Yeah was going to hit on a similar point. When you hold your breath, that feeling that you need to breathe again isn't because you don't have enough oxygen, but because you have too much CO2. This is why things like hypoxia (lack of oxygen) can be so deadly in environments where it can be a thing (pilots, scuba, etc). Early onset symptoms include things like euphoria which isn't the most helpful warning from your body for 'death: imminent.'