[K0balt]

You don’t need co2 in the air to tell you to breathe. Your body makes co2, which is what you feel. Once you run out of oxygen to make co2, you are already unconscious.

If you go into an area with no oxygen, you will pass out very quickly with little warning except being dizzy, but you will not fail to feel like you need to breathe.

It might be slightly more safe to enter into a room full of co2 than a room full of nitrogen, because you might be alerted by the high co2 a few seconds before the dizziness takes over, but that doesn’t make rebreathers dangerous because you will forget to breathe. If you run out of oxygen, you’re unconscious in a few seconds anyway, co2 or no co2.