No Biochemical Barriers: Difference between revisions

 

 

* Dita Liebely, the female protagonist of ''[[Vandread]]'', samples the food from Hibiki Tokai's home planet, but she finds it revolting and inedible. Believing the opposite must be true for Hibiki, she feeds him food from her home world, which he found to be delicious. Following this, Dita has little problem spending time with Hibiki on the condition that she feed him. Of course, although Dita considers Hibiki to be an alien lifeform, most outsiders would point out that they are the same species, just different genders.

* [[Please Teacher|Aliens love Pocky.]]

 

* Tenzil Kim and his fellow Bismollians from ''[[The Legion of Super Heroes]]'' can eat ''anything'', except certain materials he specifically can't use his power against.

* In ''[[Tom Strong]]'' #16, it's established that while unaltered humans can eat Devil's Footstool food, the opposite is ''not'' true. (With a few exceptions, such as coffee.)

* In ''[[Transmetropolitan]]'' [[The Greys|the Vilnius]] and late-stage transients (humans turning into Vilnius) can't eat human food, which helps influence the transient secession movement in the first volume.

 

* Many films where aliens, particularly sentient ones, eat humans (humans being eaten by alien beasts can be explained by the alien eating things that act like prey without knowing that it's bad for the alien's health).

* The ''[[Men in Black (film)|Men in Black]]'' films and cartoons imply that all alien immigrants and visitors to earth can eat (or appear to eat, in the case of the [[Mobile Suit Human]]-wearing Arquelliens) human food, although most of them ''need'' to in order to live as [[Muggles|ordinary humans]].

* Averted for comic effect in [[Galaxy Quest]], when the actor that the aliens think is an alien is treated to food from his "native" planet... seaweed-like stuff and ''live bugs'' in a bowl of water. It also has the [[Only Sane Man]], Guy, pointing out that going out on the alien planet without checking its atmosphere first is insane. They do it anyway (and survive).

 

* ''[[The Hitchhiker's Guide To The Galaxy]]'' has approximately 85% of sentient races having a drink called a "gin and tonic" (or "jynnan tonnyx", or "jinond-o-nicks", or "gee-N'N-T'N-ix", or "chinanto-mnigs", or "tzjin-anthony-ks"...). However, all these drinks are radically different, ranging from a gin and tonic to ordinary water at slightly above room temperature to a drink that supposedly kills cows at a hundred paces. Yet they all have the same name, and were all named such before the races had discovered space travel. This may also apply to Ouisghian Zodahs.

** Also specifically averted, as Ford tells Arthur that things like nuts and berries native to alien planets may well kill you. (Or they may not, but there's normally no way to tell except to try them, so only try them when you're at the point of dying if you DON'T find something edible.)

* This trope goes all the way back to the first [[Space Opera]], ''[[Skylark Series|The Skylark of Space]]''. Though the main characters are initially wary of eating Osnomian food, Seaton gives some Earth spices ([[Everybody Smokes|as well as cigarettes]]) to an Osnomian without being at all worried for the alien's health.

 

* ''[[Babylon 5]]'' features several examples of characters consuming food from alien societies. Sometimes there's no problem, other times... Well, this is what happens when Centauri try human fast food: "It tastes ''so good'' going down. Coming back up, it's not so good." And just about ''everybody'' can (and does) eat [http://babylon5.wikia.com/wiki/Spoo#The_Other_Grey_Meat spoo]. Even pak'ma'ra, who are carrion-eaters with a strict religious-based dietary code.

** At one point G'Kar claims that every race has independently created their own version of Swedish Meatballs. According to the card game, even the Vorlons have one and it's apparently a sentient lifeform. Very much [[Played for Laughs]].

** In TNG, it was revealed that the transporters have a "biofilter" that is capable of removing pathogens (but to look at it sardonically, it basically doesn’t ever work any time an episode is about bringing a plague onto the ship).

 

* Partially justified in ''[[Traveller]]'', where the Vilani (a race of [[Transplanted Humans]] brought to another planet in ancient times by meddling [[Precursors]]) have a culture shaped by the immediate incompatibility with their environment; one of their governing castes grew out of those with the necessary skills to chemically reprocess alien foodstuffs into human-suitable nutrition. One of the disadvantages of this history was that their biotechnology suffered from lack of compatible pathogens, and once they went to war with the Terran Federation many died of disease.

