Most gases that are toxic to breathe also do nasty things to the skin when they're in the atmosphere at dangerous concentrations - and one is walking around without sealed protective clothing; sulfur and nitrogen oxides cause chemical burns when they mix with the water in perspiration, or on one's eyes, large amounts of ozone cause similar problems by different mechanisms.

So my question is what are the key gases to use and, probably more importantly, at what levels to create an atmosphere that is lethal if you get a breath or two but won't burn your skin off or poison you anyway when you walk out the door without your moon suit?

Priority is given to gas mixes that are made up of gases that occur in/are produced by a biosphere almost identical to Earth's, so Water Vapour, Oxygen, Ozone, Methane, Nitro-oxides, Organic Carbon compounds, Carbon-oxides, Sulfur compounds etc... The best answer is the atmosphere that is most similar to our existing composition while being deadly to breath but otherwise safe for one to wear their birthday suit, apart from a breathing mask supplying safe air.

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    $\begingroup$ You mean with just a helmet? $\endgroup$ – nzaman May 21 '19 at 15:47
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    $\begingroup$ @nzaman I'm hoping for just a mouth-and-nose breather but full helmet is acceptable. $\endgroup$ – Ash May 21 '19 at 15:52
  • $\begingroup$ I am not sure you really need the hard science tag, anyway. science based answer would be fine $\endgroup$ – L.Dutch - Reinstate Monica May 21 '19 at 16:34
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    $\begingroup$ @L.Dutch Maybe but this is one of few times when I actually want some hard numbers to hang an explanation on. $\endgroup$ – Ash May 21 '19 at 16:42
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    $\begingroup$ @Ash: you'll need a helmet, or at least goggles, for extended exposure to any of these atmospheres. The cornea of the human eye gets its oxygen directly from air, which dissolves in the tear fluid. Positive pressure in a helmet is in any case a very good idea for safety in an asphyxiating atmosphere, let alone a poisonous one. $\endgroup$ – John Dallman May 22 '19 at 10:59

Any atmosphere that contains gases that are inert to a human metabolism, but no Oxygen or not enough Oxygen.

One would be an atmosphere containing 5% oxygen instead of the usual 20%, with the 15% made up of Nitrogen and Argon as here. At one atmosphere, it's equivalent to a depressurized airliner at cruising altitude. You don't notice anything wrong, but rapidly become first confused and then unconscious. You die after a few minutes from oxygen deprivation.

There are creatures here on Earth that can breathe this. Geese and vultures can fly at aircraft cruising altitudes. Birds have a superior variant of haemoglobin in their blood.

It's not directly lethal, though. You can be rescued after a minute or maybe two if there is someone else around with an oxygen mask. After this time you will have suffered irreversible brain damage.

For near-instant lethality I'd suggest an alien biochemistry that puts Phosgene into the atmosphere. It's not entirely odourless (smells of mouldy hay) but it causes fatal lung damage that is not immediately apparent. Read the section on its use as a war gas. I think it's also cumulative in which case a small undetected leak would be fatal after minutes or hours.

  • $\begingroup$ Cool that's a front runner as far as anything I've read so far and you've answered my next question too, I'll post it next week anyway. $\endgroup$ – Ash May 22 '19 at 12:00

an atmosphere that is lethal if you get a breath or two but won't burn your skin off or poison you anyway when you walk out the door without your moon suit?

take our atmosphere and remove all the oxygen, replacing it with an inert gas like nitrogen.

Asphyxia will ensue after a couple of breathes.

Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body that arises from abnormal breathing. An example of asphyxia is choking. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can induce asphyxia, all of which are characterized by an inability of an individual to acquire sufficient oxygen through breathing for an extended period of time. Asphyxia can cause coma or death.

This is the precise reason why, when using nitrogen to purge certain machinery, working in enclosed spaces can be lethal if proper precautions are not taken.

Otherwise such an atmosphere is perfectly safe to walk in it naked.

If you wonder what is the minimum oxygen concentration needed to sustain human life, you can refer to this

Serious side effects can occur if the oxygen levels drop outside the safe zone. When oxygen concentrations drop from 19.5 to 16 percent, and you engage in physical activity, your cells fail to receive the oxygen needed to function correctly. Mental functions become impaired and respiration intermittent at oxygen concentrations that drop from 10 to 14 percent; at these levels with any amount of physical activity, the body becomes exhausted. Humans won't survive with levels at 6 percent or lower.

