratman comic, firmly establish Deadly Neurotoxin as GLaDOS' Weapon of Choice.
 * The Metal Gear series often will fill rooms with poison gas, which necessitates either very brief stays or a gas mask.
 * In Command & Conquer: Tiberian Sun, the Brotherhood of Nod weaponizes Tiberium in the form of a Chemical Missile, which detonates over the target area and saturates it with noxious clouds of highly-corrosive gas. Its effects on infantry are... unsettling, to say the least.
 * Before that, the Covert Ops addon for the first game had Chemical Troopers, a variation on the normal Flamethrower infantry who sprayed Tiberium gas, instantly fatal to any non-Chemical Trooper infantry.
 * The Intruder Excluder level in Battletoads has traps that shoot small puffs of gas, instantly fatal.
 * The Borely Haunted Mansion in Silent Hill 3. In one section, Heather must avoid a strange red gas that follows her from room to room. No matter what difficulty you're playing on, if the gas touches you it's instant death.
 * The Jagged Alliance PC games let both your mercenaries and the enemy chuck mustard gas grenades around, unleashing clouds of ridiculously lethal yellow gas. Not instantly lethal, but it may as well have been since characters would collapse after one or two rounds of exposure and invariably die from poisoning in the middle of the gas cloud.
 * Iron Storm, an Alternate History FPS that extends WWI-era fighting and technology many years after the real world's WWI had ended, features mustard gas grenades.
 * Doom 3 includes a level centered around escaping a location which is slowly filling with poison gas. Said gas obeys all traffic laws in that it is properly green and slowly drains your health (complete with choking sounds)
 * Harvest Moon Magical Melody, in the mines, rocks sometimes give off certain gasses with different effects, some bad, a few good.
 * This also happens in Tree Of Tranquility.
 * In Resident Evil: Outbreak File #2, you must search the Raccoon City Police Station to find all the necessary items to escape the zombie siege. As a "riot control" measure, the crazy police chief has wired up his own police station with random nerve gas dispensers.
 * Parasite Eve 2 used this by trapping Aya in a sterilization room and releasing the gaseous chemicals that slowly drain her HP unless she jumps in the garbage chute to escape. Also used again later on when she and are trying to escape from Golems that fill the room with gas. While Aya isn't affected, despite her coughing, it hurts  and saps her HP; having her die results in a Game Over.
 * The old FPS Rise of the Triad had several rooms fill up with deadly gas shortly after you walk in. There was always at least one gas mask somewhere in the room and picking it up would protect you from dying. Since the enemies were never clever enough to do the same you could then just wait for them to die. The gas drained away by itself after a minute.
 * In Hexen the Flechette item works differently depending on your character class. If playing as a Cleric dropping a Flechette creates a cloud of Deadly Gas that will severely damage anything trying to walk through it (including your character).
 * Also, certain levels in Hexen contained special mushrooms that emitted Deadly Gas. The mushrooms were covered in green pustules, made weird squeaking noises, and released the gas when damaged.
 * Fox gets trapped into a room full of poison gas in Star Fox Adventures, where he has to solve a Block Puzzle in order to escape.
 * In Final Fantasy VII, Tifa is nearly executed this way.
 * In an old Apple II text adventure game with graphical aid, Mask Of The Sun, at one point you are locked in a room filled with gas (which the game presents by showing an entirely green screen). To get out without dying (and you have a limited time), you have to figure out the exit is on the southwest, a direction atypical to text adventures at the time.
 * In Breath of Fire V: Dragon Quarter, after Ryu, Nina, and Lin return to the End Sector Borough to find three Rangers have ambushed them with a tank of poison gas.
 * All over the place in Knights of the Old Republic. Sort of subverted in that you can break into computers to use the gas systems against your enemies.
 * The Metroid Prime games included Puffers, which upon exploding, released green gas that was harmful to Samus.
 * Also, in ''Metroid Prime 2: Echoes", Dark Aether's entire atmosphere, as well as the gas that Emperor Ing's second form releases into the room.
 * In Dead Space, one mission has Isaac fighting his way through the ship's hydroponics lab, killing necromorphs that were poisoning the atmosphere.
 * In Crystalis there was an entire swamp filled with toxic gas. If you didn't have a gas mask, you would probably die before you reached the town hidden inside because of how rapidly it drained your Health Points.
 * In The Suffering, one of the bosses,, is anthropomorphic poisonous gas, specifically the kind used in prison gas chambers. The green color is justified almost immediately; while the actual gas used for executions is invisible, he likes to make his gas visible to let people he kills know they are about to die.
 * The Gas Grenade from Syphon Filter. In the first game, it was the only way to beat the Final Boss, who was wearing full-body armor that made him Nigh Invulnerable, but still required him to breathe.
 * In The white chamber, it's possible to die from toxic gasses that result from not getting rid of
 * IG-88 in Star Wars Empire at War: Forces of Corruption has, as his land special abilities, an ability that irradiates an enemy unit, and one that release poison gas over an area. The gas is toxic to enemy and allied units.
 * In the N64 game for The World Is Not Enough, there is one point in the game where security camera's detect you and lock you in a room with poison gas coming in through vents, which you can only escape by using the grappling hook in your wristwatch. It is, however, possible to escape this by shooting the camera's, which promptly explode, before they detect you and gas starts pumping into the room.
 * Resident Evil games are ridiculous with poison gases - ineffectiveness. They are often used if you fail to solve a puzzle correctly. In many cases, gases pumped into a room won't do anything until after some arbitrary time limit passes and kills you instantly. In one case, gas is pumped into a room where it sits as a yellow cloud at knee-height and never moves until you vent it. As this Let's Play says: "Diffusion? Who needs it?"
 * The second run through the detention block level in Republic Commando ends with you defeating the ambush with the anti-jailbreak system, which releases Deadly Gas in a rather fruity pink colour. Fortunately your squad is immune since the designers remembered why Storm/Clonetroopers are Faceless Mooks and have your armour provide HAZMAT protection. The enemies...aren't so lucky.