* Averted in ''Centauri Knights.'' The surviving Centaurian ecosystems on the space colonies are home to incompatible foods and creatures, but a biomod is available to change a character over from Earth food to Centauri food permanently, with no mix-and-match. The planet itself has no ecosystem left, as it was wiped clean to the bedrock by [[Nanomachines]] thousands of years before, and thus humanity is able to begin [[Terraform|terraforming]] it without contamination.

* The number of half-X, half-Y creatures in [[Dungeons and Dragons]] borders on mind-boggling. Depending on the edition and setting, you could encounter a half-elf, half-orc, half-dragon, half-demon, half-devil, half-angel, [[Buffy-Speak|half-robotic-thingy]], half-undead, half-eldritch abomination, half-god, half-halfling, half-giant, half-fey, half-chaos frog thing, half-elemental, half-djinn, half-dwarf, half-furry, half-golem, half-mutated egg-laying cousin of humanity, half...oh, you get the idea. Most of them can eat and drink the same things (if they do indeed eat and drink.) And the idea of the act bringing some of these creatures into existence ranges from hilarious ([[Biggus Dickus|half-giant]]) to pure ''[[Squick]]''. [[Double Entendre|Sometimes]], [[A Wizard Did It|a Wizard]] [[Double Entendre|really did]] it, and how!

 

* Justified in ''[[Sid Meier's Alpha Centauri]]'', where you have to research the appropriate technologies in order to harvest resources from xenofungus squares. This quote from an in-game character sums it up:

{{quote|'''Deirdre''': Juicy ripe grenade fruits may look appealing, but a mouthful of highly toxic organonitrates will certainly change your mind in a hurry.}}

** The Forerunners can safely eat earth-based food as well

 

* The huge and scary [[Kaiju|People-Eating Poly-Sorbate Insectoid]] in ''[[The Inexplicable Adventures of Bob]]'' is perfectly capable of swallowing a man-sized creature whole, but cannot digest Earth life. Hence, a swallowed character can punch the critter's stomach lining until he is... forcefully regurgitated.

* In ''[[Freefall]]'', the incompatability of amino acids on Pfouts with terran biochemistry was the reason for Dr. Bowman's experiments with uplifting nonsapient species, in lieu of ''very'' expensive space travel and [[Terraforming]]. The Bowman's Wolf (including Florence) was to be a "proof of concept" test.

** Played with in regards to Sam's biochemestry; since he comes from a planet with a relatively benign climate that forced few selection-pressures on his species and lacked anything like the asteroid strike believed to kill off dinosaurs on Earth, his protein structure is surprisingly simple, and Deeee-licious to most Terran animals.

 

 

* Averted in ''[[Ben 10]]'' of all things. Ben feeds an alien chocolate, and nearly kills him in the process. This particular alien was a pastiche of [[Superman]], thus making chocolate his ''[[Weaksauce Weakness|kryptonite]]'' and only ate it because Ben offered, and he didn't want to seem rude (though neither of them seemed to KNOW that this would happen).

* Completely averted in ''[[Invader Zim]]''. Water is caustic to the eponymous alien, and meat actually fuses to his flesh. He does attempt to build up a tolerance, but GIR, being a robot, can (and does) eat anything.

{{quote|''"GIR, your waffles have sickened me! '''[[Large Ham|Fetch me the bucket!]]''' ''}}

 

* Caffeine. To us, a mild stimulant. To a bug, a deadly poison of [[Doomy Dooms of Doom|doom]]. Same for nicotine.

** Even among mammals, you run into problems. Chocolate chip cookies with macadamia nuts and raisins are a rather easy thing for humans to eat, but to a dog the chocolate will cause potentially fatal arrhythmia and raisins are essentially kidney failure pills, while the macadamia nuts are a powerful paralytic. Some humans don't like onions and/or garlic, but most canines and felines will have their red blood cells dissolve if they eat enough.

* There are very very few life forms on Earth that are able to digest plastics. This is why our garbage is expected to last for millions of years before it decomposes. Recent observations of the North Pacific Gyre suggest that we're providing enough food to allow the few we do have to start to thrive.

 

* In ''[[Dragon Ball]] Z'', Goku apparently contracts a disease while on the planet Yardrat, weakening his heart and which would have eventually killed him. He's saved by a vaccine brought over from an alternate future where he ''did'' die. Of course, while Goku is an alien himself, the Saiyan biochemistry is identical to a human being's as far as [[Big Eater|nutrition]] and [[Half-Human Hybrid|reproduction]] are concerned.