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    $\begingroup$ If this is going to be an outside naked kind of world, I want nitrous oxide in the mix. $\endgroup$ – Willk May 21 '19 at 16:02
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    $\begingroup$ This was essentially the Earth's atmosphere for the first billion years that life existed $\endgroup$ – Joel Keene May 21 '19 at 20:55
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    $\begingroup$ @Ash a biosphere would absolutely not be a problem, it just can't be one based on Oxygen. But remember that there were organisms before oxygen, so you could have something evolve from that (see simple.wikipedia.org/wiki/Great_Oxygenation_Event ) for stuff before it $\endgroup$ – Hobbamok May 22 '19 at 10:23
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    $\begingroup$ Just make sure the organism don't emit oxygen as a waste product (like early life did on Earth), otherwise in a few million years you'll get a breathable atmosphere, and no one wants that. $\endgroup$ – GreySage May 22 '19 at 16:54
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    $\begingroup$ Only problem with this answer is that it is inaccurate due to the assumption of STP. The actual measure of oxygen that is important is called partial pressure. With high enough pressure, even 5% oxygen is breathable. In the technical diving scene, this is very important as one need to avoid something called Oxygen Toxicity. $\endgroup$ – Aron May 24 '19 at 3:27

My suggestion:

  • 60% Xenon
  • 20% Oxygen
  • 15% Nitrogen
  • 5% Carbon dioxide

Why Xenon?

Xenon is a noble gas. It has very few common chemical reactions, and is frequently used in situations where air is too reactive to be safe. It's safe to touch and even safe to breathe.

It's also a very effective anesthetic. Present-day doctors in Europe use it because it is remarkably side-effect free, if quite expensive. A few breaths of Xenon and you'll be out cold.

Why Oxygen?

20% oxygen is around the level we have in our atmosphere. This way, fire still burns and iron still rusts in your world the same as on Earth.

Why Nitrogen?

You have enough carbon dioxide to grow plants, but they need N2 in the air to keep the nitrogen cycle running. 15% is too little for the nitrogen-fixing bacteria in the soil, but your crew could have genetically modified soil bacteria for farming.

Why Carbon Dioxide?

This is how you die. CO2 is very stable and inert, but at concentrations of around 40000 ppm (4%), it becomes quite toxic. At these levels, it's so far above the lethality threshold that if you breathe this, you will die.

How does it work?

An astronaut is working on a new construction for the planetary base when her breathing mask undergoes a malfunction, disabling the low-air alarms. The astronaut continues working as her mask slowly runs out of air, and loses track of time. When the tank is almost empty, the broken alarm fails to remind her to refill, and without realizing, she begins to breathe the atmospheric air.

After three or four breaths, she begins to feel unusually drowsy. As soon as she realizes what happened, she holds her breath, but the xenon has already taken effect. Two shaky steps towards base later and she collapses on the floor, unconscious. Her brain, fooled by the apparently welcoming air, resumes breathing, taking in 13 times more carbon dioxide than she can handle. Instead of carbon dioxide from her lungs breathing out into the air, CO2 diffuses from the atmosphere into her alveoli, flooding her bloodstream with carbonic acid. Her blood pH plummets out of control, and she dies of carbon dioxide poisoning in under a minute.


  • No damage to skin or clothing
  • Lethal after a few breaths
  • Behaves effectively the same as our atmosphere in most everyday situations
  • You can grow crops with minimal modifications.


  • You need to explain how the hell the atmosphere became 60% Xenon. A comet, maybe, or it orbits a star with lots of Xenon?

Edit: To clear up the issue of where did all the xenon come from, here are a couple possible sources:

  • Presence in solar system: The planet orbits a star that burns hot enough to produce Xenon (our sun can't produce anything past iron), a nearby Xenon-rich star went supernova nearby a long time ago. This means that there's a lot of Xenon in the solar system to begin with, and its stability means that most of it will hang around. The planet either formed from a gas-and-debris cloud that contained xenon, which bubbled up to the surface during formation, or received it from xenon bubbles in porous ice and rock from asteroid and comet bombardments. This is where we got most of our atmospheric gases, because they exist in abundance in the space surrounding our planet. As long as there's xenon in the system, the planet will get a decent share of it.

  • Radioactive decay: Xenon is produced by nuclear decay from iodine-135 in our nuclear reactors (this actually causes problems because the gas blocks the nuclear chain reaction). The xenon produced by this is radioactively unstable, but there is another process that turns iodine-129 into xenon-129, which is very stable. The half-life of iodine-129 is 16 million years, so a planet rich in iodine-129 would steadily produce Xenon gas, bubbling up to the surface. This process could have already ended, resulting in a Xenon-rich atmosphere even after the decay has slowed to a crawl. Iodine-129 is a common product of nuclear fission, which occurs slowly in nature in underground uranium deposits. So a large amount of uranium, under immense geological pressure, slowly produces Iodine-129, which slowly decays into Xenon-129, which is stable and floats up to the atmosphere in geothermal vents, volcanoes, and hot springs.






