
Death by carbon monoxide poisoning is not considered a humane way of euthanasia for animals. It takes a long time for the animal to die and the animal may cry in fear, vomit, or only appear to die. There have been several reports of animals surviving the gas chamber and waking up in the freezer or even at the land fill where the bodies are taken.
http://www.change.org/petitions/stop-gassing-animals-in-branch-county-mi
