I have a roughly Earth-like planet in a solar system some distance from ours. The atmosphere of this planet is largely similar to that of Earth, but particularly, has a somewhat higher oxygen content; currently, I'm thinking about 26-28% oxygen (O2(g)) and 65-68% nitrogen (N2(g)), compared to Earth's 21% oxygen and 78% nitrogen. The surface temperature of the planet varies between about 230 K (-43°C) and 320 K (+47°C) depending on location and time of year.

Compared to Earth's atmosphere, I would like to add something to the planet's atmosphere that has the following properties:

  • Concentration: a few percent by mass
  • At least extremely foul-smelling, ideally poisonous, to humans as we know them; doesn't need to be foul-smelling if poisonous; the higher the toxicity, the better, but beggars can't be choosers so I'll take what I can get
  • Does not pose a problem for local creatures that have evolved in such an atmosphere (this is an absolute requirement)
  • Reasonably evenly distributed among the other atmospheric gases (I don't want it to form a separate layer at some altitude where, if you can just avoid that, everywhere else is basically fine)
  • Can be maintained over time in a high-oxygen environment, possibly with geological replenishment (for example through volcanic eruptions) or through insolation breaking up some other gas that does get replenished on an on-going basis
  • Doesn't react with the rest of the atmosphere at the first opportunity

I haven't really decided all that much about the planet's geography or geology, so if some aspect of that makes it easier or for that matter harder to pull this off, I'm open to input.

I was thinking about methane, but that massively fails at least the requirement of non-reactivity in a high-oxygen environment.

Hence two very closely related questions:

  • What gases might be good options to satisfy all the criteria above?
  • If all of the criteria aren't satisfiable using a single gas added to the atmosphere, then what is the closest I can reasonably get?
  • $\begingroup$ Carbon monoxide would be very lethal to humans, but wouldn't for any animals which evolved in such atmosphere. However I don't know if it satisfies stability condition. $\endgroup$ – M i ech Apr 21 '17 at 7:34
  • $\begingroup$ @Miech Yes; it isn't so difficult to satisfy some of the conditions, but I'm at a loss for anything that can satisfy all of them. That said, carbon monoxide could perhaps be a decent answer if you can propose a biological or geological mechanism to maintain a level of CO in the atmosphere. $\endgroup$ – a CVn Apr 21 '17 at 7:36
  • $\begingroup$ You can scratch my idea. I had a vague memory that CO+O2 mix is volatile, but I didn't check it before posting first comment. After searching, I found that CO+O2 mix is even suggested as fuel in some specialised situations, so such atmosphere would be explosively unstable. $\endgroup$ – M i ech Apr 21 '17 at 7:44
  • $\begingroup$ @Miech CO concentration high enough to be explosive will kill a human in less than a minute. You can drop concentration by two orders of magnitude and still kill humans within few hours. $\endgroup$ – Ville Niemi Apr 21 '17 at 8:18
  • $\begingroup$ CO in an oxyge rich atmosphere would be oxidised to CO2 so would have to be continously replaced $\endgroup$ – Slarty Sep 19 '17 at 19:32

Carbon Dioxide

The simplest and most straightforward solution.

  • Deadly to humans at concentrations of around 10% (this is on the high end of 'a few percent by mass' but you said you will take what you can get)

  • Hypercapnia kills you by messing with the carbonic acid concentration in the blood. Since this is just a pH balance thing, it seems pretty easy for local wildlife to evolve to manage it.

  • CO$_2$ will disperse through the troposphere and stay evenly mixed (see: Earth)

  • CO$_2$ is one of the most common planetary atmospheric gasses (Venus, Mars) so it is highly likely to be present on planets to start with. On Earth, it was mostly replaced by oxygen through photosynthesis. However, if you go with an arid planet with few oceans and an order of magnitude or two less biomass of plants and algae, then you can explain why the equilibrium CO$_2$ levels are so high. Geological and biological processes (the carbon cycle) will then maintain this level.

  • CO$_2$ is stable in an oxygen-nitrogen atmosphere (see: Earth, also, too bad it does).

As a note, a higher CO$_2$ planet might be expected to have a higher greenhouse effect. However, if you also made the planet more arid and removed a lot the oceans, then you would have much less water vapor in the atmosphere. Since water vapor is an even more powerful greenhouse gas than CO$_2$, you can argue that there is some point where these effects balance out, and distance to the sun is about the same.

Regarding making the planet arid, I propose this breakdown. The Earth is about 70% ocean, 20% usable land, and 10% wasteland (Sahara, Antarctica, Siberia, etc). If you alter the ratio to 20% ocean, 20% usable land, 60% wasteland (giant continental deserts), you end up with the same relative inhabitable size of planet as Earth, and yet have highly reduced biomass of plants and algae.

