An extremely popular form of the Faceless Goons, achieved by taking your enemy Mooks and giving them all gas masks. Note that this almost always occurs in situations where the gas masks serve no real practical purpose, as chemical weapons aren't even a factor in whatever conflict is involved.
In situations (especially video games) where the majority of Mooks are notFaceless Goons, gas masks are often the mark of Elite Mooks. Despite the prevalence of gas masks amongst the enemy forces, it never seems to occur to them to use chemical weapons against the heroes. This is even though 1) being evil, they presumably have no qualms against doing so and 2) the heroes typically are wearing nothing except civilian clothes and a smile. If they're lucky.
Funnily enough, in many video games, enemies wearing gas masks are still affected by gas-based weapons.
Let us be brutally honest, though — it looks either damned cool or damned scary. There's a reason Vader's face looks like a stylized gas mask. May also show up as the Mask part of Coat, Hat, Mask, and more commonly as part of a Malevolent Masked Man ensemble. See also Hazmat Suit.
The lack of an obvious need for gas masks may be justified as preventative measures. After all, nobody's going to use gas against an army issued gas masks, but they might well use it against one that isn't, leading to the illusion of gas masks being useless. British citizens were issued gas masks during World War II for this reason, even though (or as a result) Germany never used gas in battle.
A rare good guy example is the S.W.A.T.-like troopers of the Armored Division Police who wear gas masks when going into combat, even though they mostly fight cybernetic killer machines which never actually use gas as a weapon. It's possible that they only wear those masks in order to prevent the viewer from becoming too attached to the Red Shirts.
There is a certain scene in the first movie hinting why they wear them every time. A short view from the perspective of one trooper shows them having H.U.D.s in the goggles. So they might be a cheap way of ensuring gas protection, combat information, and radio communication.
Also note that they are normally not meant to fight military-grade boomers; Dialy in the first episode even suggests letting the military handle the boomers. The ADP is normally meant to fight terrorists or criminals using high tech or civilian type boomers which went rogue. In the former case, gas attacks might happen and smoke grenades might be used by the police itself. Also, since they have riot shields in their arsenal, they might also be used as riot control, in which tear gas might be used by the police.
Used weirdly during the chuunin exam: Team 7 + Kabuto run into some ninjas from another village, but there are only three of them; they're wearing cloth masks separate from their gas masks which only cover their mouths, and they can be easily told apart.
They all make a reappearance in a filler episode where they use their masks for breathing underwater, and we actually see one without his mask. It later turns out that Hanzo, their village leader (until Pain takes over), wears one as well. Even later, we find out it's not just to let him breathe underwater; it also prevents others from being affected by his poison breath and he himself from being temporarily paralyzed by the poison released if the transplanted organ he implanted in his abdomen which make it is ruptured.
When we first see these (book 2 or episode 6), they're worn by the Demon Brothers from a rogue faction of the Hidden Mist village. There, they're more clearly compact rebreathers and are used as such (the Brothers were hiding in a puddle).
The samurai from the Land of Iron eventually turn out to be this, though it's a long time before we find out those helmets have gas masks in them (they're also Red Shirt rather than Mooks, being good guys when we see them).
In Trigun, the bandit villain Brilliant Dynamites Neon has an army of bandits who serve him, all of whom wear gas masks. This is mostly due to the fact that he uses gas to knock out crews on the sand steamers that he robs. It turns against him when Meryl and Millie beat up two mooks and steal their outfits.
Both played straight and inverted in Desert Punk: the mooks wear gas masks when they're outside, but so does everyone else. They also serve an actual purpose, as, besides working as actual gas masks against frequent poison attacks, they have built in binoculars and personal air-conditioning units.
The regular-level soldiers in Hellsing wear gas masks. The Nazi Vampires also wear them...as protection from the sunlight. At night, they don't wear them, though many of them pull scarves up over their face, presumably because it looks totally awesome.
