How can we make an alien atmosphere breathable, but uncomfortable?

In the project I’m working on, sufficiently advanced aliens terraformed a large number of planets in the distant past, giving them an atmospheric pressure and composition similar to Earth. This “galactic standard” is breathable by all alien species without any difficulty. However, because Earth’s atmosphere wasn’t terraformed but developed independently, I want the “galactic standard” to be different enough to breathable but humans but otherwise uncomfortable. I’m imagining a scenario in which most humans can acclimate given time, but many prefer to use supplemental breathing devices or implants, especially if they’re old or have health problems. I also want to this problem to be mutual, so aliens have similar problems breathing our atmosphere.

However, I want the “galactic standard” to have a high oxygen content to be conducive to larger creatures and more frequent fires, so just giving humans altitude sickness is not an option (even though it would be the easiest solution). I’m also hesitant about increasing carbon dioxide levels significantly, as its pretty potent greenhouse gas. Given these limitations, what are my option to make an alien atmosphere breathable, but uncomfortable?

Edit: I wish I could accept multiple answers, because all of these are great and I'll probably be referring back to this question a lot for reference. Ultimately, I've accepted Mike Serfas's answer because it's a very elegant solution and dovetails nicely Christopher James Huff's comments about CO2 being a go-to terraforming agent. This won't work for every world, but I imagine between excess CO2 on some worlds, excess Ozone on others (maybe a byproduct of plant respiration?), and excess humidity in general, I'll get the effect I'm looking for.

• It is a lot easier to warm a world that's too cold than it is to cool one that's too warm. Adding a bit of greenhouse effect would be a perfectly reasonable way to accomplish the former. If CO2 is their go-to gas for moderate greenhouse adjustments and most of the worlds they terraformed were on the cold side of ideal, their typical atmospheres would have elevated CO2 levels specifically because of its greenhouse gas effects. It wouldn't be standard, but might be common enough to be an issue. May 21 at 18:22
• Not really worth a full reply, but I'll leave this little science experiment here. You feel like you're drowning, but never actually drown. nature.com/articles/32312 May 23 at 15:39
• Do you really mean that very strange Question or rather, perhaps, "How can we make an alien atmosphere breathable, even if it is uncomfortable?" May 25 at 22:46
• @Christopher James Huff Where is it written that it's easier to warm a world than to cool one? May 25 at 22:56

Carbon dioxide.

According to some tellings, Earth arose very near the inner limit of the "habitable zone" around the Sun. Earth life sucks up nearly every trace of carbon dioxide available to it in the atmosphere, provided somebody doesn't dig up entire geological strata of pure carbon and send it up into the air.

Most habitable planets formed further out - an Earth-sized planet in the orbit of Mars, or halfway. Their ecosystems were limited in the amount of carbon they could fix before the seas froze, so the organisms there became accustomed to having a few percent of CO2 in the air, at least during during most geological eras.

An excess of carbon dioxide in the blood is called "hypercapnia", and can happen for many reasons, but having too much CO2 in the air is surely one of them. Below 1%, the effects are subtle, mostly of concern to air conditioning technicians looking to sell you larger machines, but living on a planet with that going all the time you would tend to start blaming headaches and sluggishness on the CO2 in the air, perhaps justifiably. Go much above that, and the urge to breathe faster sets in. Our bodies use CO2 as the first signal to tell us to breathe, so we respond to high levels by assuming we need to breathe more, no matter how much oxygen is in our blood. High levels of CO2 also make the blood acidic, because it reacts with water to make carbonic acid.

The regulation of this response is a bit complex, because there are multiple locations where CO2 is being sensed and the mechanisms overlap with regulation of blood pressure and other vital physiology. Nonetheless, for your implant suggestion, the neurons are known, and so you can simply rattle off the names in there and say you have a way to alter their signalling. You could mention cruder mechanisms such as inhibiting the sympathetic nervous system response with beta blockers or even severing the vagus nerves, but let's not do that. Ideally, it should be possible to develop a drug that very specifically targets the same neurons you would target with an implant, providing relief in a gentle, non-invasive, and reversible manner.

