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.
Another variation is a gas that is not inherently harmful, but fills the space instead of breathable air. In this case, the danger is simple suffocation, and a breathing mask with an oxygen supply is sufficient protection.
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.
This trope is always present in a Gas Chamber. A villain using one has crossed the Moral Event Horizon.
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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.
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.
He also has a Blob Monster pet named Smiley, made from the poisonous gas that he unleashed on Punk Hazard four years before the story began. He later fed it a candy that changed it back into its original gas form, only MUCH deadlier this time around.
The Titans in Mobile Suit Zeta Gundam used poison gas on space colonies in an attempt to prevent them supporting the AEUG.
Haman Karn used an earring full of cyanide gas in a failed attempt to assassinate Titans leader Jamitov Hymen at one meeting.
In one issue of G.I. Joe: Special Missions, the Joes have to find a way to stop a Nazi bomber full of experimental nerve gas frozen in a glacier in Greenland from falling into the hands of COBRA.
Captain Mar-Vell once had to fight a supervillain who had stolen a container of a deadly nerve gas called Compound 13 and received a dangerous exposure while sealing the leaking container. Although he received an antidote to save his life, the gas also to be carcinogenic and Mar-Vell died from cancer resulting from that exposure.
In Worldwar: War of Equals the Ukrainian government authorizes the use of untested nerve gas on Race forces, killing all who are exposed and in the event that Race forces overrun Jordan, the Syrian government will break out their formerly secret chemical weapon program.
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.
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 they were shown later in the movie to have been carrying breathing apparatuses the entire time. Wait, what?
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).
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.
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.
In The Assassination Bureau, General von Pinck is asphyxiated when his saber pierces one of the hydrogen balloons in the airship.
The Wild Geese. As they're fighting a war with no Hague Convention, the mercenaries use cyanide gas to silently kill guards in their sleeping quarters.
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 but which will nurture billions of Sontaran clone warriors.
You also have "Gridlock", which features most of the population of New New York all crammed into flying cars in an underground interstate. The exhaust fumes are pretty dangerous for any New Earthlings (or Time Lords) who step out of their vehicles, but the Giant Alien Crabs like it just fine.
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.
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. We are told that the number 2 is deadly to humans, and it can be seen floating in the tanks.
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 WW1 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.
One of the best Death Trap scenarios in the Adam WestBatman was when the Duo was captured and challenged to avoid drowning in an industrial smokestack for period of time by The Joker. However, the Joker doublecrosses them by filling the smokestack with a deadly heavier than air gas, noting "Who said anything about water?" When Robin protests that you can't float in gas, the Joker responds, "No, but you can drown in it."
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.
The villain of Galaxy of Fear: The Doomsday Ship had some such gas at his disposal, but held off on using it because it would kill the heroes too quickly, which wouldn't be as much fun.
Blaine the train from The Waste Lands releases this upon the citizens of Lud just before leaving the city. When we find out why, it's because they were boring to him.
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 Lensman 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.
In Dead Beat, the White Council has evacuated their wounded to a hospital in the Congo. The vampires cannot attack directly because it's broad daylight (and because the Council has thrown up some very strong wards), so they have their thralls bomb everything within a six-block radius of the hospital with sarin gas. Tens of thousands of completely uninvolved mortals were wiped out, along with dozens of Council Wardens.
Dungeons & Dragons includes numerous poison gases, ranging from the merely tear gas-like Stinking Cloud to the extremely lethal Cloudkill.
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 40,000 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.
The gas you can throw in MAG is substantially less dangerous than the other examples on this page.
Weaponized by the Forsaken in the form of Blight, a virulent plague hurled in barrels by their catapults. To make matters worse, over time large concentrations can settle into pools that may spawn sentient slime.
In Banjo-Tooie, this 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.
Gruntilda: Mmmm, cyanide and mustard gas flavor. My favorite!
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.
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.
Although not itself deadly, there is a purple poison gas in The 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.
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.