 * Little Big Planet: Horrible Gas. If a Sackperson touches it (even with their arms or feet, not just their head), they dissolve. Horrible Gas actually comes in six colors, but green is the most common. The others are used mostly to make the player think it's something else, like gray gas for fog, or purple gas for some kind of monster-related thing, as seen in the "Monsters" level pack. Unless it's deliberately attached to something else (done by gluing a material to a Piston or another material before lethalizing it with the Horrible Gas Tool, as shown by the of the third level of The Gardens), it stays in place and cannot be moved. Anything that isn't a Sackperson can move through the gas, however. A lot of level creators have this as their hazard of choice, though electrified materials and the plasma balls of the Metal Gear Solid pack are just as lethal. Fire, on the other hand, takes two hits to kill a Sackperson.
 * In Ratchet and Clank, the planet Orxon was covered in green, noxious gas from the pollution, making it impossible to explore without the oxygen mask.
 * Green gas barriers are a standard obstacle in the Overlord series, usually put in place to keep you from advancing too far until you've gotten your Green Minions back.
 * In Perfect Dark, Joanna Dark infiltrates Area 51 to rescue an alien survivor. When she reaches the room where he is located, the scientists flood the room with poison gas. Of course, the scientists kind of forgot they were still in the room as well.
 * Prototype features Bloodtox, a red gas that causes necrosis in infected tissue. It's apparently completely harmless to humans, but it drains Alex Mercer's health (but its stated he's growing resistant to it).
 * Stubbs the Zombie. It doesn't matter if the main character sets off the poison gas, because he is a zombie. It's a problem if you are remote controlling a human and still have some more victims to kill. Oh well.
 * The main area of the Twin Labyrinths in La-Mulana has a Deadly Gas trap represented by clouds drifting along the screen and a 30-second timer. To eliminate the gas, you must weight two pedestals before the timer runs out; curiously, if you fail, you get booted back to the previous area instead of dying.
 * "Oh no! Green gas! That's the worst kind!"
 * Green gas makes players faint in Backyard Football.
 * Samara's recruitment mission in Mass Effect 2 involves storming a mercenary base full of tanks of a tainted gaseous drug which an unscrupulous merchant sold to the mercs. The deal is, inhaling the gas gives the user a sizable boost to biotic abilities, but causes death after prolonged use, a fact that the merchant happened to not mention. Gameplay-wise, the canisters can be detonated during the level, and standing in the gas temporarily improves the squad's biotic ability, but standing in it for too long will lead to a Game Over.
 * Worms features the worlds least-deadly deadly gas, as it doesn't actually kill affected worms, just slowly reducing their life points to 1.
 * Call of Duty Black Ops has Nova-6, a chemical weapon originally developed by the Nazis then refined by the Soviets. A cloudy, green gas that causes near-instant necrosis all over a victim's body, the latter version capable of killing in seconds with minimal exposure,
 * Modern Warfare 3 also has deadly green gas used on the battlefield.
 * The Pokémon Koffing and Weezing have bodies that contain very poisonous gas and there are several Poison-type moves related to gas, like Smog, though some (despite being dubiously named) are perfectly harmless. Not to mention there's also a move specifically named "Poison Gas." It poisons things.
 * No One Lives Forever has cyanide gas, which gives you hallucinations while draining your Hit Points. Oh, and it looks green. The cyanide bullets have the same effect.
 * In one level of Star Trek Elite Force II, your team is trapped in a room that is being filled with poison gas (how the aliens knew it would affect any humanoid is not explained). You, as Alexander Munro, have to use your Type 2 phaser (which has a continuous beam) to fuse the gas vents. After that, the gas that has already been pumped into the room is, apparently, rendered harmless.
 * In Dragon Age II, some thieves steal a Qunari poison gas called sar-qamek in the hopes of creating enough deaths in order to have the residents of Kirkwall riled up to drive out the Qunari.
 * Gas grenades in Syphon Filter kill instantly.
 * Runescape: In "The Path of Glouphrie", brightly-colored poison gas is used as part of a Death Trap.
 * Nerve gas is a tactical aid option in multiplayer matches of World in Conflict. True to real life, it's horribly lethal to infantry while leaving structures and vehicles unharmed. Useful for digging infantry out of buildings you want to use for yourself.
 * Golden Eye 1997 has a room full of gas tanks in the Facility level, but if the tanks are destroyed the gas will slowly kill you and you will fail the level.
 * In Dragon Quest IV, there's a yellowish gas coming out of Mamon mine . While it makes the locals sick and eventually kills them, it has no effects on player characters.
 * In Ys II, one area is filled with toxic gas, which the Roda Tree Leaf protects against.

