As described in many, many, many books and videos (but especially accessibly by this book, in case you’re interested), large slaughterhouses can process so many animals per hour that workers on the assembly line can have to deal with as many as one animal every five seconds (depending on species). This causes “sloppy workmanship” on the part of the workers, manifesting itself in animals “stunned” badly or not at all, injuries to animals and workers as conscious animals are “stuck”, “legged”, or dunked in tanks of scalding water…um, anyway, high speeds in large slaughterhouses are bad, primarily because handling so many animals, so quickly, makes it possible for humane killing methods to “miss”, and for animals to go through the butchering process not only alive, but conscious.
There seems to be a movement afoot that at first looks like a great idea: some factories here and there have picked up on the idea of “sedating”/”stunning”/”gassing” the animals with carbon dioxide to make sure they are unconscious before being killed. On the one hand, this is a great step forward. A slaughterhouse is actually considering the needs of the animals it is processing and trying to make the experience less stressful for them. Yes, the slaughterhouse will benefit from this: the birds will be less damaged on packaging (less wasted meat) and of course they will be able to put “humanely slaughtered” on all their packaging, so it’s not as though it’s all altruistic, but still: baby steps.
On the other hand — speaking as someone who has personally “euthanized” hundreds of mice via carbon dioxide and seen others do so — this is not a foolproof answer. CO2 euthanasia is only a quiet, peaceful death if it is performed very slowly, very carefully, on one animal at a time. (Even that’s debatable — there’s still a happy section of science cheerfully turning out papers on whether or not CO2 euthanasia is “euthanasia” at all.) I strongly suspect that the fast-paced world of the high-volume slaughterhouse is not going to combine well with the slow and careful process of humane CO2 euthanasia. Captive-bolt stunning, which is already used by the factories, is, in itself, a humane procedure. However, the speed at which a large slaughterhouse operates renders captive-bolt stunning inhumane because proper procedures are unable to be followed and the animals are not properly stunned. I think the same issues will affect CO2 euthanasia if it is used in high-volume slaughterhouse operations.
First, the size of the operation itself is going to induce pressure to make the process of filling the gas chamber, which should be a carefully monitored, slow procedure, a quick-and-dirty one. The chicken factory mentioned in the link above, at least, appears to be talking about processing hundreds of birds at a time — according to the article, the containers in which the birds are shipped to the plant will go into a huge CO2 chamber, which will need to be room sized at least. How to fill such a room? Recommendations vary, but the average is something like 10% of room air replaced with CO2 per minute — that’s at least ten minutes to fill the chamber with gas. (There are arguments for and against pre-filling the chamber with gas, or using faster filling speeds, but that discussion must be saved for a different article.) The chamber will then need to be cleared of gas before workers enter and remove the birds (or the workers will need breathing apparatus). This is all going to take time, which high-speed workers do not have.
Second, the size of the room is going to affect the facility’s ability to deliver gas properly to every animal. Birds further from the gas vents will be affected at different rates than birds next to the vents. Also, since CO2 is heavier than air, the birds at the top of the room will be affected differently than birds at the bottom. What will assure that every animal: a) is rendered completely unconscious, b) remains unconscious for butchering? In a room of sufficient size it is possible for the outermost birds at the bottom of the room to be dead while the birds in the center on the top are still conscious. If workers are filling and emptying the room at high speeds, this problem is actually quite likely. (There are already documented reports of dogs surviving similar gas chambers at humane shelters.)
Third, many laboratory euthanasia standards recommend euthanizing animals by CO2 one at a time (or in small familiar groups in their home cages). (However, it should be noted that the official AVMA line on this is that the gas “chambers should not be overcrowded”, and indeed, some facilities routinely toss a bunch of animals all into one cage for euthanasia to save time.) This is because animals can detect the intrusion of CO2. They often become alarmed and disoriented before they go down, running, kicking, squealing, and generally acting agitated. This does not promote quiet, calm, peaceful behavior — this promotes a room full of shrieking, flapping, alarmed and disoriented birds.
Apparently they’ve been trying CO2 euthanasia on pigs too. This article suggests that all the agitation takes place after loss of consciousness and that the pigs are completely out cold while “convulsions, vocalization, reflex struggling, breath holding, and tachypnea” occur. I’ll admit I’ve never seen a pig euthanized by CO2, but I must say I have some reservations about how “unconscious” the mice and rats I euthanized were while they underwent “Guedel’s second stage of anesthesia“.
I applaud these factories for trying to make the treatment of the animals with whom they are working more humane. (I am an animal trainer at heart, and I know to reward “baby steps”, no matter how small.) However, I believe these factories are not treating the correct problem (which is generally that the facility is trying to process too many animals for its capacity, and using pressured, underpaid, and under-trained workers).