* It is speculated that the parasite behind the [[Higurashi no Naku Koro ni|Hinamizawa Syndrome]] actually came from another planet. Then again {{spoiler|the person who said that is also the one who wanted to turn it into a bio-weapon, so it's questionable how trustworthy this is}}.

* ''[[Crossbone Gundam]]: Ghost'' has an alien microbe that the villains plan on releasing on Earth, which the heros are trying to prevent. The logic is that since the microbe has a completely alien biochemistry, nothing on Earth will be able to defend against it, and it will spread like wildfire and wipe out any life it comes across. This totally ignores the fact that the opposite should be equally as true: the microbe should be just as defenseless against Earth's microorganisms. Or, in the most likely case, absolutely nothing happens, and the alien microbe dies due to lack of its native environment.

 

* The [[Green Lantern]] [[Arc]] Sinestro Corps War had a virus called [[Names to Run Away From Really Fast|Despotellis]] capable of killing more than one species, even ones normally resistant to disease. Probably a [[Justified Trope]] since Despotellis is ''sentient''.

* Played reasonably straight in [[Alan Moore]]'s [[Top Ten]] with Sudden Traumatic Organic Rapid Mutation Syndrome or S.T.O.R.M.S. This unpleasent sexually transmitted disease mutates sufferers into a new, invariably [[Body Horror|non-viable lifefrom]] ( and in one case [[A God Am I|a god]]). Interestingly it is said to be the result of a city full of [[Fantasy Kitchen Sink|humans, super human, mutants, aliens, Robots, magical creatures and all the variations from alternate universes]] all [[Fetish Fuel|interacting sexually]] with each other.

 

* The Facehuggers in the ''[[Alien (franchise)|Alien]]'' films are somehow able to impregnate species (like humans and dogs) that would not have been encountered in the environment in which they evolved or by the species that engineered them (the backstory is unclear on which is the case) as long as it is warm-blooded and alive. This is [[Hand Wave|unsatisfactorily explained]] by claiming that it is capable of adapting itself to the DNA of the host and incorporate features of said host into the Chestburster it produces, but this does not account for how it is able to do so with a completely unknown species with a vastly different biology. If we suppose it was engineered rather than evolved, it seems likely that its creators would have designed it to target a specific species, because making it so adaptable that it could infest completely unknown species would surely mean that it could adapt to infest the species that created them. (Which, according to some sources, is exactly what happened...) As far as in-movie canon goes, ''Resurrection'' makes it clear that the Alien and host's DNA are somehow mingled, even in the host's own blood. Of course, that's still crazy from a biochemistry point-of-view (why would aliens even ''have'' DNA?), but at least they try to explain it.

** Somewhat justified by a suggestion that the Xenomorphs were created as either a bioweapon or a terraforming construct (which is basically the same thing, when you come down to it). Step 1 of any biosphere replacement is going to be getting rid of the existing biosphere, and my aren't these efficient at slaughtering things... This idea presupposes that a simple, but non-obvious, kill switch exists in the Xenomorph biology allowing some easy-to-handle chemical to kill them all. Now imagine a strain that mutates so that it doesn't WORK any more...

 

* In [[H. G. Wells]]' original ''[[War of the Worlds]]'' the Martians are killed off by a terrestrial microbe within several weeks of their arrival. This is almost a reversal of the trope, as it's rather explicitly stated that the aliens are ''more'' vulnerable to Earth microbes than humans, due to not having evolved and adapted alongside them; this is based upon the outbreaks that can occur when two previously isolated human cultures begin interacting and exchanging diseases, unintentionally or otherwise. Of course, this is because [[Science Marches On]]; the very idea of viral transmittance was new when the book was first written.

** ''[[War of the Worlds]]'' manages to hit the trifecta on this. In addition to the above virus example, the Martians are also capable of breathing Earth's atmosphere (to say nothing of the pressure. The only effect is that the higher concentration of oxygen invigorates them!) and "feed" (having "given up their digestive systems") by injecting themselves with human blood. Oh how science has marched on.