  • $\begingroup$ I kind of like this idea, actually; especially because the CO$_2$ level could easily be tuned to get the desired effect on humans: everything from barely noticable through clearly problematic to deadly. $\endgroup$ – a CVn Apr 22 '17 at 12:22
  • $\begingroup$ Is excess CO2 detectable without heavy technical support? $\endgroup$ – Ash Sep 19 '17 at 18:31
  • $\begingroup$ @Ash Yes, and for cheap $\endgroup$ – kingledion Sep 19 '17 at 19:10
  • $\begingroup$ @kingledion That would be exactly what I meant by "heavy technical support", human 1.0 has no ability to detect high concentrations of CO2 though, it doesn't have a smell or a taste or anything like that? $\endgroup$ – Ash Sep 19 '17 at 19:16
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    $\begingroup$ Good answer easily produced naturally in excess including volcanos. Planted with carbon dioxide atmospheres are known eg Venus and Mars and meets your poison requirements. I would say it’s the closest you will get. Only drawback is you need a fair quantity of it, although I would think even 2-5% for an extended period would not be healthy for humans. You might find this link of interest: thoughtco.com/carbon-dioxide-poisoning-608396 $\endgroup$ – Slarty Sep 19 '17 at 19:42

Nitrogen dioxide.


From http://www.healthhype.com/silo-fillers-disease.html

Nitrogen dioxide damages the lower airways in particular and the lung tissue. It breaks down into nitrous and nitric oxide within the airways and lungs damaging the ciliated cells that line the airways and the epithelial cells (pneumocytes) that maintain the air sacs (alveoli) of the lungs. Type I pneumocytes are primarily affected in silo filler’s disease. This results in inflammation of the bronchial and bronchioles walls (bronchitis and bronchiolitis) and alveoli (pneumonitis) along with fluid accumulation within the lung (pulmonary edema). It is important to note that neither bronchitis nor pneumonitis in silo filler’s disease occurs due to an infection. However, patients with silo filler’s disease are at a greater risk of developing infectious bronchitis, bronchiolitis and pneumonia.

I have read about silo disasters where workers encounter a pocket of NO2, are overwhelmed, and then additional workers coming in to rescue them are also overwhelmed. I think in concentration like this the NO2 is just visible - a heavy brown gas.

In lesser concentrations it is an important component of smog.

I was skeptical to read in this excerpt that the active moieties are actually nitrous and nitric oxide. I am skeptical about that - certainly nitrous oxide is inhaled essentially at 100% by kids at the dentist. Nitric oxide is a vasodilator and also not that toxic. Wikipedia states that

NO2's health effects are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species which is more plausible.

NO2 can exist in equilibrium with O2 and N2 which is good for your world. Humans can put up with small quantities of NO2 for a while which is good for narrative possibilities. Biting, unpleasant atmosphere is more compelling that if your humans just drop dead from a breath.

Re the natives: our scavenging /detox mechanisms can get rid of some NO2 and we can repair damage from the rest. A matter of degree like other toxins. If your natives have much more radical scavenging ability in their cells they will be able to tolerate more NO2. For humans you could have your atmosphere anywhere from the silo bottom (rapid death) to Beijing smog (no fun). You could have it vary day to day and place to place.


You could throw a bit arsenic in the atmosphere. A few years back there was talk of a earthbound lifeform that used arsenic instead of phosphor.

Wikipedia says (highlight by me):

In 2008, bacteria were discovered that employ a version of photosynthesis in the absence of oxygen with arsenites as electron donors, producing arsenates (just as ordinary photosynthesis uses water as electron donor, producing molecular oxygen). Researchers conjecture that, over the course of history, these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive. [...] Although the arsenate and phosphate anions are similar structurally, no evidence exists for the replacement of phosphate in ATP or nucleic acids by arsenic.

Now I am no chemist. Arsenic is mainly dangerous for earthlings if dissolved in our drinking water, but it has forms that are dissolved in the air. High concentrations of arsenic (a few percent should be more than enough) should make your planet deadly for humans. It could be replenished by local phenomena just as it is replenished on earth and should stay in your atmosphere (or at the very least - your drinking water and soil). As for the smell ask chemistry student Ryan Curry:

Several sources claim that most arsenic salts are tasteless, but may change the flavor of a food they contaminate (bitterness is often reported). One symptom of arsenic poisoning (like other metal poisonings) is a metallic taste in the mouth.