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    $\begingroup$ Does it need that much Xenon? Xenon is doable, a lot of Xenon even but that high a fraction is extremely hard to justify. $\endgroup$ – Ash May 21 '19 at 18:33
  • $\begingroup$ @Ash medical concentration is 60-70 percent xenon, the rest oxygen. Any less and the anesthetic could take minutes to set in. Carbon dioxide won't put you to sleep, you can survive a few minutes of it and get to safety. The xenon makes it much more dangerous. $\endgroup$ – Adrian Hall May 21 '19 at 19:02
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    $\begingroup$ Could you perhaps justify a much more potent anesthetic that requires much smaller atmospheric percentages, that's produced by some species of plankton as a by-product, and that the local species have simply evolved around $\endgroup$ – Nicholas Pipitone May 22 '19 at 5:21
  • $\begingroup$ Do you also have to come up with a plausible (geological? chemical?) back-story how so much of a rare gas exists in abundance in this atmosphere? Wouldn't that imply an incredibly radioactive planet? How did the xenon get synthesized? $\endgroup$ – smci May 22 '19 at 12:33
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    $\begingroup$ Great answer because it does what the OP wants even with normal Earth levels of O2. $\endgroup$ – Cyn says make Monica whole May 22 '19 at 14:49

Pure nitrogen is harmless -- except that it won't support life. A breath or two will do no harm, but you won't even notice you're suffocating, because the carbon dioxide will clear from your blood as if you were breathing air -- but you won't be gaining any oxygen. You'll fall over unconscious after three or four breaths, and you'll die in four minutes (give or take whatever level of exertion was going on before, and whether you knew what you were getting into and could hold your breath for a while). The same would be true of nitrogen/oxygen, if the oxygen mix is less than about half what it is at sea level on Earth -- except it'll take longer to fall over, and longer to die.

Now, you wanted "lethal, but not corrosive" more or less. Hydrogen sulfide is biological in origin, more toxic than prussic acid, and although it has a very noticeable odor, humans lose the ability to smell it well below lethal levels, alarms are needed for concentrations as low as 5ppm. It claims a victim (or string of victims) on a fishing boat every so often, when refrigeration in the hold fails; a crew member is sent below to fix the system, doesn't come back, another goes to check on him, doesn't come back -- I've read about as many as eight people dying in this kind of scenario before anyone caught on.

A concentration of 800 ppm is considered 50% lethal, and 1000 ppm (that's 1/10 of 1%) is instantly lethal -- one breath will kill. As little as 100 ppm (1/100 of 1%) is considered immediately dangerous -- in part because this is the level at which loss of sense of smell begins. These levels can be formed by decomposition in natural settings, as long as there is a depression to collect the heavier than air gas.

Now, hydrogen sulfide will, eventually, form sulfuric acid, by oxidation and combination with water (vapor or liquid), but this is a slow process compared to its lethality.

  • $\begingroup$ Some science about lethal concentrations, rates of reaction, etc... is really needed to meet the Hard-Science tag but this is probably a good idea. $\endgroup$ – Ash May 21 '19 at 15:58
  • $\begingroup$ Sadly, I'm at work at present and can't readily look up the concentrations for loss of ability to smell, lethal concentration, etc. If you have that handy, I'd welcome an edit. $\endgroup$ – Zeiss Ikon May 21 '19 at 15:59
  • $\begingroup$ @ash Okay, never mind, I got it. $\endgroup$ – Zeiss Ikon May 21 '19 at 16:11
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    $\begingroup$ At 50 ppm, the smell will drive you out of the concentration before skin irritation/burns set in. Above 100-150 ppm, you can't smell it any more, and it's poisoning you with every breath; you'll likely be dead before skin damage means anything. $\endgroup$ – Zeiss Ikon May 21 '19 at 16:50
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    $\begingroup$ Except I'm looking at a gas that causes no damage at concentrations that you can't breath without dropping dead. $\endgroup$ – Ash May 22 '19 at 15:08

Carbon monoxide is a nice easy one. Colourless, odourless, non-corrosive, lethal at concentrations above 500 (or thereabouts) parts per million. It will oxidise over time to carbon dioxide in an oxygen atmosphere, so you’ll need some biological source to keep replenishing it.

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    $\begingroup$ More like 12,800 ppm for the "if you get a breath or two" in the question, at that kind of level it's effects as a reductant are going to be extremely noticeable. $\endgroup$ – Ash May 21 '19 at 16:12
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    $\begingroup$ I don't know about carbon monoxide but the carbon dioxide is an eye irritant, $\endgroup$ – jean May 22 '19 at 18:33

Helium. It is very inert, and used by deep divers to replace nitrogen due to it being safer at high pressures. It can enter all your tissues without causing harm. The only side effect to breathing it is a funny voice.

