[gorkonsine]

How is CO2 a problem in breathable air? To humans, it's a harmless and inert gas, just like nitrogen. The real problem is too little O2, which humans require for respiration. Obviously, too much CO2 could displace O2 and reduce O2 levels, but it's not the CO2 that's an actual problem (i.e., if the CO2 displaced only N2, and O2 levels stayed the same, you wouldn't have a problem).

So having a CO2 scrubber doesn't seem to make any sense to me, you need something that keeps the O2 levels up. That may be a CO2 scrubber (pulling the carbon atom off the CO2), or it may be more feasible to just carry an oxygen bottle as many elderly people do today. I suspect the latter is the case; if it were feasible to have a portable CO2 scrubber to improve your O2 levels, they would have done it already for all these people.

[Bartweiss]

This relies on several common misconceptions.

Earth's atmosphere is only about 0.03% carbon dioxide. CO2 levels high enough to cause catastrophic global warming still wouldn't displace enough oxygen to matter to humans - it would be a smaller change than a few thousand feet of elevation, which is entirely safe.

The second issue: CO2 is not inert in the human body. It's not used for any crucial reactions, but it's more relevant to us than N2 (largely inert at STP, not while diving) and vastly more reactive than He. That's why divers commonly use Trimix (O2, N2, He), and deep divers use Heliox (O2, He). CO2 is present in solution in your bloodstream at all times, partly as an output of chemical reactions and partly absorbed from the air.

Among other roles, CO2 levels are detected in human lungs - high CO2, not low O2, is what makes you feel the burning "need to breath" sensation. That's why hyperventilating is a common cause of shallow-water blackouts - it lowers blood CO2 levels more than it increases O2 levels, allowing you to run out of air without feeling it.

There's some data suggesting that elevated CO2 levels cause cognitive impairment, which can become quite serious. There's other data (e.g. from nuclear submarines) showing that humans can tolerate CO2 with minimal effect. No one has reconciled this neatly, but it's definitely not true that O2 displacement is the problem.

[gorkonsine]

Very informative, thanks.

[fulafel]

It is harmful in large concentrations as it displaces oxygen (cf. dry ice pool accidents).

[Bartweiss]

True, and dangerous; this even happens naturally when volcanos displace oxygen and create lethal stretches of depressed ground. (https://en.wikipedia.org/wiki/Mazuku)

But not the issue here. CO2 is a tiny fraction of the atmosphere, and apocalyptic global warming happens well before O2 displacement becomes an issue. Bear in mind that humans can function effectively up to several thousand meters - the O2 loss from low pressure is vastly larger than any possible CO2 displacement.

If CO2 is going to have physiological effects via the atmosphere, it's based on chronic exposure and high blood concentrations. There are some studies suggesting that causes cognitive harm, and others suggesting it doesn't.