Return to the electric chair is a thoroughly bad idea. That method was held unconstitutional by the Georgia Supreme Court, hardly a bastion of liberal judicial activism, in 2001 and by the Nebraska Supreme Court in 2008. The last thing we need to do is reopen that can of worms.
The gas chamber is a different matter. The constitutional attacks that were made on the gas chamber were not based on the method as such but on the particular gas, hydrogen cyanide. Those attacks had some validity. Cyanide is a bad way to go. There are, however, other gases available that are both painless and easy to acquire.
The preferred method is barbiturates. That is what we have with the single-drug method of lethal injection using pentobarbital. If the supply problems could be resolved, which could be done by forbidding resale restrictions as an illegal restraint of trade, this would remain the preferred method. This is how animals are euthanized every day in this country.
The "conditionally acceptable" methods are all gases: inhalant anesthetics, carbon dioxide, carbon monoxide, nitrogen, and argon.
Inhalant anesthetics would present the same supply problems as pentobarbital, so we can pass on that one. The other four are all readily available in pure form in gas cylinders. Here are the advantages and disadvantages of each as listed by the AVMA:
Advantages--(1) The rapid depressant, analgesic, and anesthetic effects of CO2 are well established. (2) Carbon dioxide is readily available and can be purchased in compressed gas cylinders. (3) Carbon dioxide is inexpensive, nonflammable, nonexplosive, and poses minimal hazard to personnel when used with properly designed equipment. (4) Carbon dioxide does not result in accumulation of tissue residues in food-producing animals. (5) Carbon dioxide euthanasia does not distort murine cholinergic markers or corticosterone concentrations.
Disadvantages--(1) Because CO2 is heavier than air, incomplete filling of a chamber may permit animals to climb or raise their heads above the higher concentrations and avoid exposure. (2) Some species, such as fish and burrowing and diving mammals, may have extraordinary tolerance for CO2. (3) Reptiles and amphibians may breathe too slowly for the use of CO2. (4) Euthanasia by exposure to CO2 may take longer than euthanasia by other means. (5) Induction of loss of consciousness at lower concentrations (< 80%) may produce pulmonary and upper respiratory tract lesions. (6) High concentrations of CO2 may be distressful to some animals.
We don't need to worry about disadvantages 1, 2, 3, and 5. Number 4 is not a major problem. Number 6 might be an avenue for challenges, so let's look further.
Advantages--(1) Carbon monoxide induces loss of consciousness without pain and with minimal discernible discomfort. (2) Hypoxemia induced by CO is insidious, so that the animal appears to be unaware. (3) Death occurs rapidly if concentrations of 4 to 6% are used.
Disadvantages--(1) Safeguards must be taken to prevent exposure of personnel. (2) Any electrical equipment exposed to CO (eg, lights and fans) must be explosion proof.
Advantages 1 and 2 are what we want. The method should be bulletproof to Eighth Amendment challenges. Disadvantage 1 is not difficult. "[I]f the chamber is inside a room, CO monitors must be placed in the room to warn personnel of hazardous concentrations." The monitors are readily available and inexpensive. Disadvantage 2 is an engineering problem, but it should not be a major barrier to implementation.
N2 and Ar:
Advantages--(1) Nitrogen and Ar are readily available as compressed gases. (2) Hazards to personnel are minimal.
Disadvantages--(1) Loss of consciousness is preceded by hypoxemia and ventilatory stimulation, which may be distressing to the animal. (2) Reestablishing a low concentration of O2 (ie, 6% or greater) in the chamber before death will allow immediate recovery.
As I have noted before on this blog, my personal experience in Air Force flight training is that hypoxia is painless. However, that may take a while to achieve by displacing oxygen with nitrogen or argon. Given the AVMA cautions, this may not be the way to go. Also, in states such as California where the statute says "lethal gas" and the Legislature can't be counted on to clarify, an argument can be made that a gas that is not inherently lethal but only becomes so when it displaces oxygen is not a "lethal gas" within the meaning of the statute.
Carbon monoxide is probably the best way to go for any state that wants to establish an alternative to injection.