CO2 or carbon dioxide poisoning simply chokes the animal to death. They loose consciousness prior to death which is why many people think it is more humane. In actuality it is no more humane than leaving them in a plastic bag. The effect is the same, only quicker because CO2 is produced faster with dry ice than with chronic exhalation.

A far more humane method would be CO poisoning, or carbon monoxide. Instead of causing a choking feeling it simply robs the animal of oxygen and puts it to sleep gently. Same process as leaving your car running in the garage with the doors closed.

People often mistake the two, but yea...