* In ''[[Lucky Starr|David Starr, Space Ranger]]'', a series of poisonings occurs in people who ate Mars grown food. A (human) Martian scientist says it could have been a poisoning by the local bacteria. {{spoiler|Subverted later, when it turns out he was the [[Big Bad]] and was telling a deliberate lie.}}

 

* ''[[Babylon 5]]'' had the Markab people fall victim to a universally fatal plague to which humans turned out to be completely immune, as it acted on a type of nerve cell that humans don't have. The Pak'ma'ra, however, turned out to be similar enough that it also affected them, though less severely. In fact, that similarity allowed Dr Franklin to figure out how to cure the disease - {{spoiler|too late to save the Markab}}.

* In an episode of ''[[Stargate SG-1]]'', the local alien population of a planet are falling ill after the arrival of the main team. As the good guys desperately try to find a cure, General Hammond [[Lampshade Hanging|points out that they're lucky they mostly deal with human civilizations who have the same diseases as Earth do and that's it's a small miracle they haven't run into a problem like this before]]. The episode was ultimately an aversion, since it turned out that the illness wasn't caused by a disease.

* ''[[Mighty Morphin Power Rangers]]'': In the opening three-parter of the third season, the [[Monster of the Week|Repellator]] is briefly afflicted with Kimberly's cold after she sneezes on him a couple times. Lord Zedd's reaction is hilarious.

 

* The Life-Eater virus in ''[[Warhammer 40000]]'' fluff can scour entire planets clear of life in minutes, no matter what kind of life inhabits said planet. Even Eldar (with ''[[You Fail Biology Forever|quintuple helix]]'' DNA) and Tyranids (which may have local DNA salvaged from corpses, but the race as a whole comes from a ''different galaxy'').

** The fluff implies that it's a sort of [[Grey Goo|nano weapon]].

* Played with in the Classic D&D game's diaboli, a race from an alternate dimension where many aspects of the cosmos are reversed. Not only are the normally-harmless bodily secretions of diaboli deadly poisonous to humans, but human saliva is equally toxic to them.

 

* In ''[[Warcraft]] III'' and ''[[World of Warcraft]]'', although the Plague of Undeath created by the Lich King only affects humans and not any of the other intelligent species in the game universe, it still has mutagenic effects on plant and animal life. Probably because it's [[A Wizard Did It|magic]] and was made to prepare the land for undead invasion. In at least one quest it's explicitly stated to affect some other races, but not fully as intended; one quest involves getting bones from Murlocs that were turned into undead by the plague... but weren't robbed of their free will by it as humans would be.

** ''Warcraft'' also has [[Half Human Hybrids]], despite diseases apparently not translating well among species on the same planet. For example, Garona Halforcen who is half orc and half DRAENEI. Yup, alien parents from different planets. And then she had a son with MEDIVH, who is, biologically, pretty much human. So that's 3 species from 3 different planets who are all compatible with one another. Draenei and Orcs in particular are interesting, since their hybrids are clearly still fertile.

* In ''[[Sword of the Stars]]'', the disease bio-weapons need to be invented only once and are equally useful against all species. They can be fired in a first-contact situation (before you'd logically be able to dissect a member of said species and find their disease markers) and will not lose any efficiency. The only exception is the [[Super Soldier|Zuul]], who are immune to all plague weapons except for the [[Grey Goo]] missile no matter what you try.

 

 

* ''[[Justice League]]'' had a rogue amazon [[Gendercide|trying to wipe out all males on Earth]] with an engineered gender-specific virus. It even affected Superman and the Martian Manhunter, who aren't human (The latter barely even qualifies as male by human definitions). The worst part is that the "disease" is finally stated to be an engineered allergy. Allergies ''do not work that way.''

* Somewhat averted in Invader Zim, where when lice broke out, Zim was the only one not infected (Well, there was Miss Bitters, but she ended up getting them at the very end, too.) Turns out there's something in his skin that kills the lice. This is probably the only time in the show where something earthly wasn't harmfull to him in some way. Of course, it might be possible that Irk once had lice or a similar creature, and their lice killing skin is an adaptation.

* ''Star Wars Clone Wars'' has a virus called "the blue shadow" which is highly contagious to all species. This ability is exactly why it's so feared in universe, as not only is no one immune, it's ''extremely'' deadly.

 

* Several diseases can do this. The most notorious include rabies, mad cow disease, foot-and-mouth disease, and the infamous avian flu. There's also distinct possibility that virtually all notable human diseases originated from livestock vectors. Smallpox and anthrax are of bovine origin, influenza of avian and porcine stock, the common cold may be from horses, HIV a variant of SIV from African green monkeys, etc. The only ones that don't show strong relationships with livestock are STD's, with the exceptions of HIV (as above) and syphilis (originally limited to sheep). But on the whole, these cross-species diseases are still fairly limited. Rabies, for instance, is stunning in its ability to cross species lines, but is still limited to infecting mammals. It would be unlikely to be found in a reptile, inconceivable in an octopus, and beyond ridiculous in an alien.