In an atmosphere made of pure helium, a human would die of asphyxia. That would be the only damage caused by such an environment.

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    $\begingroup$ @AdrianHall pressure does not depend just on gravity. Simply add more gas. $\endgroup$ – The Square-Cube Law May 21 '19 at 18:07
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    $\begingroup$ @AdrianHall Venus has less gravity than Earth and it's 900atm at ground level. $\endgroup$ – Ash May 21 '19 at 18:19
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    $\begingroup$ @AdrianHall "Venus air is much denser than Earth air." my point exactly, atmospheric density is weird and complex, it doesn't quite follow any one rule. $\endgroup$ – Ash May 21 '19 at 19:14
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    $\begingroup$ @AdrianHall The density of the gas has zero effect on the atmospheric pressure. You can have dense gases with a thin atmosphere (example: our moon. It's atmosphere is mostly argon but would be considered a hard vacuum) or light gases with a dense atmosphere (Jupiter.) Holding onto gases is purely a function of the escape velocity. (Not even the surface gravity. Build an unobtainium shell around a galactic core and you could have a breatheable atmosphere in nanogravity on it's outer surface.) $\endgroup$ – Loren Pechtel May 22 '19 at 3:46
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    $\begingroup$ @AdrianHall Pressure is due to the weight of the gas above, it has nothing to do with the density of what's around you. $\endgroup$ – Loren Pechtel May 22 '19 at 15:18

Wait, nodoby mentionned Avatar? Pandora is exactly what you describe: survivable but deadly if you breathe.

Pandora's atmosphere is a mixture of nitrogen, oxygen, carbon dioxide (>18%), xenon (>5.5%), methane, and hydrogen sulfide (>1%) and is about 20% denser than the atmosphere on Earth primarily due to the high percentage of Xenon; a heavy, colourless, odourless, and generally unreactive noble gas. The high concentration of carbon dioxide in the Pandoran atmosphere makes it extremely poisonous to humans, rendering them unconscious in about 20 seconds and causing death in about 4 minutes when they venture out unprotected by specialized breathing masks. The hydrogen sulfide present is also quite poisonous; concentrations over 1000 ppm (0.1 %) can cause immediate collapse with subsequent loss of breathing, even after inhalation of a single breath.

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    $\begingroup$ Someone mentioned Hydrogen Sulfide but it starts to cause surface damage around 50ppm so it's iffy as a candidate. $\endgroup$ – Ash May 22 '19 at 15:04


You cannot breath pure O2 for long, after a few hours it will start to destroy alveoli causing lungs permanent damage. It's also very toxic for your nervous system and can cause seizures and reach convulsions and unconsciousness. In fact, oxygen poisoning is a common cause of diving accidents due to the increased pressure in the diver body and from experience, I can tell you it feels like a serious hangover.

A 90% O2 in 2 atm pressure is more than enough to do the trick. It can become a "public health" problem because young people from the colony can try it as an alcohol substitute because it can make you feel drunk.

Bonus: Megafaun, insects in special can go very huge in an oxygen-rich environment.

Cons: Fire, increasing oxygen will increase risk of fire because any fuel will burn most easily. Wood can burn like gasoline and even stuff normaly hard to get fire in earth will readily burn if theres enough oxydant. Be sure to make your planet lightning free.

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    $\begingroup$ Yeah giant bugs do not endear this option to me at all but that's okay it does make a good point. That fire problem is so bad that an atmosphere with more than 35% O2 is basically impossible with carbon based land flora and fauna. But if the only photosynthesis is going on in the ocean an overloaded atmosphere could be a thing maybe. It appears that while this is a fun thought oxygen toxicity, even at very high doses takes a long time to effect people. $\endgroup$ – Ash May 22 '19 at 18:06
  • $\begingroup$ @ash no so long, I was an amateur diver long time ago and depending on quantity it can take seconds to get you dazzled and disoriented, also you lose your judgment capacity what makes it more dangerous. $\endgroup$ – jean May 22 '19 at 18:10
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    $\begingroup$ Yeah unfortunately I'm looking for near instant "you took off your mask there's no saving you now" material. $\endgroup$ – Ash May 22 '19 at 18:12
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    $\begingroup$ At 90% O2, dry forest will catch on fire in the hot sun and burn down entirely. So I doubt there could be a lot of flora, at least densely packed flora. $\endgroup$ – Adrian Hall May 22 '19 at 20:11
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    $\begingroup$ @AdrianHall LOL you are right, the big problem here is spontaneous combustion. Sure we can low the oxygen and yet got it lethal (in the long run). Also if there are moisture iron alloys will rust really quickly. $\endgroup$ – jean May 22 '19 at 20:15

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