• +1 this is the easiest way. Because we use CO2 as our respiration sensor, the unpleasantness is not via the sense of smell, which easily habituates away, but a really primal choking/gasping/dizzy whole-body response. I’ve been in rooms with a CO2 leak (dry ice freezers) and it’s unpleasant in a way that no garlic, durian, sewage or surströmming can remotely compete with. And you can’t actually really smell it, so it’s harder to avoid and, in low concentration, affects your mood and focus without you even realising. May 22 at 21:28
• I think this answer is the closest one to what the OP is looking for, which I took to be something like "you can breathe and stay alive, but it just doesn't work as well". Also, major atmospheric constituents tend to be simple small molecules like O2 or CO2. May 22 at 23:30
• @Ottie and human pollution has now doubled the amount of CO2 here on Earth. In another century, people will (apart from being cooked alive) get those unpleasant mood responses all the time with no way to get away from them and they will think it's just how humans are. May 23 at 13:20
• @Ottie yes, on reading the question it was my first thought, and it would be really horrible - I was imagining the feeling when swimming underwater just before time when I must surface - but all the time. It suggests luxury venues for humans, with lower CO2 than ambient, as well, perhaps useful to the story May 24 at 12:17

Uncomfortable is a matter of tastes and culture, but I guess thiols make a good candidate for making an uncomfortable environment: proximity of rotting eggs, sewers, rotting algae are just few examples of uncomfortable locations, because they release sulfur based compounds.

They smell foul at low concentration and are toxic at high:

The odors of thiols, particularly those of low molecular weight, are often strong and repulsive. These compounds are detectable by the human nose at concentrations of only 10 parts per billion.

I remember during my university time I had a colleague who, from time to time, would be busy synthesizing quantum dots using thiol: despite he using a state of the art chemistry hood on the first floor, one could smell that awful thing from ground floor.

• I imagine your colleague's social life must've suffered to some extent. May 22 at 14:53
• that was my first thought too. But it doesn't satisfy the "mutual clause". Can't imagine aliens having a problem with the lack of smell on earth May 23 at 14:16
• That depends. It may be a purely psychological component to them - a message that they are in the midst of the herd or that there is plenty of vegetation to graze on. Most humans really dislike the "taste" of distilled water with no dissolved gasses in it. May 23 at 14:27

There is a discussion about how much of various gases are necessary for humans to survive in Stephen H. Dole Habitable Planets for Man, 1964.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

On pages 13 to 19 Dole discusses atmospheric composition and pressure. Table 2 on page 16, table 3 on page 18, and table 4 on page 21 give upper and/or lower limits for various gases.

And of course research in the last 60 years might have modified those limits.

And there are no doubt many thousands of other compounds found in nature or synthisized by chemists which are gaseous at normal Earthlike temperatures.

In the Harry Turtledove story "The Road Not Taken", Analog, November 1985, a Roxlani ship lands in Los Angles. And when the hatches open, the Roxlani find that polluted Earth (or Los Angles at least) air smells bad, while the humans find the air from the ship smells bad (though the Roxlani were unable to keep the ship's air smelling like their homeworld which might have smelled better to humans).

Anyway, mutually bad smelling air is pefectly possible.

You want to select gases which are relatively plausible to naturally occur in a planet's atmosphere, so that the native planet of the species that created the galaxtic standard atmosphere has that gas in its atmosphere and that species evolved with that gas.

If that gas has a detectable oder, they might be used to that odor and not really notice it until it is gone. That gas's odor might mask the sents of other gases on their world, and so they might be troubled by the odors of those other gases which are more common in naturally occurring atmospheres. Thus they make sure to artifically put that gas in the atmosphere of all planets they terraform, even though lifeforms that evolve on planets without that gas tend to find its odor offensive.

Or possibly that species was civilized for so long before they began to terraform other planets that they are very used to the subconsious odor of some artifical pollutant which they accidentially released into their atmosphere that air feels bad without it, so they terraform all planets to have that pollutant in their atmospheres.

Or maybe the odor which they desire is a combination of the odors of several different chemicals. Possibly most worlds have one or two of those chemicals in their atmospheres, but the home planet of the galactic terraformers was a rare planet with three, four or more chemicals in the atmosphere. Thus they would terraform planets to have all of those scented gases in the atmosphere.

Since Earth has only one of those gases, the combined odors of the other gases are offensive to humasn.

