[andbberger]

There's a good chance I have that wrong and they use nitrogen. Like I said, I don't work in animal labs, just adjacent to them.

[nothrabannosir]

I knew someone who worked with lab rats; her job was to kill them and she had to use CO2.

[HorstG]

afaik (second hand) CO2 is used for safety reasons, humans can sense a CO2 poisoning but not an excess of nitrogen.