It also makes a brief appearance after you break out of the testing chambers. GLaDOS will attempt to get you back into the chambers by opening up a chamber with a heart on the wall. If you are stupid enough to go into the chamber, she will flood it with neurotoxin, killing you instantly. This leads to some Fridge Logic: Why did it take the neurotoxin 6 minutes to kill you in the first game, but only 3 seconds in the second game?
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. note Mustard gas is strongly mutagenic and carcinogenic, and causes severe chemical burns and bleeding in the repiratory tract - there's a reason it kills so quickly. The vesicating effect is what is most lethal. Victims might well drown in fluid released from the blistering in the lungs.
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) up until the second you finally hit the "Vent Gas" switch, at which point it is instantly drained from the entire location and ceases being harmful. Apparently the Mars base has some powerful gas vents.
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 and Animal Parade.
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 Eve are trying to escape from Golems that fill the room with gas. While Aya isn't affected, despite her coughing, it hurts Eve and saps her HP; having her die results in a Game Over.
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.
In an old Apple ][ 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.
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.
Crystalis had 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, a former warden responsible for gas executions and killed himself in the gas chamber, 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.
In the white chamber, it's possible to die from toxic gasses that result from not getting rid of the dead body inside the fridge.
The Powder Toy has caustic gas, which dissolves everything it touches and is sometimes used in bombs.
IG-88 in 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.
In Star Wars: Republic Commando, the second run through the detention block level 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.
Scorch: That's the last one! I guess we know how long Trandoshans can hold their breath.
LittleBigPlanet: 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 ghosts 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 & Clank, the planet Orxon is covered in green, noxious gas from the pollution, making it impossible to explore without the O2 Mask that you obtain from Pokitaru. When it reappears in Ratchet: Deadlocked, the atmosphere is apparently capable of sustaining life again.
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 Zero's Laboratory Rescue mission, the laboratory gets flooded with green gas that slowly drains your health and prevents it from regenerating until you activate the ventilation.
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, in the original version, if you fail, you get booted back to the previous area instead of dying; in the remake, you lose health very rapidly instead.
TimeSplitters: Future Perfect features a gas-chamber deathtrap on Khallos' train. Lampshaded by Harry Tipper:
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, and planned by Dragovich to be released all over the United States. It's available as a tactical grenade in multiplayer, though it's nerfed to an incapacitant rather than instant death.
Modern Warfare 3 also has deadly green gas used on the battlefield. Makarov's men used dirty bombs in most major European cities, crippling the nations and paving the way for Russian invasion.
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 hoping to incite anti-Qunari feeling steal what they think is the formula for Qunari blackpowder. It's actually a decoy - the formula for a poison gas called sar-qamek. Not only does it kill, but it first drives the victims mad and turns them against each other.
RuneScape: In "The Path of Glouphrie", brightly-colored poison gas is used as part of a Death Trap.
In Ys II, one area is filled with toxic gas, which the Roda Tree Leaf protects against.
The original Alone In The Dark 1992 has a smoking parlor where the smoke will drain Carnby's health unless you extinguish the ashtray with a water jug.
In Fallout: New Vegas: Dead Money, the Sierra Madre is contaminated by the Cloud, a highly toxic red smog that also corrodes metal. This may have inspired the Pink Cloud in Fallout: Equestria. On Hardcore mode, it slowly saps the Courier's HP outdoors, with more concentrated pockets causing faster health loss.
Pops up a few times in horror adventure game The Cat Lady in the form of the deadly hydrogen sulfide. Mitzi's boyfriend uses it to kill himself, Susan uses it to kill the second Parasite, and the Eye of Adam uses it to attempt to kill Susan and Mitzi, claiming his own father's life in the process.
Reisen in the Touhou fighting games has a spellcard called Poison Smokescreen "Gas-Woven Orb". She smashes a vial on the ground, releasing a drug that turns into a Life Meter-sapping poison gas that lingers close to the ground like a cloud of green death. It alone can't KO an opponent, but its damage can add up.
