I have been brainstorming for my sci-fi world building project and liked the idea of having a planet where there persisted a heavy poisonous gas over the majority of the land that would force almost all animals and plants to live on higher altitudes. However I don't know nearly enough chemistry to immediately know what sort of gas it might be or how it would form on this planet. Perhaps it is something environmental, maybe an organism is intentionally producing it to spread its territory and only its own species survives in it.

Any ideas for certain chemicals?

  • $\begingroup$ Verik, please be aware for future questions that the help center states, "Questions must be specific as well as answerable. If you are looking for discussion, brainstorming, or an overall process rather than specific questions and answers, the Worldbuilding Stack Exchange might not be a good place for your question." Fishing for ideas questions are generally difficult to ask here because you are required to ask with the intent of objectively picking a best answer (you need to provide limitations, restrictions, conditions, and expectations). $\endgroup$
    – JBH
    Commented Jan 1, 2023 at 21:23
  • $\begingroup$ @JBH Is where a specific stack exchange site better suited for this question? If not, is there any other places you could suggest? $\endgroup$ Commented Jan 1, 2023 at 22:18
  • $\begingroup$ @Venik Hue This reminds me of the planet in Poul Anderson's Orbit Unlimited and the planet Plateau in Larry Niven's Known Space series. So maybe you should check the atmospheric compositions of those worlds. $\endgroup$ Commented Jan 2, 2023 at 17:40
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    $\begingroup$ @SamKitsune There is no stack for brainstorming questions. Stack Exchange's principle model is one-specific-question/one-best-answer. Worldbuilding is a curious outlier in that it hosts a tremendous number of subjective questions. It's actually a miracle that SE let it graduate from Area 51. On the other hand, we list a number of discussion forums that might be better suited in the List of Worldbuilding Resources. (*Continued*) $\endgroup$
    – JBH
    Commented Jan 2, 2023 at 19:30
  • $\begingroup$ @SamKitsune ... I'm testing whether or not it's possible to remove the brainstorming restriction for the Stack (see this Meta post), but it's failing (mostly, it appears, because people don't like how I asked the question). $\endgroup$
    – JBH
    Commented Jan 2, 2023 at 19:32

4 Answers 4


Sulfur dioxide and Nitrogen dioxide

It seems to me that you're asking for a gas that fulfills three core requirements: (a) it is toxic to plants and animals (b) it is denser than air, so it will settle at lower altitudes (c) you want an explanation for how it would form on a planet. Sulfur and nitrogen oxides fulfill these requirements, assuming your planet resembles Earth.

(a) Toxic to humans and plants

Sulfur dioxide (SO2) and nitrogen dioxide (NO2) are both gases that are toxic to human and plants. They have severe effects on the respiratory tract in humans and can inhibit growth in plants:

Sulfur dioxide is a severe irritant to the respiratory tract, eyes, mucous membranes, and skin. Exposure to high doses can cause pulmonary edema, bronchial inflammation, and laryngeal spasm and edema with possible airway obstruction. There is no antidote for sulfur dioxide.

Sulfur dioxide inhibits photosynthesis by disrupting the photosynthetic mechanism. The opening of the stomata is promoted by sulfur dioxide, resulting in an excessive loss of water. The cumulative effect of sulfurous pollution is to reduce the quantity and quality of plant yield.

Elevated levels of nitrogen dioxide can cause damage to the human respiratory tract and increase a person's vulnerability to, and the severity of, respiratory infections and asthma. Long-term exposure to high levels of nitrogen dioxide can cause chronic lung disease.

At high concentration levels, nitrogen dioxide is potentially toxic to plants, can injure leaves and reduce growth and yield. In combination with either ozone (O3) or sulphur dioxide (SO2), nitrogen dioxide may cause injury at even lower concentration levels.

As indicated by the last quote, sulfur dioxide and nitrogen dioxide are most harmful in combination. When combined, you can get the same effect at lower levels.

(b) More dense than air

SO2 and NO2 are both denser than air. Air at sea level has a density of about 1.2 kg/m3. In contrast, the density of sulfur dioxide is 2.86 kg/m3 and nitrogen dioxide is 2.05 kg/m3. They will easily settle to lower altitudes.