I note that sometimes I notice the smells of various plants in the air. Possibly the home planet of the galactic terraformers has very slight seasons, and plants emit pollen all year long, and so the galactic terraformers had to change the seasonal cycles of planets they terraformed, so that the plants from their homeworld which they planted on terraformed planets could release pollen all year long.

I note that breathing a high concentration of helium makes human voices high pitched, and apparently that concentration of helium is not high enough to suffocate someone. So I can imagine that if an Earthlike planet somehow manages to have a high enough concentration of helium in the atmosphere to change people's voices, humans might find that annoying. Since helium is the second lightest gas, and thus would escape very fast from an Earthlike planet, I don't think that a high concentration of helium would be very probable on the home planet of the galactic terraformers.

I note that the surface gravity of a habitable planet has to be low enough for human comfort, while the escape velocity of a planet has to be high enough to retain all the gases for long times - including helium if that is what makes the humans uncomfortable there. There are different formulas for calculating the surface gravity and the escape velocity of a planet.

A planet with a lesser density than Earth could have a surface gravity low enough to be tolerable with an escape velocity which was higher relative to Earth's than the surface gravity was.

I note that Uranus and Neptune both have lots of helium, and even hydrogen, in their atmospheres despite having escape velocities similar to the 11.186 kilometers per second of Earth. Uranus has a surface gravity of 0.886 g, and an escape velocity of 21.3 kilometers per second, 1.904 that of Earth. Neptune has a surface gravity of 1.14 g, and an escape velocity of 23.5 kilometers per second, 2.1008 that of Earth.

But their great distances from the Sun, and thus low temperatures and speeds of particles in their uppr atospheres, help them to retain hydrogen and helium. If they were close enough to the Sun to be warm enough for liquid water using life, they might not be able to retain helium.

If you decrease the density of a planet while increasing its mass and volume so it can have a low enough surface gravity for human and a high enough escape velocity to retain helium, it might turn into a water planet with its entire surface covered by tens or hundreds or thousands of kilometers of water, or even turn into a giant planet with no solid surface.

i note that in the Star Trek episode "Amok Time" there are two features of the planet Vulcan which make it uncomfortable for human:

KIRK: It's lovely. I wish the breeze were cooler.

MCCOY: Yeah. Hot as Vulcan. Now I understand what that phrase means.

KIRK: The atmosphere is thinner than Earth.

MCCOY: In this climate? If the heat doesn't get you, the thin air will. You can't do it!

MCCOY: Is this Vulcan chivalry? The air's too hot and thin for Kirk. He's not used to it.

http://www.chakoteya.net/StarTrek/34.htm

So the temperature and the thin air combine to make exertion on Vulcan uncomfortable for humans.

So possibly galactic standard terraformed worlds have higher or lower temperatures than most Earth humans are comforatble with, as well as slightly different atmosphere, and together they combine to make galactic standard planets rather uncomfortable for humans.

However, I want the “galactic standard” to have a high oxygen content to be conducive to larger creatures and more frequent fires, so just giving humans altitude sickness is not an option (even though it would be the easiest solution).

So maybe your galactic standard planets give Earth humans reverse altitute sickness which happens when people from high altitudes move to lower altitudes or sea level.

http://yaccatravels.com/blog/reverse-altitude-sickness-symptoms-cure-prevention-and-more

https://www.newscientist.com/lastword/mg24732983-000-is-there-such-thing-as-reverse-altitude-sickness/

The average atmospheric pressure at sea level is defined by the International Standard Atmosphere as 101325 pascals (760.00 Torr; 14.6959 psi; 760.00 mmHg).

https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Pressure_and_thickness

760 mmHg is 760 millimeters of Mercury. According to Dole on page 15, the sea level pressure of oxygen is 149 millimeters of mercury (mmHg). The accepted limit of oxygen for patients who needed extra oxygen was 40 percent of the total atmosphere then, though many persons could & can tolerate higher levels. Dole set an upper oxygen limit that would be tolerable for at least some if not necessarily all humans.

Following this philosophy, for our purposes we may state that the imspired partial pressure of oxygen must be greater than about 60 millimeters of mercury but less than about 400 millimeters of mercury.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

So if the oxygen content of the atmosphere is close to about 400 mmHg, some people should have problems breathing that much oxygen.

