Interesting. One thing that strikes me is that CO2 is a better asphyxiating agent than nitrogen: it is inert to combustion and it is 47.3% denser than N2, so i

Interesting. One thing that strikes me is that CO2 is a better asphyxiating agent than nitrogen: it is inert to combustion and it is 47.3% denser than N2, so it will stay put if you can keep it under 147.3% of ambient temperature. And people are looking for places to sequester CO2, anyway. It will asphyxiate animals and people too, though, so you'd better not have any leaks...