(c) Produced by fossil fuel consumption

Sulfur dioxide and nitrogen dioxide are byproducts of fossil fuel consumption. They are formed during the combustion of fossil fuels such as coal, gas, and oil, especially from fuel used in cars.

Fossil oil is largely made up of carbon and hydrogen which form carbon dioxide (CO2) and water following combustion in air. The combustion of fossil oil also creates sulphur oxides (SOx) and nitrogen oxides (NOx) and these are believed to have a detrimental effect on the environment, the atmosphere and the ozone layer.

Sounds like exactly the combination you want.

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    $\begingroup$ Judging by your figures there, you've given the density for liquid sulfur and nitrogen dioxide, not gaseous, so for the purposes of atmosphere formation you're out by a couple of orders of magnitude. The biggest issue with both is that they readily react with water to form acids that tend to be liquid at STP and rain out onto the surface. You need a continuous and energy-intensive process to keep enough in the air to remain toxic to breathe. $\endgroup$ Commented Jan 2, 2023 at 14:17
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    $\begingroup$ @StarfishPrime You're right. I changed the values of the densities to the vapor densities. My bad. I knew the numbers didn't sound right ;) Doesn't change the science, however, as both are still denser than air. $\endgroup$ Commented Jan 2, 2023 at 20:11
  • $\begingroup$ @StarfishPrime You are right about the acid rain. Some spur-of-the-moment ideas that might solve this: (1) Sulfur/nitrogen dioxide does turn into acid rain, which pollutes water sources and causes deforestation. As a result, inhabitants migrate to higher altitudes seeking fresh water and plant life. (2) Climate change affects the temperature enough to affect the phases of these substances (ex: extreme global warming keeping large amounts of nitrogen dioxide as gas at atmospheric temp). (3) Enough fossil fuels have been produced to cause a shift in the equilibrium of this reaction [...] $\endgroup$ Commented Jan 2, 2023 at 20:24
  • $\begingroup$ [...] cont. (3) If there is excess sulfur/nitrogen dioxide, water and air would become limiting reactants. Obviously this is an extreme case and would probably be unlikely. (4) This occurs on a different planet with different atmospheric temps/pressures. OP never specified Earth (this one's my last resort...;) $\endgroup$ Commented Jan 2, 2023 at 20:25
  • $\begingroup$ I believe NOx disproportionates when a certain concentration is reached. SO2 doesn't though. $\endgroup$
    – user86462
    Commented Jan 4, 2023 at 7:43

a heavy poisonous gas over the majority of the land that would force almost all animals and plants to live on higher altitudes

The problem here is that either a) the atmosphere has been like that for evolutionarily-interesting periods of time (in which case things will have evolved to cope with it) or it has been produced over such a short timescale that it will have required some very energetically intensive industrial-scale process to produce... basically a xenoterraforming effort which has worked very successfully and will probably soon kill any remaining vestiges of the old ecosystem.

The solution I'd propose is that you have some or more species of interest (eg. humans and other terrestrial plants and animals, etc) that can't survive "at depth", and either native flora and fauna that are just fine or lifeless depths because the planet in question is being actively terraformed and there was no life to speak of beforehand.

As I mentioned in the other answers, having some stable toxic gas that covers huge areas without the need for implausible amounts of rare elements, and/or industrial scale hugely energetic chemical processes to maintain it, is difficult.

What I'd suggest instead is pressure. The gas mix you're breathing right now is pretty toxic when breathed for long enough under high enough pressure... nitrogen narcosis and oxygen toxicity are both things that can disable or kill you and similar terrestrial organisms. Some gases are even more narcotic than nitrogen. Most of these are cosmically rare, but argon already forms ~1% of the air you're breathing and is only ~15 times more rare than nitrogen in the solar system. It is also more than twice as narcotic as nitrogen.

Therefore, I suggest you have a world with a very thick atmosphere, rich in argon and maybe even with increased amounts of oxygen relative to Earth. There are probably other gases you could throw into the mix just to make things more unpleasant, such as increased CO2 as well, which is also narcotic to humans at high pressures. Surface pressure should be at least 4 bars, and preferably higher. Humans and their fellow terrestrials are just fine at altitude, but at depth they become more and more intoxicated until they become unable to act sensibly and end up getting lost, or crashing their vehicles, and ultimately dying (probably of thirst) unless rescued in good time. High oxygen pressures can cause seizures and unconsciousness and given enough time, lung and nerve damage that can kill or disable. It might also be possible to suffer from decompression sickness if ascending from the lower altitudes too quickly, and gas embolisms can definitely kill or cripple in very short order.

