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.
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.