It should be noted that there appears to be a fair amount of ambiguity about the phrases "controlled atmosphere stunning" (CAS) and "controlled atmosphere killing," (CAK). Common sense suggests that these terms should mean very different things – and it is clear that when some researchers refer to "stunning," they are referring to a system in which unconscious but live birds are shackled and then cut across the neck (this has led to concerns about quickness of recovery from stunning and period between being stunned and being killed. This is sometimes called the "stun-to-stick" issue). However, other researchers use CAK and CAS interchangeably (for example, some use terms such as "stunning/killing systems" and "gas stun/kill"). Some research papers appear to be about CAS, as CAS is explicitly mentioned in the title, but the substantive text may be about CAK.
I decided to look into the welfare claims about CAK – especially as I noticed that PeTA, even in their "case for CAK video," seemed rather keen on not letting their supporters see chickens being stun-killed by taking away their oxygen, even though this was supposed to be the major welfare advance they have been campaigning to bring about for five years. Looking at some of the research, I think I can see why PeTA are reluctant to fully inform their members and sympathisers. They have been careful and somewhat misleading in their claims about CAK. For example [see 1], they claim that asphyxia and anoxia are very different experiences, which is extremely doubtful. The claim that anoxia is "a painless process" is dubious too – anoxia certainly seems associated with pain in human animals.
However, when PeTA state that, “With CAK workers never handle live birds, so there are no chances of abuse,” they are being downright disingenuous or else engaging in wishful thinking. PeTA have themselves documented how workers sadistically treat animal property such as chickens: being trapped inside a transportation crate hardly protects one from all abuses and rights violations – and this is only half of the story in the first place. The chickens and hens have still to be taken from battery cages, cage-free facilities and broiler houses and placed in the transportation crates. They are still subject to rights violations at the farm end of the process [see 3 for one student’s sickening experience in Ireland].
It appears that there is a great deal of academic debate about the welfare benefits of placing chickens in "controlled atmospheres." There seems to be a consensus that there should be welfare benefits compared to the live shackling and neck cutting of chickens but there is dispute and an absence of agreement about the extent and circumstances of these benefits. For example, there is ongoing research and discourse about gas mixtures, length of exposure, ways of stopping chickens regaining consciousness, and how they behave when they are exposed to gases and mixtures. There appears to be particular troubling "behavioural responses" such as "gasping," "wing flapping," "vocalising," "headshaking," "loss of posture" (meaning becoming unstable and struggling to regain balance), and "convulsions" which scientists are trying to eliminate in the use of gas chambers.
Some convulsions are thought to occur after some individuals are unconscious. However, Simmonds reports in 2005 that, “birds being killed by anoxia may be subject to these uncontrollable movements [loss of posture and convulsions] while still conscious.” There are further concerns raised because CAS/CAK systems have more moving parts compared with electric bath stunners and it appears breakdowns are expected. Indeed, in Britain in 2007, DEFRA insisted on the use of both audible and visible breakdown signals, including warnings of when gas mixtures and concentrations are incorrect. DEFRA also want checks when birds exit the chambers in case some are still alive and insist that a "back-up slaughter method" is available for such circumstances. They also want a means of visually monitoring the inside of the gas chamber and a way of getting birds and gases out of the chamber in the event of "failures." They also want birds who are taken out of transportation crates before being put into a gas chamber to be ‘handled with care’ and designated workers to take "corrective action" if it is found that live birds are exiting the chamber.
Apart from problems in gassing chickens and hens, researchers are also looking into the gassing of "laboratory animals," turkeys, unwanted pets and pigs (with groups like the RSPCA Australia and FAWC monitoring results – the FAWC have recommended the phasing out of gassing pigs).
According to Dorothy McKeegan there are several common "gas methodologies."
• Anoxia: the replacement of air by argon or nitrogen
• Hypercapnic anoxia: application of a mixture of argon and carbon dioxide (CO2). CO2 has been shown to be painful in human use.
• Hypercapnic hypoxia: use of high levels of CO2 (as much as 80%)
• Hypercapnic hyperoxgenation: combination of high levels of oxygen and CO2
• ‘multiphase systems’: a mixture of some of the above, often to attempt to prevent chickens recovering before they are killed.
The expected welfare benefits have been that chickens will not be shackled until made unconscious by the "gas environment" and McKeegan says, “gas stunning eliminates the possibility that some birds will not be stunned adequately before bleed-out.” As ever, much of this concern is as much to do with economics, carcass damage, as with bird welfare. However, contrary to McKeegan’s studies, other research shows that not all chickens and hens may be stunned properly, resulting in the recommendation of finding ways to create "turbulence" in gas chambers to reduce the number of pockets of air which some birds may find.
McKeegan essentially shows that it is hard to get everything right – it is apparently difficult to avoid some chickens "gasping" and in "respiratory distress," often because they can smell or taste the gases and sometimes they seem to experience facial pain. The research seems to indicate that chickens and hens react to gases in similar ways to humans. These research worries about "potentially averse gaseous environments" have slowed down the implementation of legislation because there is concern that one "potentially painful and distressing slaughter method" may be replacing another.
A great deal of research on gas stunning/killing has been carried out at Bristol University under the leadership of AB Mohan Raj. This work, which began in the late 1980s, has also raised some welfare issues. First, however, what does this research look like? How should we picture it? Mohan Raj describes an experiment in which turkeys are gassed. The researchers have constructed a "feeding chamber" and a "roosting chamber" connected by a descending tunnel with windows so the gassing can be observed. Mohan Raj graded "aversive reactions" to gases as follows.