Web Comics

 * MAG ISA -- Fr. Jose gets poison gas sprayed at him by a demon in the confessional.
 * In Darths and Droids, following the plot of the Star Wars prequel trilogy, the room where the characters were supposed to hold some negotiation is filled with toxic gas. The players continue talking in character, while holding their breath. Lampshaded of course, by the DM.

Web Original
"Pom Pom was filled with a deadly gas, which killed the Popsmith very fast."
 * Carina Appelbaum from v2 of Open Blue included vials of Deadly Gas amongst her many other mysterious chemical weapons, which explained why the crew and marines on her ship all wore gas masks.
 * As his name implies, the villainous Belcher of the Global Guardians PBEM Universe can let loose with weaponized burps. Mostly he uses noxious gases, but has been known to let loose with a gout of fire occasionally.
 * In one Homestar Runner cartoon, Strong Sad imagines all the rest of the cast meeting horrible fates. He drops an O from the Poopsmith's name and has him pop Pom Pom, leading to:


 * In Salvation War, during the battle for the banks of the Phlegethon river in Hell, the Russians drop sarin gas shells on their own tank positions when they are overrun by Beelzebub's fire-breathing harpies; most of the crews were safe inside their sealed positive-pressure tank hulls, but the harpies were completely unprotected and were killed in their thousands.