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    $\begingroup$ Wikipedia seems to claim that 5 mg/m^3 is "immediately dangerous to life and health". Because I'm too lazy to run the exact numbers right now, if the atmosphere is approximated as nitrogen (about 1.251 g/dm^3 = 1251 kg/m^3 at standard temperature and pressure), if my math is right then that's about 4 ppm! Makes me wonder what a few percent (tens of thousands of ppm) would be like for unprotected human visitors; almost certainly immediately debilitating, at least. en.wikipedia.org/wiki/Arsenic#Legal_limits.2C_food.2C_and_drink and en.wikipedia.org/wiki/Nitrogen $\endgroup$ – a CVn Apr 21 '17 at 21:24
  • $\begingroup$ Gaseous Arsenic compounds are too toxic for the OP's purposes, the least toxic are universally lethal at a few hundred ppm. $\endgroup$ – Ash Sep 19 '17 at 18:27

You could try hydrogen sulfide. It's not stable in the long-term in an oxygen atmosphere, but it could be continually renewed from biological sources, and you don't need a high concentration. You can have it be either smelly or fatal, but not both, since fatal concentrations paralyse the sense of smell and so can't be smelled. You'd only need 1 part per million to make your planet smell very bad, or maybe 100 to 300 parts per million to be lethal.

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    $\begingroup$ First thing I thought of too, so this earns you plus one from me. Actually the planet can be both smelly and fatal. Some locations will have fatal atmospheres, say, due to a high concentration of biological sources of H2S, while others don't have so much. As long as you can smell the foul aroma of rotten eggs it's safe, but if the air smells fresh you die. $\endgroup$ – a4android Apr 22 '17 at 1:56

Sulfur dioxide. https://www.cdc.gov/niosh/idlh/7446095.html Site requires at least 30 characters. Why? I don't know. Venus has traces of SO2 in its atmosphere, it's certainly a possible gas. It would be an immediate health threat at about 500 ppm which is 0.05%, so a couple of % would be deadly. An alternative, or just an additional toxic gas is carbon dioxide. Its tolerated in much higher %, but between 7 & 10% produces unconsciousness. Note, Venus' atmosphere is mostly CO2.

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    $\begingroup$ Site requires 30 characters because link only answers aren't really answers - if link is inactive, they are we worthless. $\endgroup$ – Mołot Apr 21 '17 at 7:45
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    $\begingroup$ Welcome to WorldBuilding! As has already been said the site requires you to write at least 30 characters as we don't want link-only answers and answers that look more like comments. Those would be deleted if the user didn't flesh it out. You can comment once you reach 50 reputation. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Apr 21 '17 at 7:59

What about Ozone? It has a sour smell reminiscent of Chlorine, it's toxic at fairly low doses but not lethal until the dosage is much higher, it forms naturally in the presence of UV Radiation and Methane and what it doesn't kill adapts to otherwise lethal doses in a space of hours. Ozone decomposes in the presence of Oxygen but it forms fast enough in the lower atmosphere to be a health risk in modern day cities where smog gives it unburnt organic molecules for a catalyst. A planet with a lot of UV from a bright star, say white or blue spectral type, and an active anaerobic carbon cycle, so plenty of methane and even ethane in the atmosphere, would have a diurnal toxicity cycle. On such a world the air would stink of excess Ozone by night and be lethal during the day when the sunlight was producing bulk quantities of fresh gas.

As an additional note Ozone at high concentrations is both corrosive and explosive so bodies left out in the sun would dissolve and/or burn up in the Ozone bathed daylight hours.


I have found chlorine planets to satisfy such requirement. Here is one. There are other possibilities involved:

  • sulfur dioxide if sulfuric acid replaces water
  • carbon dioxide: if the planet is too far out from its sun, it will need more greenhouse gases, such as CO2. An ecosystem in equilibrium must maintain a higher level, which may be harmful to humans.

I think carbon dioxide is your best bet as already described. If you want something more exotic then, Chlorine, Bromine or hydrogen cyanide might work but would need replenishing. Xeon is used as an aesthetic and would ultimately be fatal but high concentrations would be needed. Failing that have a look here: highly toxic gases


I am going to make the most cruel suggestion here. Have your world's crust have large amounts of surface uranium. How large? Let's say continental amounts.

But I asked for a gas, not a solid material!

Wait for it. Uranium naturally breaks into radon over time, which is a gas. Wikipedia has some things to say about radon:

It is a radioactive, colorless, odorless, tasteless noble gas. It occurs naturally as an intermediate step in the normal radioactive decay chains through which thorium and uranium slowly decay into lead; radon, itself, is a decay product of radium.


As radon itself decays, it produces other radioactive elements called radon daughters (also known as radon progeny) or decay products. Unlike the gaseous radon itself, radon daughters are solids and stick to surfaces, such as dust particles in the air

Being a noble gas, radon is mostly non-reactive.

So why is the presence of surface uranium required? Radon is short lived (it has a half life of 3.8 days), so it has to be replenished from a natural source.

I like this idea because a simple gas mask or astronait suit will not be enough to protect a human in this environment. And anything you use to shield yourself from it will be covered in radioactive soot.

As for how life could cope with it, just look for plant and animal populations around Chernobyl. Also for radiotrophic fungi that grow inside Chernobyl's reactor - they use melanine to extract energy from radiation in a process similar to photosynthesis. And these are life forms that evolved in our world - in a world that has always had high levels of radioactivity from the beginning, life would be much more resistant to it.


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