Howdy there Slayer, and welcome!
AHHHH yes the cursed CO2 bug. I've definitely had the pleasure of it's company FAR too many times. From what I've noticed, and from what other modders have suggested, it's usually caused by alt-tabbing while you're on patrol. Sometimes I've had the alt-tab do nothing, and other times I've had the CO2 constantly rise. WHATEVER the case though, I've only seen this bug when I alt tab to go do something else.
Fortunately, whenever this happens I just save wherever I currently am, and reload that fresh save. This gets rid of the bug and YAHOO now my men can breathe again! :P
*Not a PERFECT solution, but it's the only one I know of, and it's not too terribly difficult
Hope this works for you! If you're not using alt-tab or minimizing the game in any way and STILL getting the bug, well that's just screwy. And now I'll defer to any others who might have more advice