Real Life

 * As mentioned before, chlorine is distinctly green, and rather nasty to breathe. Chlorine, however, will generally not try to eat its way out of its container, unlike its sibling one row up. Fluorine is a yellowish-amber color, but if you ever see enough of it to see the color, you'd best hope it's properly secured or you're already dead. The heavier halogens are easier to handle since they don't evaporate at room temperature, but if you do heat them up the vapours are almost as nasty.
 * Even fluorine pales in comparison to chlorine trifluoride, which will happily set things like water, bricks, sand, and asbestos on fire. Said reactions will inevitably produce dangerous by-products like hydrofluoric acid.
 * As a definition of 'dangerous' above; at university students are told that putting your hand into hydrofluoric acid would be painless as it dissolves the flesh faster than the nerves can transmit the pain signals. Stopping putting your hand in is going to sting a bit though.
 * Although the above is false. HF will pass through skin, leaving a mild burn...and painfully dissolve your bones, spreading along your skeleton.
 * A good number of toxic gases were used as weapons in World War One along with chlorine -- phosgene, lewisite, mustard gas, among others. Adolf Hitler experienced this personally and refused to allow their use in World War Two.
 * The Russian authorities in the 2002 Moscow theatre siege used (tragically ineptly) a Knockout Gas called Kolokol-1, believed to be a highly potent derivative of fentanyl, an opioid so much more powerful than morphine already that it can only be given to cancer patients who have become morphine-tolerant.
 * Fentanyl (and a five to ten times more powerful variant called Sufentanil) are extremely commonly used IV analgesics for surgical anesthesia. Sure, you wouldn't use it for extended periods, but that's mainly because it's powerful enough to easily cause respiratory suppression. (As the Moscow police learned, we might note.) Terminal, drug addicted cancer patients are hardly its core demographic.
 * The 1998 Tailwind scandal, based around a (possibly but not definitely discredited) CNN story of Vietnam-era war crimes supposedly committed by US troops in Laos, revolved around a dispute over whether chemical weapons were used, including nerve gas and/or tear gas. At least one account of the story included a chemical code-named "Glink", which was supposed to paralyze everyone on the ground where it was spread; the idea was that medics were supposed to show up on rescue helicopters and give the antidote only to friendlies. (Disputes over the accuracy of the report led to a round of firings at CNN, but the original producers of the report stand by their content.)
 * According to experts, hydrogen sulphide (the rotten eggs smell gas) is odourless in lethal concentrations (no idea how they discovered this). So it you stop smelling it then you are either perfectly safe, because it has gone, or going to die, because it hasn't.
 * It's primarily an issue of time, not concentration - your nose adapts to it after you've smelled it for a while. So if you're exposed to a slowly increasing concentration, yes, you'll stop smelling it before you die; but at a high enough concentration you'll drop dead before you stop smelling it, and at a low steady concentration you can stop smelling it and not be harmed. Sub-lethal concentrations of hydrogen sulfide occur commonly enough that it was easy for people to learn about this property.
 * Oh, and don't mix ammonia and bleach together. You'd think it would make your cleaning job go faster by combining two cleaners. In reality, it's more likely to create a nasty gas called chloramine, which can knock you unconscious and/or kill you. (This is especially true if your mixture is very concentrated, and/or the room poorly ventilated.)
 * And, in fact, people have died for this exact reason. In Philadelphia in early January 1995, a 61-year-old man mixed bleach, ammonia, and a commercial drain cleaner to try to unclog an upstairs toilet. The result was chlorine and phosgene gas, which quickly knocked him unconscious followed by death due to suffocation. This may have been the inspiration for the 1000WaysToDie story "Trailer Trashed".
 * Zyklon B, the gas used by Those Wacky Nazis in so many gas chambers during the Holocaust.
 * Semi-averted by the series of nerve agents. While they are a chemical weapon, their gaseous form is invisible to the naked eye. If there is a high enough concentration that you can actually see the nerve agents, you are probably to busy dying in a horribly painful manner too care.