But at the same time you are suffocating the animal. Have you ever seen the first Resident Evil movie? In it the Red Queen/computer locks some of the people in a chamber and fills it with CO2 to suffocate them and quite frankly I would rather die by being smacked in the neck with a ruler( If I was a rat) Then again I know how much you adore your rodents but honestly CO2 isn't as humane as first believed I mean.. Living things need oxygen, if you fill a chamber with something other than air what happens? They suffocate. Just my two cents nothin on ya sweetie