There are threads on the subject peppered thoughout SubSim, back into the archived material... Nowhere can I find anything "concrete". As examples:
Medium-old
https://www.subsim.com/radioroom/sho...82#post2200082
Newer-old
https://www.subsim.com/radioroom/sho...12#post2784112
SH5, but should apply to 3 & 4 also
https://www.subsim.com/radioroom/sho...00#post2381000
Suffice to say that you will get a "warning" in the Text box for "CO2 level is over 10%!", and then nothing more until "CO2 level is critical, sir!", which I do not remember when you encounter that (70%??). The rate increases exponentially as time passes, at an ever increasing rate. When you do get that warning though, you do not have a lot of time to get to the surface. I think it is 30% where you will begin to notice a lag between "giving an order" and its resultant "compliance". In other words, you order PD from 150 feet, and you do not notice a change in the bubble or the depth for a bit. You might have to "re-issue" the order. Your crew is suffering from CO2 poisoning. They're still alive, but reacting slowly, and often inaccurately. If you go to the crew screen (F7 in FotRSU), you'll see health and fatigue issues. A crewmember may also have a yellow border around them when the level gets high, indicating they are near death. Hopefully, there is nothing above your boat at that time... else you will shortly encounter the slow spin, screen growing darker phenomenon called "EndGame Sequence", as your crew passes out at the helm and dive planes and begins that descent to crush depth...
A really BIG thing to remember about the CO2 level, is that it is one of the first things to get "stuck" in the game, as far as anomalies encountered after shelling-out to the desktop from the active game. If after you surface and your CO2 level does not clear inside of 30 seconds, then Pause the game, Save the game, Exit the game. Re-start the game, and load your Save, and your CO2 should restore.
