I've read all 3 books, and loved them except for one small claim that keeps bothering me. Specifically, is Carbon Dioxide as poisonous as we are lead to believe? I always thought that if we were in an environment of Earth's atmospheric pressure, and, say, 60% nitrogen, 20% oxygen, and 20% carbon dioxide, that it wouldn't cause us any negative effects, but the Red/Green/Blue Mars trilogy says differently.

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    I think the limit is somewhere around 5%CO2. In diving rebreathers are units that absorb the CO2, and let you reuse the remaining oxygen (i.e. only some of the O2 is used when we breathe. Apr 29, 2011 at 19:10
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    See this Meta answer, its comments and its links for discussion on the acceptability of this broad type of question. On this basis, I'm inclined to allow this question, but I don't feel strongly about it.
    – user56
    Apr 30, 2011 at 10:58
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    Another point about CO2 in diving: The deeper you go, the denser your air mixture, and the more CO2 you have in your blood stream and the more confused you get. Divers refer to Martini's Law, which states that for every additional 50 under water you go, the extra CO2 is equivalent to drinking another Martini. Deep divers use HE (Helium) to replace CO2 so they don't get CO2 poisoning. In real life any rig as deep as in The Abyss would have some kind of mixture like HE/O2 for survival -- which means the crews' voices would sound like Mickey Mouse.
    – Tango
    Dec 21, 2011 at 7:34

2 Answers 2



Hypercapnia or hypercapnea (from the Greek hyper = "above" and kapnos = "smoke"), also known as hypercarbia, is a condition where there is too much carbon dioxide (CO2) in the blood. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.


The balance of the atmosphere is a very delicate one. We have around 0.04% in the atmosphere, a very small amount, but an important amount - without that much we would die. However significantly higher levels would also kill us, because it replaces the oxygen in our blood. So yes, it does need to be just right, and very low.

Of course, the same is true of the oxygen in the atmosphere - even though we need it to live, we would die if we tried breathing pure ( or nearly pure ) oxygen. The 20% level we have in the atmosphere is just perfect for us, and any significant change either way causes us problems.

  • Not strictly true, 100% O2 is not a problem... at sea level, but if the oxygen partial pressure gets to high O2 does in deed become toxic: en.wikipedia.org/wiki/… (which - funnily enough - means that with a 100% O2 tank, which of course would be best for SEALs and the like, because a standard tank could store way more compressed O2 than air, but they could not go deeper then 20 feet lol).
    – BMWurm
    Sep 17, 2014 at 6:44
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    Carbon MONoxide replaces oxygen in the blood and so is far more dangerous than CO2. But CO2 is not without risk.
    – Oldcat
    Oct 31, 2014 at 18:39

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