Update:  The bills in New York, West Virginia, New Mexico, Illinois passed and are now law!

Original report: A number of states this year are considering legislation to ban animal gas chambers:

New York (would mandate euthanasia by injection or EBI), Pennsylvania (would also ban chloroform, ether, halothan, fluothane, or similar substance when used in an airtight chamber or plastic bag, North Carolina (would mandate EBI),  Georgia (EBI mandated), West Virginia, New Mexico, and Michigan (would also ban CO2).

A bill in Illinois, S.B. 38 would strictly limit the use of carbon monoxide gas chambers and ban the use of carbon dioxide and decompression chambers. (Update: This bill has passed and is awaiting the governor's signature.) 

The following states already do not allow use of carbon monoxide gas chambers in public shelters to kill homeless or unwanted animals or those deemed "unadoptable".

Arizona actually mandates one of 3 methods for euthanasia:  sodium pentobarbital or a derivative of sodium pentobarbital, nitrogen gas, or  T-61 euthanasia solution or its generic equivalent. A.R.S. Section 11-1021

California prohibits the use of carbon monoxide gas. Also heartstick or intracardiac injection of a euthanasia agent on a conscious animal is prohibited unless the animal is heavily sedated, or the circumstances justify it.

For other than newborn dogs and cats, California prohibits high altitude decompression chambers and Nitrogen gas.

For newborn dogs and cats only chloroform vapor or inoculation of barbiturates is permissible for euthanasia.  Cal Civ. Code, Penal Code 597u,v

Delaware mandates use of sodium pentobarbital or choloroform after approval by a licensed veterinarian. The use of a decompression chamber is specifically prohibited.  Del. Code Ann. Section 8001; Del. Code Ann. Section 8005

Florida mandates use of  sodium pentobarbital, a sodium pentobarbital derivative, or other agent the Board of Veterinary Medicine may approve for the euthanasia of cats and dogs. Fla. Stat. Section 828.058

Warm blooded animals in pet shops can only be put to death by sodium pentobarbital, a sodium pentobarbital derivative, or a substance or procedure which acts on the central nervous system and is clinically proven to be humane. Fla. Stat. Section 828.065

Maine only allows veterinarian or other certified persons to use barbiturate overdose to euthanize cats a dogs. Revised Statutes Section 17-42-1042 

Maine does allow shooting an animal when human or other animal life is endangered or the animals is suffering unduly and the usual method cannot be used quickly.  Section 17-42-1043.

Maryland bans use of carbon monoxide and decompression chambers as well as curariform drugs in killing dogs and cats.  Md. Code § 10-611

New Jersey does not permit the use of a decompression chamber or lethal gas except an inhalant anesthetic in killing shelter animals. N. J. Statutes Section 4:22-19 New Jersey also prohibits the use of succinylcholine chloride, curare, curariform drugs, or any other substance which acts as a neuromuscular blocking agent. N. J. Statutes Section 4:22-19.3

The primary recommended method is an intravenous injection of a barbiturate. New Jersey also allows administration of injectable solutions approved for use in dogs and cats, inhalant anesthetics, and intravenous injections of combinations of chloral hydrate, magnesium sulfate and pentobarbital for large animals, such as horses. New Jersey Administrative Code 8:23A-1.11

Oregon law states that dogs and cats can only be euthanized by a lethal injection of sodium pentobarbital or other substance approved by the Oregon State Veterinary Medical Examining Board. Revised Statutes Section 609.405

If the animal to be euthanized is an imminent threat to other life, and administrating sodium pentobarbital is inappropriate given the situation, the animal may be destroyed by another humane method. This decision is reviewable by the Oregon State Veterinary Medical Examining Board.

Rhode Island prohibits the use of carbon monoxide gas chambers for shelter animals and states the only acceptable means of euthanasia is lethal injection., R.I. Gen Laws 4-19-12

There is an exception for emergencies where there is no alternative. In those situations the animal may be shot by a well-trained shooter if the animal is restrained humanely and maximum precaution is taken to minimize suffering.  

Rhode Island state law also specifies racing greyhounds can only be killed by lethal injection or by some other humane method approved by the AVMA, which would seem to leave the door open for gassing as long as it is not done by a public shelter. R.I Gen. Laws. 4-1-34

Tennessee law specifies methods that can be used for euthanasia of non-livestock shelter animals which are basically injection of sodium pentobarbital or other agents approved the the state board of veterinary medicine. Code Ann. Section 44-17-303

Virginia specifically prohibits killing of shelter animals by use of a decompression chamber or gas chamber.  Code Ann. Section 3.1-796.77.

Washington only permits euthanasia of shelter animals by injection of sodium pentobarbital. Washington Administrative Code §246-887-050

Wyoming law specifically prohibits high altitude decompression chambers and carbon monoxide gas chambers using gasoline engines. Statutes Ann Section 6-3-203

Wyoming law also defines euthanasia to mean "the act or practice of providing a humane death for animals through the use of euthanizing drugs". "Euthanizing drugs" means "any pentobarbital-based drug labeled by the manufacturer for the purpose of euthanizing animals" with some excepted drugs. This means in Wyoming euthanasia of shelter animals can only be performed using a pentobarbital-based drug.  Wyoming Statutes Ann Section 33-30-216

While Connecticut does not ban the use of animal gas chambers in public shelters except high altitude decompression chambers, state law does require pet shops to euthanize warm-blooded animals only by lethal injection of sodium pentobarbital.  Conn. Gen. Statutes Section 22-344a; Conn. Gen. Statutes Section 29-108h Euthanasia on wild animals must conform to the Commissioner of Environmental Protection's orders which are based on the recommendations of the 1993 report of the American Veterinary Medical Association panel on euthanasia. Conn. Gen. Statutes Section 26-47