The ordinary soldiers of the Britannian army in Code Geass wear gas masks as part of their uniform. They don't get attention in the series often, in battles that are mostly decided with Knightmares, but for some reason, their opaque masks never gain significance even though they could block the protagonist's mind control powers.
In Cowboy Bebop, the temperatures on Callisto are so frigid that many wear masks outdoors. A large gang of them mistake forever-broke Spike for a cash-laden Vicious and attempt to rob him. Awesomeness ensues.
In Nausicaš of the Valley of the Wind, the manga and the movie, almost everyone carries gas masks in order to deal with the highly toxic atmosphere of the Sea of Corruption. Wormhandlers and Torumekian grunts, however, rarely take them off.
Digimon Xros Wars has Troopmon. Supplementary material elaborates that they are actually comprised of energy stolen from other Digimon stuffed in the rubber suit-gas mask combo, and when destroyed, that energy is just recollected and reused, making them an interesting variation of this trope.
Caesar Clown's mooks in One Piece were introduced this way, wearing rather creepy gas masks. It's justified in this case, however, because Caesar's powers (and their armaments) are all based around gass based weapons.
Nort soldiers in Rogue Trooper almost always wear gas masks. Justified because the atmosphere of Nu-Earth is poisonous to normal humans.
Used, lampshaded, justified, subverted, and used as a Chekhov's Gun in a Buck Danny comic, when he is escorted by russian guards in full chemical warfare gear. He manages to escape from them briefly to give a warning signal. The Big Bad demands why his lieutenant dressed the Mooks like that, and the man cites that the KGB handbook says it has a good intimidation factor. The Big Bad is angry that the man thinks that someone like Buck would be frightened of some overalls. In the end, it turns out the lieutenant was on Buck's side from the start and meant for him to give the warning.
The Front Line Group wore gas masks to hide their identities when they first showed up in No Hero.
In All Fall Down, the Order of Despots' minions are all equipped with their stand issue, needlessly creepy gas masks.
In the Disney animated feature Atlantis: The Lost Empire, the villain's loyal mooks wore gas masks throughout the entire film (even though it's set in 1914 — before gas warfare was invented). Likely reasons include ease of animation and dehumanization to make them acceptable casualties. (Plus, while gas warfare didn't exist, running into gas pockets was a likely hazard).
The soldiers of the evil Wizard Blackwolf in Ralph Bakshi's animated film Wizards. Yes, you saw it right, they work for a wizard and they wear World War I gas masks. Two of these mooks get the best scene in the whole movie:
Max: Fritz, get up for God's sake! Get up! They've killed Fritz! They've killed Fritz! (draws gun) Those lousy, stinking yellow fairies! Those horrible atrocity-filled vermin! Those despicable animal warmongers! They've killed Fritz! (starts shooting off-screen) Take that! Take that! (Fritz gets up) Take that, you green slime! You black-hearted, sharp, bow-legged— Fritz: Max, Max, I'm okay. I'm okay, Max. Just a scratch; look, I'm all right. Max: Oh. Oh, damn. There you go again, stepping on my lines, raining on my parade, costing me medals. Oh, damn. (gun misfires, killing Fritz) Ooh. Oh, Fritz?
Interstella 5555: The 5tory of the 5ecret 5tar 5ystem: The mooks sent by the Earl de Darkwood to kidnap the band are wearing gas masks. Justified, however, as they do use sleeping gas weapons to put down the opposition and capture their quarry alive and unharmed. It also hides for some time the ironic fact that they are humans kidnapping aliens.
The first Resident Evil movie begins with a squadron of faceless goons abseiling in the windows via different camera angles.
In Iron Sky the Nazis are seen wearing gas masks as they load up into their flying saucers.
The Crazies: In both the original and the remake, all of the soldiers wear gas masks, although the Trixy virus is spread via drinking infected water. The soldiers wear them in case the virus has gone airborne and to hide their identities from the public.
