# How long could a human survive in a near-pure CO2 atmosphere?

Imagine Mars's atmosphere has been mostly terraformed. By that, I mean that the surface pressure and temperature have been brought up to at least survivable values. (The atmosphere is still 95% Carbon Dioxide.) People can walk around outside as long as they wear masks that convert the CO2 in the atmosphere into Oxygen.

All fine and good so far.

Here's where the problem comes in: what if someone's mask accidentally comes off? It's got to happen eventually, I'm sure, no matter what precautions are taken.

I realize that the carbon dioxide atmosphere will kill them eventually no matter what, so what I'm asking is this: how long would a human have to get inside or their mask back on before recovery is impossible? It's fine if they pass out and someone else has to help them; I just want to know how long they'd have in general.

Please note, I want to see at least some form of reference with your answer. I will not accept an answer that is pure speculation.

• less time they they could in a vacuum. CO2 displaces Oxygen in hemoglobin so oxygen will be actively leaving their system.
– John
Aug 12, 2021 at 16:41
• What is the atmospheric pressure and oxygen level? Either way, it will be a quick death, just a different mechanism. Aug 12, 2021 at 16:46
• @John Only if they start gulping down the non-air. They can still hold their breath as long as they ever could, which for most people is up to a minute or so. The only relevant facts here are whether CO2 is toxic (to the eyes perhaps? don't think so) or can cause some sort of hyperventilation reaction through passive infiltration (without actively inhaling). Aug 12, 2021 at 16:46
• Don't answer in comment, thanks
– L.Dutch
Aug 12, 2021 at 16:50
• CO2 dissolves in water to make carbonic acid. With that much CO2, you may raise the temp of mars by enough to make the temperature more comfortable, but it will also melt the ice and turn what little water you have into acid making the planet much more hostile to terrestrial life, not less so. Aug 12, 2021 at 17:48

Apparently you would be unconscious almost instantly and in arrest in in under a minute. Note that highlights are mine...

Carbon dioxide does not only cause asphyxiation by hypoxia but also acts as a toxicant. At high concentrations, it has been showed to cause unconsciousness almost instantaneously and respiratory arrest within 1 min

Apparently anything over 30% is "bad"

Concentrations of more than 10% carbon dioxide may cause convulsions, coma, and death. CO2 levels of more than 30% act rapidly leading to loss of consciousness in seconds. This would explain why victims of accidental intoxications often do not act to resolve the situation (open a door, etc.)

The answer to the question of "How long does one have to be resuscitated?", might be gleaned from:

In higher concentrations of CO2, unconsciousness occurred almost instantaneously and respiratory movement ceased in 1 min. After a few minutes of apnea, circulatory arrest was seen. These findings show that the cause of death in breathing high concentrations of CO2 is not the hypoxia but the intoxication of carbon dioxide.

Additionally, treatment is not simply giving oxygen as this will initially increase levels of co2 in the blood. The article suggests use of "hyperbaric oxygen therapy"

Due to this so-called Haldane effect, an initial increase of pCO2 in the bloodstream is to be expected when giving oxygen to a hypoxic carbon dioxide intoxicated person

Lots of good (grim) details in the article but I'm not a medical professional and so some of it is beyond my paygrade.

I'm sure that some will argue that a minute or more is possible if one was to "hold their breath". I don't think this is reasonable or plausible under an accidental loss of one's mask.

At best, you would have "whatever time you have left" in your normal breath. This is going to typically be much less than taking a full breath and holding it.

Worse, without significant training, loss of one's mask is likely to result in an involuntary gasp and if the quick acting symptoms at 30% concentration are any indication then at 95% I think it likely that any little involuntary initial gasp will result in intoxication with loss of judgement leading to an additional breath and unconsciousness seconds later.

• As a note: People should not stick their head into large coolers which are cooled by dry ice. Aug 12, 2021 at 17:39
• This is fantastic, thank you! And excellent points about holding your breath, I'll take that under consideration too. Aug 13, 2021 at 1:03
• There is a lot written on the use of CO2 euthanasia for animals because it is effective, popular and cheap. Aug 14, 2021 at 21:00
• @Willk are you sure it wasn't nitrogen? Nitrogen is apparently painless - your body can't sense it and can't sense oxygen deprivation, so it's thought to be like falling asleep - whereas your body can sense CO2 and it causes severe panic. Mar 15, 2023 at 18:42