** Mad Cow, more correctly known as Bovine Spongiform Encephalopathy, is a special case as it's not caused by even a rudimentary lifeform such as a virus. BSE is a prion-based disease. Prions are mis-folded proteins that are nonetheless stable enough to interact with a biological system, and several are known to causes diseases. Since proteins are one of the most fundamental building blocks of earth life, it wouldn't be surprising to find prion-based diseases capable of infecting a ''wide'' range of earth lifeforms. BSE itself is known to have variants that can infect cows, horses, sheep and humans.

** Going back to the mention of space germs, the notable astronomer Fred Hoyle believed that human noses evolved to point down in order to help keep us from inhaling any microbes that fell from space. Mind you, he was rightfully laughed at for that hypothesis.

 

* The planet Namek in ''[[Dragon Ball]] Z'' has a ''green sky''. That alone should point out the differences in composition with Earth's atmosphere, but the humans that visit this planet, and the Namek natives that visit Earth (and the multitude of other aliens from many different worlds that congregate on either planet) have no problem breathing in it or even fighting at full strength.

** There's also a scene that [[Lampshade|lampshades]] this, where Bulma begins analyzing the atmosphere from inside their ship to see if it's breathable - only to look up and see that Gohan and Krillin are already outside.

 

* Inverted in ''[[The Scrameustache]]''. [[Human Aliens]] wear protective gears on earth ''because of our pollution''. But those who spend too much time on Earth adapt.

* A [[Fantastic Four]] comic has The Thing and Mr Fantastic visit a alien space station with an acidic atmosphere. After having his helmet broken it turns out The Thing can breath the air, it just tastes awful.

 

* Same in ''[[Star Wars]]'', where a bunch of different species mingle with humans without respiratory problems. For that matter, it seems likely that various human populations would adapt to the particular atmospheric conditions of their homeworld over the millenia and that they therefore could find the conditions on other worlds tolerable but bothersome. There ''are'' some exceptions, though, as a few aliens are shown wearing breathing rigs, and the [[Expanded Universe]] adds more--including at least one which doesn't even technically breathe.

** It is mentioned that the human Rebels find the Mon Calamari battleships annoyingly warm and humid (Mon Calamari are amphibious and will dry out quickly), but nothing worse than that.

* Partly averted in ''[[Avatar (film)|Avatar]]''. Although the atmosphere is primarily oxygen/nitrogen (albeit denser), humans can not breathe it without filters due to the presence of lethal levels of carbon dioxide and hydrogen sulphide.

 

* Averted in the ''[[Robotech]] II: The Sentinels'' novels. The Garudan's homeworld is oxygen-rich much like that of the Humans and other aliens, but also contains chemicals and microbes that for the other races are basically the equivalent of breathing LSD with your air.

* ''[[Sector General]]'' has oxygen-breathers, water-breathers, chlorine-breathers, methane-breathers, cryonic methane-breathers, oxygen/micro-organism breathers/eaters/drinkers and whatever-the-hell-the-radivores-breathe-breathers, and the Hudlars (classification FROB) require a different kind of oxygen-atmosphere, having evolved on a high-gravity planet with a very thick, high-pressure oxygen atmosphere (the make-up of which is an occasional minor plot point, albeit purely because Hudlars "eat" by absorbing micro-organisms and nutrients through their skin). However, the extremely tough tegument they are covered in (among other things) means that they can actually survive in a vacuum if need be, provided they get sprayed with nutrients often enough.

* There is a short story by [[Isaac Asimov]] where a race of aliens can breathe quite well on Earth... so long as they have a flask of hydrogen cyanide to breathe from every few minutes. Another story has aliens who, when they visit human planets, require filters to absorb carbon dioxide - it is highly toxic for them.