Grade 1: the bird entered the feeding chamber without pausing but exhibited some gasping and/or head shaking in the tunnel, and was killed by the gas in the feeding chamber;
Grade 2: the bird entered the feeding chamber without pausing but exhibited gasping, vocalisation and head shaking in the tunnel, and was killed by the gas in the feeding chamber;
Grade 3: the bird did not enter the feeding chamber but paused or turned back in the tunnel without exhibiting gasping, vocalisation or head shaking, and was killed by the gas in the tunnel;
Grade 4: the bird did not enter the feeding chamber but paused or turned back in the tunnel or re-entered the tunnel exhibiting gasping, vocalisation or head shaking, and was killed by the gas in the tunnel;
Grade 5: the bird did not enter the feeding chamber but after each attempt returned to the roosting chamber after its initial exposure to the gas in the tunnel, exhibiting gasping, vocalisation and head shaking.
Nice work huh?
Back to gassing chickens, Mohan Raj (and N.G. Gregory) stunned 320 "broiler chickens" in one experiment for 2 minutes each. They found a number of survivors, between 8 and 28 out of 100; and in one experiment none of the birds had lost consciousness after 2 minutes and the test had to be stopped. They found survivors by measuring time to eye opening after gassing and by pinching the birds’ combs to see if they could get a response. The researchers recommended “that care should be taken to ensure that there is sufficient turbulence within the stunning chamber to avoid air pockets being trapped between the birds.”
In another experiment, Mohan Raj and Gregory measured "convulsive episodes" and time to "sustained eye closure" and found that hens took longer to gas than "broilers." The same experimenters also discovered that bleeding is higher in electrically stunned birds compared to gassed ones. This means that the whole killing system in slaughterhouses needs to be slowed down when gassed chickens are "bled-out," otherwise they will still be bleeding when entering scalding tanks to loosen feathers. This appears to be at the centre of Simmonds’ concerns about ensuring CAS/CAK systems are "suitable for a commercial environment," noting, for example, that the "gas killing" of chickens was not even thought a possibility in Britain until the 1980s due to line speeds. Since then, the FAWC has insisted that the research be done.
It should be pointed out, of course, that all or at least the vast majority of the results I have cited come from tests in carefully controlled experimental conditions and not in commercial settings where time and speed means money. Mohan Raj at Bristol seems to have been busy trying to discover which system will function best "under commercial conditions." There is also an apparent further problem of damaged wing bones during gassing operations, presumably due to the reported wing flapping. Mohan Raj, Gregory and Wilkins suggest that electric stunning created bleeding and bone fractures in chickens but wing bone damage was "significantly higher" in chickens subjected to gas chambers.
Clearly there is no consensus as yet to the best way to gas chickens and other animals and chickens have not been mass gassed "under commercial conditions" as yet. It may be far too early for groups such as PeTA to claim that gassed chickens merely "go to sleep" even with the use of argon alone and notwithstanding that argon alone appears to create other welfare issues. How long will it be before we see a shocking video expose of the very system PeTA pushed for? About as long as it took before we saw the rights violations taking place in "RSPCA-approved" systems of animal use I reckon.
 Please note that I am not claiming to have reviewed all the available literature on CAK/CAS. I merely report on initial findings; a dipping of the toe into the subject.
 “ I spent one night working in a chicken farm in westmeath when i was in college. Our job was to move the chickens from the warehouse they were fed in, onto crates driven in and out of the warehouse on forklifts, and then loaded on to trucks. I seen chickens kicked around like footballs, their necks snapped by workers when closing the drawers of crates, and forklifts driving over them & bursting them like balloons etc. I remember vomiting 2 or 3 times during the night.. no idea how people do this for a living”. http://www.indymedia.ie/article/83640#comments
 McKeegan, D.E.F. (2004) ‘Sensory Perception: chemoreception’ in G.C. Perry (ed.) Welfare of the Laying Hen: Poultry Science Symposium 27. Wallingford, Oxfordshire: CABI.
Other sources used.
Mohan Raj, A.B. & Gregory, N.G. (1990) ‘Effect of rate of induction of carbon dioxide anaesthesia on the time of onset of unconsciousness and convulsions’. Research in Veterinary Science, 49(3): 360-363.
Mohan Raj, A.B. & Gregory, N.G. (1991) ‘Efficiency of bleeding of broilers after gaseous or electrical stunning’. The Veterinary Record, 128(6): 127-128.
Mohan Raj, A.B., Gregory, N.G. & Wilkins, L.J. (1992) ‘Survival rate and carcass downgrading after the stunning of broilers with carbon dioxide-argon mixtures’. The Veterinary Record, 130(15): 325-328.
Mohan Raj, A.B. (1996) ‘Aversive reactions of turkeys to argon, carbon dioxide and a mixture of carbon dioxide and argon’. The Veterinary Record, 138(24): 592-593.
Lambooij, E., Gerritzen, M.A., Engel, B., Hillebrand, S.J.W., Lankhaar, J. and Pieterse, C. (1999) ‘Behavioural responses during exposure of broiler chickens to different gas mixtures’. Applied Animal Behaviour Science, 62(3-3): 255-265.
‘Welfare and other aspects of controlled atmosphere stunning’, N. Simmonds. 2005: http://www.animalscience.com/uploads/additionalFiles/QualityOfPoultryMeat/172.pdf