Lots of plants and insects could survive at lower altitudes of course, but if this is a lifeless world being terraformed then they won't have had time to spread down there yet. Humans would stick to the high mountains and plateaux and carry gas monitors if they venture down to lower altitudes.

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    $\begingroup$ You might want to add mentions of the planetary atmospheres in Poul Anderson's Orbit Unlimited and the planet Plateau in Larry Niven's Known Space series. As I remember they were too dense to breathe at lower altitudes but breathable at higher altitudes. $\endgroup$ Commented Jan 2, 2023 at 17:42
  • $\begingroup$ @M.A.Golding not familiar with those. The world of Fulgor in John Meaney's To Hold Infinity had uninhabitable low-altitude "hypozones" but didn't discuss the whys and wherefores at all (which is usually the best approach) is the only one I remember coming across myself. $\endgroup$ Commented Jan 2, 2023 at 18:13
  • $\begingroup$ Isn't this kinda the situation on Venus? The surface is absolutely lethal, but 80 km up humans can go outside an airship with an oxygen supply and eye protection. Well, for a short time at least, full body protection is still advisable. $\endgroup$ Commented Jan 5, 2023 at 10:10
  • $\begingroup$ @TheDyingOfLight Saturn is similar, though floating is harder and you'd need to bring warm undies with you. $\endgroup$ Commented Jan 5, 2023 at 16:25

Sulfur Hexafluoride

Sulfur hexafluoride (SF6) is an inert, invisible gas with no smell. It's so dense that if you breathe even a mixture of it and air in, it can't escape out of the bottom of your lungs, and you gradually asphyxiate. Because it's so heavy, it stratifies nicely and will sit in low areas; it's also very stable so it will persist for a long time.

Chlorine and Bromine

The opposite of SF6, in some ways. Chlorine (Cl2) is a green gas, stinks, and kills people painfully, with lots of obvious signs. It's relatively reactive, so you need a way to continuously make it; but it's also easy to make. It's heavy enough that it will mostly sit in low areas, but will waft up pretty readily if there's much wind.

Bromine (Br2) is even more toxic and brown, and is liquid at room temperature, boiling into gas when it's warm. It's much heavier than chlorine even when in the gas form. Between it being much heavier and having a liquid form, it will stratify much better.

However, chlorine is very abundant on many real life planets and bromine isn't, so it's harder to believe from that point of view. (Think a silver planet versus a gold planet).

Between SF6 and one of Cl2 and Br2, you ought to be able to match what you are imagining.

The weakest point is that plants probably won't care about these gases unless there's so much of them that they can't get CO2. At the very least, they will grow deeper than most animals live.

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    $\begingroup$ Fluorine is depressingly rare... its cosmic abundance is low and its reactivity is high so the problems here is that there won't be much fluorine to work with in the first place, and mass producing stable gas-phase compounds is going to be very energetically expensive. $\endgroup$ Commented Jan 2, 2023 at 14:19
  • $\begingroup$ @StarfishPrime The quantity needed for an atmosphere's worth of gas isn't really a problem. SF6 has a highly exothermic formation reaction. Cl2 and Br2 are energetically expensive, but no more so than O2. We've had several photosynthetic schemes for Cl2 imagined over the last year or two. $\endgroup$
    – user86462
    Commented Jan 2, 2023 at 18:06
  • $\begingroup$ @StarfishPrime Fluorine has a relative abundance similar to chlorine on Earth; both are 2 to 3 orders of magnitude more common than Br. SF6 has a whopping formation enthalpy of -1200 kJ/mol. $\endgroup$
    – user86462
    Commented Jan 2, 2023 at 18:16




Your world has a lot of radon. It is a gas but it is very heavy. In our world it is mostly of concern when it accumulates in low lying unventilated areas. In your world there is orders of magnitude more radon and not much wind. Low lying areas are not compatible with life because of the radiation emitted by radon.

A side product of all the radon is that many surface areas are dusted with lead, which is a decay product of radon.


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