Bravely Default has the Toxic Mist, a weapon capable of killing one hundred thousand soldiers, friend and foe alike, and rendering the land uninhabitable for years. Dr. Qada, its inventor, is a firm believer of We Have Reserves, as he doesn't feel any guilt over the massive friendly fire the weapon causes.
In EverQuest the gods created a poison gas (the "Greenmist") to wipe out the Shissar
The Land of Tombs and Xenon in Homestuck has this kind of atmosphere, a (likely deliberate) bending of the laws of chemistry as xenon in real life is non-toxic but asphyxiant (and tends to pool at the bottom of the lungs as it's difficult for the body to expel). However, the characters refer to it as poison gas and use gas masks to protect against it (rather than oxygen tanks, which is what you'd actually need).
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:
Pom Pom was filled with a deadly gas, which killed the Popsmith very fast.
In The 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.
SCP Foundation has an odd example in the case of SCP-681, a malevolent form of helium (in real life a non-toxic asphyxiant) that intentionally coats the inside of the lungs of living beings in the most efficient possible manner, causing them to suffocate. Luckily, it can be trapped in balloons.
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.
Elemental fluorine is one of the most difficult chemicals in the world to handle properly because it reacts quickly and violently to almost everything except some metals and chemicals. It will happily consume even the most invisible quantities of water and eat straight through glass. It is the most electronegative chemical known, meaning it bonds to almost any other element and absolutely will not let go without some serious chemical prodding. (It also bonded with a good number of 19th century chemists trying to isolate it, who are now remembered as the "Fluorine Martyrs".) Fluorine's sheer reactivity, in fact, makes it ideal for creating nonreactive substances like Teflon and non-polluting refrigerants, as well as a number of oddball uses in medical chemistry. That roll of Teflon sealing tape in your toolbox? Nearly 80% fluorine by weight... and, chemically, possibly the single safest thing in your house.
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, which painlessly passes through skin leaving a mild burn...and painfully dissolves your bones, spreading along your skeleton. And then goes into your blood, giving you a heart attack due to hypocalcemia.
[CF3]'s attempted use as a rocket propellant oxidiser was documented in the book ''Ignition'' by John Clark, a history of US propellant research with some very funny anecdotes about the people involved; it seemed that it was fine held at rest in most metal containers, but it started eating through everything as soon as you tried to move it anywhere. Clark mentioned a story about a substantial spill of the stuff that ate through a three-foot-thick concrete floor and created a cloud of nasty reaction products in the process.
And, if this isn't horrible enough for you, during WWII Nazi Germanyweaponized it: N-stoff was, in effect, a chemical weapon that set its victims on fire. Thankfully for the battlefields of Europe, only 50 tonnes were ever produced and these were captured unused by the RKKA in 1945.
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 smelling gas) is odourless in lethal concentrations. A high enough concentration of hydrogen sulphide numbs your nose completely (and for several days, by the way). So if you stop smelling it then you are either perfectly safe, because it has gone, or going to die, because it hasn't.
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".
Bleach is commonly used as a cheap alternative to toilet cleaner, and urine is high in ammonia. This has lead to many accidental deaths and injuries, including burnt genitals.
This is used in a episode of King of the Hill when Peggy was writing some general housekeeping tips in a newspaper article and wrote that bleach and ammonia make a great stain remover. Hank had to gather all the newspapers before it killed everyone (she had been previously been fed real tips for articles so everyone would have believed her).
Mercury gives up vapors at room temperature, which are invisible, odorless and can easily kill you. Mercury should never be spilled or held in open containers.
Zyklon B is not a gas by itself, it's a way to store and transport the gas. It's absorbent diatomous earth permeated in the poisonous gas itself (hydrogen cyanide), which is released from it by heating.
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 too busy dying in a horribly painful manner to care.
VX Gas is so potent that only 30 micrograms will cause lethality. To give you an idea on how small that is, find a pin and look at the tip. It only takes a droplet on that pin tip to kill one person.