[samatman]

While this is standard, it shouldn't be.

Mammals sense CO2 buildup, not oxygen depletion. By smothering with CO2, the mice spend their last moments desperate to breathe and unable to.

If they were to use nitrogen instead, the mice, having no way to know they've been deprived of oxygen, would just pass out and die.

[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.