And many other other known gases, especially those which are common on Earth or other planets, have been tested to find upper limits of pressure tolerable for humans. Thus your galactic standard planet could have a higher than Earth normal concentration of nitgrogen, for example, as well as a higher than Earth normal concentration of oxygen, and humans might find breathing such dense air at least slightly uncomfortable, some humans more than others.

On page 20 Dole mentions upper limits for the concentration of dust particles in the atmosphere. However, I can't think of any reason why aliens would find lower concentrations of dust in Earth's atmosphere annoying. But possibly galactic standard worlds have so much pollen in the air (suggested above) that it would be irritatiing or harmful to humans, while aliens enjoy the scent of the pollen and miss it on Earth.

And possibly a combination of various atmosphere factors mentioned in my answer and other answers could make a galactic standard atmosphere more or less irritating to most humans and harmful to some, and make Earth's atmosphere uncomfortable for the aliens for reverse reasons.

• I think the point about particulates, pollen or pollution would make a big difference in comfort. Depending on the size of the Particulates it would also connect to lung function, or allergies. May 22 at 7:59

Heavy Air

70% of Earth air is Nitrogen. This is nonreactive and harmless to your lungs. If you only breath Nitrogen you will asphyxiate. But it is not the Nitrogen that kills you. It is the lack of oxygen.

The Standard air has some of the Nitrogen replaced with some other heavy nonreactive gas. For example Krypton is 3 times denser than air. This is uncomfortable for humans at first. But they eventually get stronger lung muscles and get used to breathing heavy air.

Edit: I am not sure about Krypton any more. Cody says it has an anesthetic quality. So the question is does there exist an equally suitable heavy nonreactive gas.

• Here is Cody breathing all the noble gases. The xenon one is actually a little bit scary. What is first aid for someone who has inhaled xenon and cannot exhale it because it has settled in the lungs? youtube.com/watch?v=rd5j8mG24H4&t=422s May 21 at 16:55
• @Willk Since it is heavy, it would be a good idea to simply put them upside down and let the xenon fall out. If that doesn't work then you perform the reverse kiss of life to suck the xenon out -- as popularised by Kirsten Dunst and Toby McGuire. May 21 at 16:59
• @Willk Sounds silly but is probably not far off what the real procedure. Only they would have obscure names for things. May 21 at 17:02
• @Willk I don't think it's possible for heavy gases to "settle in the lungs" and become impossible to exhale. I don't have a great source for that, though. My understanding is that our lungs are pretty good at mixing the gases within them, so with every breath in, the xenon will get diluted, and with every breath out, some of it will get exhaled. Once the xenon has been diluted, it will remain diluted; it will never settle to the bottom. May 22 at 17:24
• @JackAidley Maybe the denser air is harder to float away from artificial habitats. May 23 at 10:51

Simply humidity can make breathing uncomfortable.

Humidity can make the air feel heavy. Figuratively, because actually humid air is lighter than dry air. The reason is that water vapor molecules are lighter than both nitrogen and oxygen. Water vapor replaces either nitrogen or oxygen in the air. Thus, humid air has less or lighter nitrogen and oxygen. This also makes humid air less breathable.

Additionally, breathing in humid air activates nerves in the lungs that can narrow and tighten the airways. Humidity also makes the air stagnant enough to trap pollutants and allergens like pollen, dust, smoke, mold etc. It can trigger asthma symptoms as well.

• Also, if it's warm, humid air is uncomfortable in other ways as well. Downside: the air can easily be made nonuncomfortable again by dehumidification, as (e.g.) anyone from the eastern U.S. can tell you. May 22 at 14:55
• +1 because it's quite easy to imagine, for the other half of the question, "standard" life needs more water than most air on Earth contains.
– J.G.
May 22 at 21:35
• The simplest answers are usually the best and I think this answer is exactly that. The concept is easy to grasp for anyone. It's also nice that it can be negated with relative ease making it not a big deal but still quite the inconvenience. May 23 at 14:01

Ozone.