Ultraviolet (2006) features an assortment of Faceless Goons, many of whom wear gas masks as their identity-concealer of choice. These include Big Bad Daxus' personal platoon (who are specifically referred to as "the Gas Guard" in a semi-important manner, even though they turn out to be just regular Mooks wearing gas masks instead of motorcycle helmets), as well as a dozen white-clad Elite Mooks who try to fight Violet using katanas instead of firearms (and end up doing a rather lousy job of it). Although the gas masks are somewhat handwaved in that the film takes place amid massive paranoia about a highly infectious blood-based disease, it's not clear why these particular goons feel obliged to protect their purity of essence with gas masks, instead of being content with the cloth surgical masks or motorcycle helmets most of the other Mooks use.
The Russian stormtroopers in the Hitman movie wear gas masks and football-gear-based armor straight out of Jin-roh: The Wolf Brigade. Just in case they ever need to storm another opera house.
Repo! The Genetic Opera has Rotti's henchgirls donning gas masks in Things You See in a Graveyard. Justified as they're throwing tear gas grenades.
The opening scene in Ian McKellen's Richard III has Richard's commandos wearing gas masks. Of course, this is to allow a Dramatic Reveal of the villain during the opening titles. (Bonus feature: compare that opening to the first shots of the original Star Wars.)
The tractor drivers in The Grapes Of Wrath. Incidentally, the driver who talks to Muley just wears goggles.
The Stormtroopers from Star Wars. Probably the example people think of about this trope, or at least high on the list of candidates.
World War Z: In the Battle at Yonkers, the Army brass decide to invoke this trope in order to create the appearance for the news cameras that the US Army troopers are invincible killing machines that are ready to mow down the shambling zombie hordes. On top of the MOPP-4 masks was layered body armor and the latest in satellite commo gear. It fails miserably — as told by a line infantryman, all this succeeded in doing is making every grunt on the line more tired, overheated, and inaccurate due to trying to shoot while wearing all that junk. That was just the infantry: the armor, artillery, close air support, and engineering divisions had also been selected and loaded for maximum visual effect and bragging rights for the news crews as opposed to maximum efficiency in actually destroying zombies.
Brave New World features an interesting variation. Riot police wear gas masks as they spray relaxing gas ('Soma', which is a common relaxant in the story), and then play a record of calming narration to dispel mobs, which fits with the novel's theme of dystopian control without direct oppression or bloodshed.
Federation troopers in Blake's 7 — although some do remove their helmets.
The Breen from the Star Trek franchise (especially Star Trek: Deep Space Nine). Explained by their homeworld being a frozen wasteland having a rather noxious atmosphere outside of which they can't survive. Although a few characters claim to have visited the Breen homeworld and say that it's actually perfectly habitable and at a comfortable temperature. The Breen apparently wear their environment suits and masks just to appear enigmatic. It is said that no outsider has ever seen what one looks like beneath their masks and lived to tell of it.
On Babylon 5, the Babylon 5 security personnel could be considered such by certain aliens, since parts of the station are filled with atmospheres for various exotic aliens, requiring oxygen-breathers to wear special masks (or to get gill implants, as some elect to do instead).
An interesting example is Kamen Rider The First. The mooks wear Gas Masks, but are killed just as easily as in their skeleton-costume days (and they give the same "yee!" shout as they hop around!) The Shocker Greeed in Let's Go Kamen Riders has a gas mask apparently as homage, though The First and The Next aren't in main series continuity. Fewer appear in Kamen Rider The Next and mostly in a non-combat role with foodsoldier work largely left to the Shocker Riders.
Borderline example: "The Empty Child"/"The Doctor Dances" on Doctor Who set during World War II. The "monster", a small boy, wore a gas-mask and was actually a zombie, who could control technology, could only say the words "Are you my mummy?", and could convert everyone else to be just like him. Nothing says fun like watching a grown man gag as a gas-mask forces its way up his throat.
