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I have seen some questions asking whether some atmospheric compositions are survivable for humans. Most of them focus on long term survival, as in people being able to breathe indefinitely without the need for special apparatus, or at least with a breather.

I want a variation of the second case. I am writing a story that happens in a planet where people can breathe without any equipment for about a day, perhaps more, but this will ultimately kill them. So they use breathing gear when exploring the place, and they have no cause to panic in case the equipment breaks down as they can just go for a spare... But if someone gets lost and can't find a new breather, they know their hours are numbered.

Optionally, whatever is toxic for humans in the atmosphere does not cause damage nor irritation to the skin and sensitive parts such as the eyes and nose, so that a person can be comfortable without the need for special suits or masks.

In case it helps, the planet I am sending people to die a slow death is Gliese 667Cc. I am going to handwave a few of its characteristics to make it more habitable (such as making it so that it is not tidally locked to its star).

So... What kind of atmosphere would allow you a grace period of a day or more before it kills you?

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Consider carbon monoxide. Inhaling it reduces the oxygen in your blood, causing your body to die. People who die from carbon monoxide poisoning feel very tired (due to lack of oxygen) and simply fall asleep, never to wake up. Time taken to die depends on concentration, and it is possible to remove carbon monoxide from the air using a catalytic converter (they remove CO and other gases from car exhaust fumes).

In the case of an atmosphere containing carbon monoxide, a catalytic converter would be essential to produce breathable air. Without it, people would lose consciousness and die.

An alternative is mustard gas (Dichlorodiethyl sulfide). used in the second world war, this gas creates green clouds of mist. An atmosphere containing this would have severe consequences on any organism that inhaled it:

  • Red, burning skin
  • Blisters
  • Burning and red eyes
  • Sneezing, nosebleeds
  • Coughing
  • Vomiting, nausea, diarrhea
  • Trouble breathing

These symptoms typically appear 12-24 hours AFTER exposure however higher concentrations can cause the appearance of symptoms within an hour or two. A high dose like this will kill, however a light dose will be survivable (with possible blindness and other disfigurements).

Another atmospheric hazard is volcanic ash. inhaling this will cause the ash to turn to liquid cement in your lungs (causing suffocation), and smaller particles will literally scour the inside of your respiratory system (imagine inhaling sandpaper).

Ash is also very heavy, so buildings would have to be built to withstand such conditions.

Edit: Just realised you wanted an atmosphere with no symptoms. In that case, carbon monoxide is your best bet, however i will leave the other suggestions in just in case you like them.

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    $\begingroup$ I have no problem with symptoms appearing only hours after exposure, the mustard gas is a good one too (though I am tending more towards your points about carbon monoxide). The part about no exposure symptoms is optional. I can find ways to tweak and taylor the atmosphere. $\endgroup$ – Renan Jul 26 '16 at 12:08
  • $\begingroup$ If you want more details, there are plenty of websites describing the effects of mustard gas during world war 2, i can link some good ones. You could even opt for a combination of mustard gas and carbon monoxide, to add the extra factor of losing consciousness if you stay out too long. $\endgroup$ – Aric Jul 26 '16 at 12:10
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    $\begingroup$ One problem with mustard gas is that it can also be absorbed through the skin. Then, not only does it cause painful blisters everywhere it contacted your body, it mangles your DNA and makes it more likely you'll end up with cancer even if you do survive the acute chemical burns. $\endgroup$ – Dan Bryant Jul 26 '16 at 13:34
  • $\begingroup$ yes, that is a consideration i overlooked. If you are using humans in the future, consider a possible skin condition that the race has developed to defend against the atmosphere. If humans are new to the planet, they must wear some sort of protection to cover their skin. $\endgroup$ – Aric Jul 26 '16 at 15:35
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There are three options far slow death gas of choice They are hydrogen sulphide, radon or carbon dioxide. All three gases are produced by natural processes. Hydrogen sulphide and carbon dioxide are the result of decay processes. Radon by radioactive of uranium.

High CO2 levels have long-term deleterious effects. H2S is usually a bad smell, not for nothing is it known as rotten egg, however in strong concentrations it is ordourless and can be lethal. Radon is more subtle, but it's definitely a long term killer. It causes cancers, mainly lung and throat, and has a tendency to accumulate in dwellings.

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