 

* ''[[Star Trek]]'' is a frequent offender here. Given that the majority of the crews found aboard the ships the various series center around are human, it is reasonable to assume that the air they breathe is adapted to human physiology. Other oxygen-breathing species are almost never shown to find the pressure, temperature or the (presumable) presence of nitrogen and water vapour bothersome. The Benzites are the only race that's been described as needing something other than oxygen/nitrogen. It's possible the Breen count too, since those refrigeration suits aren't needed, as well as the Tholians, who naturally live in temperatures of at least 450 Kelvin (177 Celsius). This is given a [[Hand Wave]] by an episode of ''[[Star Trek: The Next Generation]]'', which explains that most of the species in the Alpha Quadrant did in fact descend from the [[Precursors]], who scattered their genetic code across thousands of worlds, apparently [[Genre Savvy]] enough to know that their own society would inevitably [[And Man Grew Proud|grow proud]]. This goes some way to explaining why [[Rubber Forehead Aliens]] tolerate nitrogen/oxygen atmospheres, as well as other forms of alien/human... [[Half-Human Hybrid|interaction]].

** There was a nice moment in a ''Deep Space 9'' episode that seemed to acknowledge this, where Garak, a Cardassian, comments that the environmental settings on the eponymous station, adapted to Bajoran/human norms from their Cardassian originals, are uncomfortable to him - "the temperature is always too cold, the lights are always too bright". So even amongst ''[[Star Trek]]'''s rubber forehead brigade there are still variations in environmental tolerance.

* ''[[Power Rangers]]'' as a whole. Not sure whether the ranger's armor acts as a protective gear (surely it does in ''[[Power Rangers in Space]]''). But anyway, humans, [[Human Aliens]] good guys and Dark Specter's forces can breath on each other's planets. Oh, and apparently you can breath on Moon, and it has the same gravity as Earth.

 

* Averted in ''[[GURPS]]: Space'' most of the possible atmospheres are both suffocating and toxic.

* Averted in [[Dungeons and Dragons]] when traveling the planes. Depending on the plane you go to, problems may include but are not limited to: having the life sucked out of you, too much life being crammed into you, obscenely high temperatures, obscenely low temperatures, lack of atmosphere, poisonous atmosphere, or in some extreme cases whatever the DM feels like screwing the players over with.

 

* ''[[Mass Effect]]'' deserves a [[Rule of Three|third mention]]. This time, it's the volus. Apparently, the volus homeworld is a high-pressure cocktail of ammonia and other caustic gasses; all the volus we see in-game wear high-pressure suits, and it's implied that, without them, their skin would split open from their bodies' internal pressure. Ew.

** There's also a planet you can go to on one of the side missions that is very close to being Earthlike. It even actually has MORE oxygen than Earth...which is part of the problem as the thunderstorms on the planet are magnified to horrifying intensities. This plus toxic pollen rampant in the atmosphere makes it thoroughly unsuitable for human life.

* ''[[Sword of the Stars]]'' at least averts this one, as all races have their own preferences for atmosphere, gravity, humidity and temperature that makes it difficult for two or more species to share a planet without high-level environmental engineering tech. The preferences are randomized each game, which can have interesting results.

 

* Lampshaded for comedic effect in ''[[Toy Story]]''. Woody opens Buzz Lightyear's helmet accidentally, and Buzz starts choking, expecting something deadly. He suddenly realizes...

{{quote|The air isn't toxic...? How DARE you open a space ranger's helmet on an uncharted planet! My eyeballs could have been SUCKED from their sockets!}}

* ''[[Fairly Oddparents]]''. Cosmo doesn't quite understand what's wrong with visiting 'The Planet Of Not Enough Atmosphere'.

 

* Fish, squid, etc. can extract oxygen from water just fine, while humans and any other animal with lungs can't and will drown. On the flipside, dry/collapsed gills cannot function. And they dry very fast. There are species of fish that can survive prolonged exposure to air, and just a slight chemical difference in the water can kill an organism with gills. And never transplant a saltwater fish to a freshwater, or vice versa. Unless it's a salmon, and even then they can't move directly from salt water to fresh water or vice versa; they need time in an area that is a mix of the two.

* Humans can tolerate all kinds of pressure and gas differences, as long as whatever we're breathing contains an approximate partial pressure of 3.0 psi oxygen. Of note, heliox (helium-oxygen mix, containing no nitrogen at all) has long been popular with technical divers at extreme depths, as the lack of nitrogen removes one possible avenue for pressure toxicities. On the other hand, oxygen toxicity becomes a real problem at those kinds of depths, so you have to carry multiple mixes, in separate bottles with separate breathing rigs. And God Himself can't save you if you breathe from the green bottle (100% oxygen) at deeper than about 20 feet of salt water.