Gas masks are a repeated visual in Frank Zappa's music, including the liner notes for Joe's Garage and the concert film Baby Snakes.
Weasels Ripped My Flesh included a song called "Prelude to the Afternoon of a Sexually Aroused Gas Mask."
A gas mask is the Iconic Item of Norwegian alternative rock band Kaizers Orchestra and is a common element in album art designs, sometimes cleverly hidden. The mask is worn by their organist during live performances.
One constant about Thrash metal band Sodom's soldier mascot Knarrenheinz is the gasmask he wears.
Armageddon Steel Legion and the Death Korps of Krieg, among other Imperial Guard legions. Then again, they tend to live in places where the masks are a good idea for survival in general.
The Death Korps also wear them to hide the fact that they're clones, a practice which is frowned upon in the rest of the Imperium.
In Dan Abnett's Gaunt's Ghosts novel Straight Silver, both sides make extensive use of gas masks. Then again, they need them.
The Chaos cultists in the Dark Vengeance box have a mixture of gas masks and other, cruder face-covering devices.
Some Plague Marines have helmets reminiscent of gas masks.
Warhammer Fantasy includes specialized Skaven wearing gas masks for protection against various airborne ills of their own devising; they are not named characters, making them mooks, but they are generally not cannon fodder, either.
A lot of d20Modern artwork shows military-looking mooks (mercenaries?) in gas masks, apparently just for Rule Of Cool. (PCs using chemical weapons are extremely rare, but it can happen.)
The Combine Overwatch soldiers and Civil Protection officers in Half-Life 2, as demonstrated above. Apparently, an early version of the script had City 17's air unbreathable, so that everyone wore gas masks. This was dropped, but the Combine gas masks weren't. It should also be pointed out that Overwatch soldiers required the mask to include their vocoder, due to their vocal cords being ripped out (visible when you see the topless Overwatch model in Nova Prospekt), and Civil Protection officers would need them for anonymity, due to being rather brutal to citizens. The Gas Masks actually work against them at times. Using the "'bug bait" on a normal person gets no reaction, but using it on a Civil Protection Officer or an Overwatch Soldier causes them to flail about, apparently choking. That might just be because they're Friendly Fire Proof.
Some of the marines in the original Half-Life also wore gas masks (including Adrian Shephard in Opposing Force).
In Far Cry for the PC, the toughest enemy soldiers are Elite Mooks who are identified by the gas masks they wear. Why they do so is something of a mystery, as chemical weapons are never a factor, and it turns out that the mutagen the bad guys are toying with isn't even airborne.
In Return To Castle Wolfenstein, heavily armored Venom Troopers wear long greatcoats and gas masks. It's unclear why they do so, as none of the other enemy soldiers wear any sort of NBC gear. The gas masks only seem to serve as a message to the player that these guys can soak more bullets than normal soldiers.
Same goes for the Drache Troopers in Wolfenstein, though, in their case, it could at least be argued that they need them to filter out the smoke produced by their flamethrowers (especially since they have a tendency to use them in confined spaces). The Veil Assassins and Heavy Troopers have no such excuse, though; rather, they wear gas masks because they are notexactlyhumananymore.
Borderlands: Some of the Atlas soldiers appear to be wearing gas masks. Far more visible (and numerous) in the Secret Armory of General Knoxx DLC.
Likewise, in Medal of Honor: Airborne, the Mini-Boss-like Panzergrenadiers (guys with rocket launchers) and Nazi Storm Elites (guys with heavy body armor wielding MG 42 machine gun turrets as portable weapons!) wear gas masks for no apparent reason other than to signal that they're supposed to be badass.
Medal of Honor: Allied Assault has a rare justified example: the only soldiers wearing gas masks are the ones guarding the interior of the mustard gas factory in the game's final mission.
