At what levels of carbon dioxide do humans have breathing problems (e.g. hyperventilation) or adverse medical effects? And what level would people need to wear breathing apparatus to survive? This is mainly a time travel question (going back to various geological periods) but the answer will also be helpful to me when designing habitable planets for my science fiction RPG campaign.

For instance some CO2 levels in Earth’s geological past are listed below (data from the GEOCARB model – there are big error bars on the real estimates, so I’ve just picked some example figures).

  • Way back at the beginning of the Cambrian the CO2 could have been as high as 20 times Present Atmospheric Level (PAL). That’s 7 x 10^-3 bar.
  • At the start of the Carboniferous it might have been 9.5 times PAL. Approx 3.4 x 10^-3 bar.
  • In the Triassic it was 5 times PAL (say 1.8 to 2 x 10^-3 bar)
  • At the start of the Cretaceous it was 4 times PAL.
  • At the end of the Cretaceous/early Tertiary it was 2 times PAL.

I know the oxygen levels will also be different from the present. I hope the CO2 question is answerable without the corresponding oxygen data. I can look up the oxygen if it is not.


1 Answer 1


Those carbon dioxide levels are not a problem. Earth's current atmosphere is about 0.04% CO2, and up to 1% is fine. So that would be about 25 times PAL.

  • $\begingroup$ A short but forceful, concise and right answer. I like it (+1). $\endgroup$
    – Ender Look
    Jun 22, 2017 at 1:25

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