The American Veterinary Medical Association, in a 2007 study, detailed the horrific physiological effects of using carbon monoxide as a form of euthanasia:
...distress vocalization (this means barking, crying, howling), struggling, attempts to escape, defensive or redirected aggression, salivation, urination, defecation, evacuation of anal sacs, pupillary dilatation, tachycardia, sweating, and reflex skeletal muscle contractions causing shivering, tremors, or other muscular spasms.
There is no question shelter workers have documented the piercing cries, howling, frantic calls, scratching and panic of animals as they are gassed. Just putting them in the chamber is frightening for animals. The chamber is hot, confining and often smells probably like death. They don't know what is happening and they immediately experience panic and distress. The buildup of gas in an animal's lungs is slower if there is decreased ventilation, a leaky valve or seal, or more than one animal in the chamber. There may be no way to know how quickly the gas reaches the required concentration of 6% before it can sufficiently build up in an animal's lungs and result in loss of consciousness.