In Deus Ex, some of the soldiers seem to incorporate light gas masks into their mouthpiece headgear. Oddly, these don't seem to provide any actual protection against tear gas grenades, or even against pepper spray. That might be because tear gas and pepper spray are eye irritants — since there's a global pandemic going on, it's probably not for protection against chemicals at all.
In Iron Storm, enemy Siberian Troopers wear gas masks and heavy body armor. As per usual, the gas masks serve mostly to signal they're badass (they carry mini-rocket launchers that gib you in one hit) rather than to protect against NBC weapons. In fact, their gas masks are purely cosmetic and provide absolutely no protection against mustard gas grenades or LSD gas grenades.
Umbrella Special Forces operative Hunk from the Resident Evil series is a rare example of a Gas Mask Mook with a name and his own story.
Some late-game mooks in Resident Evil 4 and Resident Evil 5 wear gas masks. In 4, they're disturbingly sewed to Ganados' faces and make it slightly harder for Ada and Leon to elbow off their heads, whereas the ones in 5 protect against headshots and make them immune to flash grenades. There's no reason for them in "5", although it's implied to be justified in "4": the exposed flesh of the gas mask wearing enemies is horribly mutilated, presumably from chemicals, and they can't take them off because, well, they're sewed on.
A number of the Ultranationalists in Call of Duty 4: Modern Warfare wear gas masks, apparently just because they're cool. Some of the SAS troopers also wear them, but only for the sort of room-clearing missions where real life SAS doctrine does indeed call for gas masks for everyone, not because of gas but because their SF10 masks◊ have tinted visors to help against flashbangs.
A few of the Russian Federation paratroopers in Modern Warfare 2 also wear gas masks, with no more in-story justification for them than they had in the first game.
All Nod soldiers in Command And Conquer Renegade, except for plot-important NPCs. May be more justified there since Nod likes to toy around with Napalm and Tiberium. Not that the superiors seem to care much about safety... (this was most likely due to limitations in the game as seeing the same face on every soldier you killed would get boring, and also to try and skirt around censorship in germany, much like the original C&C renaming civilians "farmbots" for the german release).
Black Hole infantry in Advance Wars come close, wearing space helmets for no real purpose other than to make them look more alien. In fact, they are aliens! It's stated outright in the second game that they've arrived from space.
The Xylvanian rifle grunts of sister series Battalion Wars wear full gas masks, both because Xylvania is dangerously polluted and because their military is big on chemical warfare — the Xylvanian equivalents to Flame Vets are Acid Gas Vets.
The Helghast wear gas masks all the time and are most likely inspired by the Wolf Brigade from Jin Roh (see above). However, the game somewhat explains them wearing these. Since their home planet has a toxic atmosphere with little light and the people on it have gotten used to it (more or less), their soldiers require these masks because normal atmosphere and light makes them weaker. They invert the normal purpose of gas masks by toxifying breathed-in air.
And yet, you get to invade the Helghast homeworld in Killzone 2 and they still wear masks and your side doesn't. The game's official website explains that the Helghast dictator's propaganda has turned the masks (which the inhabitants of Helghan wore before they adapted to the planet's atmosphere) into a symbol of their fortitude, thus they wear the masks in other planets out of necessity, and in their home planet for symbolic reasons. Some of the Enemy Chatter even hangs a lampshade on their choice of dress in a somewhat humorous fashion:
Helghast 1: Did you hear? Two grunts from our unit got executed by Radec? Helghast 2: No shit? What for? Cowardice? Defeatism? Helghast 1: Uniform violations. I shit you not. Helghast 2: "The Dress Code is the foundation of discipline..."
In Rise of Legends, the Vinci hero character Pulitore, who has poison gas-based powers, as well as the Doge's Elite Guard super infantry, all wear gas masks. The doge himself uses a lot of gas attacks, so it makes a bit of sense. Pulitore is immune to gas effects, but the elites aren't.
