Since you mentioned CO2, I'm guessing you are using this answer from Tim B.
If so, then think of dolphin moles. To store up large stores of CO2 while still being able to breathe may require the bone grass to go on a crazy oxygen converting frenzy. This would create an abundance of outside oxygen while the plants stored up CO2. Only during this time do the moles appear for air. The bone grass has learned to ignore the moles as the miles always disappear before they can pump out sufficient CO2. The mole dolphins don't have to surface for a while and are alive and well. If you want the moles to tunnel into people's lungs, it would be a bit impractical as the person collapsed because it has too much CO2... Perhaps this is what the mole eats?
Also just a side note. The CO2 could be used as a defense against fire. Whenever you try to light the bone grass, it suffocates the fire!
