Carbon dioxide (CO2) is not acceptable for routine use in animal care and control facilities for euthanasia. However, a commercially manufactured chamber using compressed CO2 may be acceptable for certain wildlife species.
The methods that The HSUS considers inhumane, disapproves of, and campaigns against include decompression, nitrous oxide, drowning, decapitation, cervical dislocation, pithing, exsanguination, electrocution, gunshot (excluding properly performed field euthanasia), air embolism, nitrogen flushing, strychnine, chloral hydrate, caffeine, nicotine, magnesium sulphate, potassium chloride, succinylcholine chloride (Sucostrin, U-Tha-Sol, Anectine, Quelicin Chloride, Scoline Chloride), and any combination of pentobarbital with a neuromuscular blocking agent.

Carbon monoxide (CO), when in the form of compressed cylinder gas and delivered in a properly manufactured and equipped chamber, is a conditionally acceptable method of euthanasia for some animals. It is unacceptable to use CO for the euthanasia of dogs and cats who are under four months of age, or who are old, sick, or injured.