Metal Gear Solid gives us the No Fourth Wall boss Psycho Mantis, who's easily the quirkiest of the Quirky Miniboss Squad. He wears a gas mask, although it's for a good reason: it keeps him from hearing the thoughts of everyone in the general vicinity. It also covers his face, which is pretty badly scarred.
The Tengu Commandos in Metal Gear Solid 2 and the Haven Troopers in Metal Gear Solid 4 wore full-body exoskeletons, with their helmets appearing to have some sort of filter. The filter is more prominent in the Haven Troopers, but it doesn't stop sleeping gas.
Then there's the mooks that actually wear gas masks in the area that the nuclear heads are stored in from MGS1. And that area actually does get flooded with poison gas if you are spotted.
Both subverted and used in Fallout 3 (and somewhat in other games of the series), where a group of enemies use gas mask-like Powered Armor, but so do a neutral group, a good group, and sometimes even the player. Lampshaded in that some power armor helmets lower charisma when you use them. Specifically, the Enclave power armor helmets. It's also justified as heavy dosages of radiation are found throughout the wasteland. Additionally, most of the soldiers that do have them aren't really mooks, they are some of the best trained soldiers on the face of the earth (or at least the US).
Actual gas mask-masked potentially hostile folks show up in the Mojave Wasteland, but the Veteran Rangers have that same "too good to really be called mooks" thing as mentioned above and, to a person working for the NCR, are a generally good group.
The Ghost People in the Dead Money expansion of Fallout New Vegas are a more traditional example, being mute, numerous, and universally hostile. They're also extremely agile and strong and don't die unless dismembered or disintegrated (or unless you learn the perk from Dog).
Also, the Lobotomites from Old World Blues. They may seem like mindless cybernetic zombies at first, but they can both dish out damage and take it, and are quite accurate with firearms.
While there are no gas-based weapons in STALKER, the gas masks and full face helmets on certain suits do make sense as those are the ones which grant the highest radiation protection, and one way they do so is by filtering out radioactive particles in the air. Just about every non-plot essential NPC wears either gas masks or face wraps, probably to save time on lip syncing.
It should be noted, however, that there are, indeed, gas based anomalies which manifest as toxic green clouds hovering in the air, occasionally with some manner of green liquid condensing and dropping down to the ground. Not something you want to breath in without protection.
Justified in the Halo series, where the grunts are methane breathers and require respirators in oxygen atmospheres. Inversed as well, as they are the weakest mooks. The Heretic Elites wear gas masks as well, although the Covenant Elites don't use them in the same environment (the gas giant Threshold).
All human soldiers in Dark Sector wear gas masks, but this is justified in that they are fighting The Virus.
Skeleton soldiers from Painkiller wear gas masks, but since they are animated skeletons already, it is just another nonsensical element. It does give them a distinct battle cry, and soldiers of a certain era could have died in those gas masks, hence the reason they're still wearing them.
The Special Forces soldiers who assault the Mall in Dead Rising are a whole different game than the zombies. They've got the firepower to really ruin a player's day.
In Prototype, the Blackwatch all wear gas masks, apparently a standard part of their gear as a division commissioned to deal with biohazards. The unusual triple configuration of the eye and mouth filters makes them look much less human, something intended by the developers.
Strangely absent in Rise of the Triad, because there are several rooms where poison gas is released. It'll eventually kill any enemies in the room.
Introduced in Super Mario Bros. 2, Snifits are best known for using their masks to spit projectiles. It wasn't until Paper Mario Sticker Star, more than two decades later, that their masks were used for toxic purposes.
Batman Arkham Asylum: Scarecrow wears a particularly nightmarish version◊. Justified, since he sprays around his terrifying gas frequently.
In Destroy All Humans! 2, some of the Soviet troops in Tunguska play this trope straight by wearing gas masks, while others do not. Gas mask or not, the soldiers are heartless, yet expendable mooks at the end of the day. Potentially justified in how Tunguska is home to some dangerous radioactive and alien gases, some of which can cause mutation in alien mooks.
