I think that balloon aliens are an interesting alien body plan. It is something practically never seen on earth and the only lifeforms I’m aware of that do anything similar are kelp that have tiny bladders filled with hydrogen to keep them buoyant in the water. But I was thinking maybe such a body plan would be better adapted for a world with a denser atmosphere with heavier gas.

I have an idea for such a world that is covered in fungal forests and such an atmosphere. There are several creatures that take on a balloon shape in order to float effortlessly through the air. Along with the floaters there are deadly fungal spores, but I wanted another good reason for human explorers to keep their helmets on.

So I need a gas that is heavy but also not very healthy for human lungs.

One idea I had was sulfur hexafluoride that seems perfect based on this video.


If you can float a tinfoil boat on the stuff, you can probably float anything. You don’t even need to try and produce hazardous hydrogen, and going up like the Hindenburg anytime lightning strikes. All you need to do is filter oxygen from the atmosphere, suck down a bladder full and start flapping.

Not to mention it isn’t exactly the best thing to breathe. While many people use it to make their voices sound deeper, it carries the same side effects of helium, first hit makes your voice sound silly, but breathe in too much and not enough oxygen will reach the lungs resulting in eventual death by asphyxiation.

But I wanted to call upon the wisdom of the crowd to make sure I have it right.

So what atmospheric composition would allow balloon animals to float and be toxic to breathe by human explorers?

  • $\begingroup$ You can float a tinfoil boat on SF6 because the gas above it, air, is less dense. Fill the boat with SF6 and it will sink just as filling it with water will cause it to sink. Thus, your Q is trivially answered with, "any gas that's toxic to humans and more dense than hydrogen." E.G., pure oxygen, methane, etc.. But that means there's too many answers for this to be a good Q. From the Help Center we read, "if your question could be answered by an entire book, or has many valid answers, it's probably too broad for our format." $\endgroup$
    – JBH
    May 27, 2022 at 5:18

2 Answers 2


Noble Gasses

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

The noble gasses are like nitrogen but denser. Xenon is the same density as Sulfur Hexafluoride..

The great thing about noble gasses is they are unreactive. Meaning you do not need to worry about things dissolving in the air the same way you would with a Sulfur Hexafluoride atmosphere.

I suggest an atmosphere with little oxygen, a lot of heavy noble gasses, and the rest being whatever you need for the floaters' metabolism. This will give a thick soupy atmosphere.

Of course it is hard to imagine how such a planet comes about in the first place. But no harder than a planet of Sulfur Hexafluoride.

  • $\begingroup$ Note that heavy noble gasses have narcotic effects, which could reasonably be considered "toxic". $\endgroup$ May 27, 2022 at 16:31
  • $\begingroup$ @LoganR.Kearsley Yes Cody says his lips become numb from breathing Xenon. I imagine full exposure would be eventually fatal, even with a 20/80 oxygen/xenon mix. $\endgroup$
    – Daron
    May 27, 2022 at 18:54

Put it farther away from the sun (or use a dimmer sun) so it doesn't boil, and fill the atmosphere with lots of carbon dioxide.

CO2 is considerably denser than O2 or N2, and so easier to float a balloon in. But it is also extremely common, unlike SF6 or heavy noble gasses. And, it is actively toxic to humans at concentrations not very much higher than what Earth has, causing hyperventilation, respiratory acidosis, and reduced cognitive function, even when there is also sufficient oxygen to support human life; i.e., it does not merely cause death by suffocation.

There are two planets in our own solar system with primarily-CO2 atmospheres (Venus and Mars), one of which is extremely thick (Venus), so forming such an atmosphere naturally is clearly easy to do--unlike naturally accumulating huge quantities of SF6 or xenon.

More exotic options:

Hydrogen Sulfide (H2S): It's just slightly heavier than air, and it would react with oxygen fairly easily so you would need a mechanism to continuously replenish it if the atmosphere is meant to otherwise support oxygen-breathing life, but it is acutely toxic at even very low concentrations.

Sulfur Dioxide (SO2): Much heavier than CO2, and it's an acute respiratory irritant--it will specifically damage the lungs, and eyes, and at higher concentrations irritates the skin. Eventually, it causes bronchial inflammation, and pulmonary edema, and laryngeal spasms. It can react with oxygen to form SO3, which is a solid at human-survivable temperatures, but not as rapidly as H2S is destroyed, and it's easy to imagine biological processes which would destroy SO3 to regenerate SO2. It also reacts with water to form sulfurous acid (a major component of acid rain). The known existence of worlds like Io, which high concentrations of sulfur on the surface, make this plausible.

You could also go with a combination of CO2 and SO2, providing multiple mechanisms to aggravate and poison humans without excluding native oxygen-breathing life.

Nitrous Oxide (N2O) and Nitrogen Dioxide (NO2): in the absence of denitrifying bacteria, all the oxygen in the atmosphere would eventually react with nitrogen to form various nitrogen-oxygen compounds. Nitrous oxide is a sedative, and nitrogen dioxide will react with water to form acids just like sulfur dioxide does, and produce similar respiratory irritation. Hal Clement's The Nitrogen Fix explores the aftermath of a catastrophe in which biological nitrogen fixation outstrips the rate of denitrification, turning Earth into this sort of world. Native organisms might not need to breathe at all, relying exclusively on dissolved nitrates as a metabolic oxidizer, but there could still be free oxygen in the atmosphere as well if photoautotrophs (plants) keep producing it.


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