Ozone is formed by the action of UV radiation on oxygen and other atmospheric components. It is a normal and protective component of the upper atmosphere. It is also a normal minor component of the lower atmosphere though its creation is much augmented by human activities.

https://en.wikipedia.org/wiki/Ozone#Acute_ozone_exposure

Acute ozone exposure ranges from hours to a few days. Because ozone is a gas, it directly affects the lungs and the entire respiratory system. Inhaled ozone causes inflammation and acute—but reversible—changes in lung function, as well as airway hyperresponsiveness.[76] These changes lead to shortness of breath, wheezing, and coughing which may exacerbate lung diseases, like asthma or chronic obstructive pulmonary disease

Ozone is a powerful oxidizer. It is like chlorine in many respects and now is commonly used to treat pool water in circumstances where chlorine used to be used. If oxygen worlds had more ozone one might expect persons used to earth to use carbon filters to capture and react away the ozone. Exposed hair and materials from Earth would be bleached. Plastics and especially rubber will crack and oxidize fast. These things (besides a "sharp tang" in the air) will be other good ways for a fiction to show that things are different.

I already upvoted one answer, but you said you wanted this galactic standard atmosphere to have a high oxygen level and that could in itself be the problem. Being so reactive, oxygen is toxic when you get too much of it - check out the Wikipedia article on oxygen toxicity.

For a double whammy, you could have both O$$_2$$ and CO$$_2$$ at high levels.

Dust or pollen in the air can make breathing uncomfortable causing frequent sneezing and such. Not enough to die from, but certainly uncomfortable.

Some people are more resistant than others, and I guess humanity can grow to adapt. Or alternatively humans might wear special masks to filter out the particles.

Overly warm and humid can make things almost unbreathable over a prolonged period. Like if you are in too hot a shower for too long. You do not want to experience that.

Helium rich atmosphere

One might say that helium will be easily lost to space from an Earth-like planet, but that takes millions or billions of years. And since the aliens are terraformers, they can probably make more whenever they want.

Humans can either use breathers or sound like cartoon chipmunks.

• It doesn't take billions of years. E.g. this paper gives residence times of the order 1 million years for helium. That's still a long time, but it's a lot less than a billion years. May 22 at 9:10
• @N.Virgo thanks, I fixed it. Still, here we are 4.5 billion years later and we still got some. May 22 at 11:40
• That's because it's being produced by radioactive decay in the interior of the Earth and slowly making its way to the surface. There's a slow flow in from the Earth and a slow flow out into space. If you started with a helium rich atmosphere it would deplete until the two flows balanced again. May 22 at 11:56

Thin atmosphere - just as at high altitudes here on Earth.

Simply make the atmosphere the same as here on Earth, at the very top of Everest or a bit higher than that.

You can survive but it's a real chore.

• +1. Actually missed your answer and posted this myself (now deleted). It's a perfect fit. I wouldn't go as high as Everest though, that goes past uncomfortable and into dangerous. A lower plateau would be perfect. May 23 at 9:41

Hot, humid ar feels heavy, but it is actually lighter.Water vapor is not respirable, but it is actually less dense than the air that we know. Hot, humid air feels heavier because it is harder to breathe. Water vapor is a strong greenhouse gas, so that would also be trouble. Neon (an inert gas) would be about as heavy as water, but without the greenhouse effect. But by replacing water that would reduce the wet-bulb temperature and allow us to sweat away some water to cool us.

The maximum for survival would be a wet-bulb temperature identical to body temperature. above that temperature, water would literally condense upon us because we are cooler. That would create severe problems as we could no longer give off waste heat easily. The water condensing upon us would be hot -- not cool. It would not cool us. We would need to be in artificial cooling to survive.

High heat and humidity wear one down fast. It might not kill (up to a point), but it would make life miserable.

Capsaicin

In humans, capsaicin irritates the eyes, nose, throat, and lungs. However, the aliens have differences in their heat receptors such that they are not triggered by capsaicin. Instead, perhaps to the original aliens, capsaicin functions either as a pheromone, or more generally as a pleasant smell (much like lavender would be to us). And more generally, other aliens mostly have a neutral or no response to the capsaicin; it's just a complete accident that humans happen to have a debilitating reaction to the compound.

I don't know about the durability of capsaicin particles in the air. In case it turns out they would settle out within a comparatively short time, I could imagine a combination with the answer by Klaas van Aarsen: alien pollen happens to use capsaicin as a signalling molecule incorporated into the surface of the pollen, to entice the alien plants' pistils to accept the genetic material.

What you describes remind me of Netflix Arcane, where topsider police usually wear gas mask when going to Zaun

So simply just make it a little toxic and saturated (low oxygen level) to make it hard to breathe for anyone not acclimatized

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