One of the enemy types in Uncharted 2 wears a completely unnecessary gas mask.
PSICOM soldiers from Final Fantasy XIII. Although they don't look exactly like real gas masks and it could just be the helmet for their Powered Armor, but it's never specified one way or another.
In the Hunter multiplayer mode of Splinter Cell Conviction, the last part of each stage has these. In the Washington Monument stage, a Gas Leak Coverup is used to evacuate the fairgrounds, and the Black Arrow troops sent in to get Sam are outfitted appropriately to the excuse.
Some of the enemies in later missions in SWAT 4 wear gas masks, making them immune to your tear gas.
The Pig Mask Army in Mother 3 is exactly this. The masks are just meant to make them look more pig-like.
In Brink, certain head/face parts turn characters into Gas Mask Mooks. The Resistance have "The Firestarter" (though it may not be an actual gasmask) while Security has "The Bouncer", "The Shield", and "The Unit", though the latter doesn't make the wearer anonymous as it's the more modern style with a large visor instead of two eye holes.
The G22 operatives from Alpha Protocol. Their blue-lit visors are remarked upon in game.
Everyone who journeys above ground in Metro 2033 needs to bring a gas mask, unless they plan on holding their breath. The proliferation of Gas Mask Mooks is actually quite helpful, as stealing their air filters keeps you alive.
Verminators wear gas masks in Rabbids Go Home, not because of some toxic hazard, just because they're obsessed with germs and neatness. However, it doesn't need so much effort from the player to put that mask away...along with every other garment Verminators wear.
The Urban Replica Soldiers in First Encounter Assault Recon wear one of these, along with the helmet standard among Replicas. They're the second weakest type of Replica in the first game, but their badass look makes up for it.
Inverted with the Black Egrets of Skull Girls; despite the presence of Gas Masks and their aesthetic, they work for Parasoul, who seeks the destruction of the Skull Heart responsible for creating the titular being, making them the good guys.
Also of note is Black Egrets member Panzerfaust, who's likely to become DLC for the game...And has a TANK for an arm/fist.
Numerous enemies in the Mass Effect games wear armor that includes a helmet that resembles a gas mask, but this serves a practical purpose despite the main character not having any chemical warfare ability because armor in the game doubles as space suits, so it would need to be sealed against vacuum.
In Dishonored, the Assassins wear Steampunk gas masks that were taken from the abandoned whale oil refinery, where the workers used them to protect against hazardous fumes.
Soldier Of Fortune II's Prometheus troopers, and the hazmat/NBC soldiers in the first game.
Every single character in Gone with the Blastwave is one of these, unless you want to count the giant mutant moth. The markings on their helmets, which seem to indicate their roles in the unit, are the only way to tell them apart. It's not clear whether this is just a defensive measure in the war they're fighting or whether the post-apocalyptic setting makes it necessary for survival. Either way, it adds to the general theme of the characters being faceless mooks in a war they don't understand or particularly care about. Page 50 filler shows "25 essential expressions" on these.
The four protagonists of Romantically Apocalyptic (Sniper, Captain, Pilot, and Engineer) all wear gasmasks, though this may be because of the rather eccentric captain. Lampshaded in a flashback scene in which a pre-apocalypse child version of Captain still insists on wearing a gasmask.
In Rudolf Ising's1939 anti-war cartoon Peace on Earth, the now-anthropomorphic animals recall how Man finally wiped himself out in one final war. The grimness of these sequences is hyper-accentuated by their never-removed gas-masks which seem to run the length of their whole bodies. The two last ones shoot each other dead before falling into the muck and mud, ever faceless.
The Equalists from The Legend Of Korra. Rather unusually, as there doesn't seem to be any particular reason for them to be wearing gas masks — at least, not yet.
As of Season 1 Episode 3 "The Revelation", they do have a reason. A pair of Badass Bikers throw some kind of gas grenades/smoke bombs to cover their escape, and their masks protect them from the green gas. The masks also fit with the anonymity cultivated by their leader Amon, who wears a White Mask of Doom, albeit an opera mask not a gas mask.
The Terras in Motorcity wear these, as their faces are mutated due to toxic waste dumped by Kane Co.
Very much a Truth in Television, as various SWAT and Counter terrorist teams wear gas masks regardless of whether they're going to use gas or not. Two reasons for this: a) preventative measures in case who they're going against does or if they themselves decide to use tear gas or smoke grenades and dust that would be thrown up by entry charges and b) the psychological effect it has on the enemy inside the building they're about to storm. Many gas masks are also compatible with flash reduction lenses, which means the wearer is less vulnerable to flash grenades.
It's not wholly fair to call them evil, but they're almost certainly the source of this trope: during World War I, a lot of German soldiers wore gas masks because of all the chlorine gas they were using in battle, most famously at Ypres.
In the awkward segue between horse and vehicle warfare, horses sported some variety of mask.
The Allies started to issue gas masks once they were fully developed. They didn't wear them constantly because they were incredibly uncomfortable, and you could actually choke in one if you didn't switch the filters often enough.
The above Philip K. Dick quote comes from the notes of one of his stories, where he is remembering his father, a World War I veteran, showing him the mask when he was a child.
The US Marines have a nickname they're fond of that they claim to have been given by the Germans, Teufel Hunden, meaning "Devil Dogs", attributed to reports from the Battle of Belleau Wood where Marines wore bulky gas masks and clawed their way up a steep hill to get at the Germans, foaming at the mouths from the heat and exertion. Supposedly, this gave them a hellish, dog-like appearance.*
Worth noting, nobody's ever found any evidence that these German reports ever existed, nor are there any accounts from the Germans of them actually calling the Marines this, plus the fact that "Teufel Hunden" would exhibit remarkably poor grammar in German. That said, the story is still highly entertaining and very cool
"Teufelshunde" is actually gramatically correct German.
Chemical weapons were not used prominently in Europe during World War II, not even by Nazi Germany, because, aside from the majority of German transport being comprised of horses, both sides had lived through such a horrible experience with them in the last war that they were terrified of using them again. There was a tacit understanding that if either side started using chemical weapons, the other side would retaliate with their own chemical weapons, and escalation would soon lead to whole armies and entire cities being nerve gassed. Each side had spent the past two decades living with thousands of wounded soldiers crippled by exposure to poison gas, the reality of these weapons had really hit home, so they never seriously considered using them.
That wasn't to say that the Germans didn't have poison gas - they made tons of the stuff, but Hitler refused to use it (because of his own experience with gas in WWI). The UK also stockpiled chemical arms, which would have been used against a German invasion.
Even the Japanese, who hadn't fought on the Western Front and who were willing to use biological warfare against the Chinese, were terrified that the United States would resort to chemical warfare attacks against their defensive tunnels on the Home Islands. This is why they never used chemical weapons, even when their home territory like Okinawa was being invaded. Their military central command issued explicit orders never to do anything that might provoke the Americans into using chemical weapons, such as specifically using chemical weapons on American troops.
Iraqi soldiers fit this well enough as they utilized chemical weapons against Iran.
During the Cold War, every country manufactured masks to protect themselves from chemical and biological weapons. The Soviets created many masks that represented human skulls which were intended to also have a psychological impact on enemy troops. These mask designs were used in Half-Life 2 and other dystopian fiction. Also, many Russian families keep these skull masks just in case, and Russian truckers like to put these masks on the necks of their trucks' diesel tanks.
Even after the Cold War, new masks are being designed and produced, as various improvements are made (usually in terms of comfort and durability) and old equipment wears out or becomes obsolete.