Wha?? CO2 sure will kill them if you leave them in there long enough just like any other animal that breathes air. How do you think CO2 chambers are made for rats. You use a paintball gun cartridge in a tub with a screw type valve. And i dont know if i agree that even gas chambers are that humane either. I mean they darn well know they cant breath and i would imagine they are stressing. It's like if you were put in a CO2 chamber lol. I mean rats arent humans but anything will know if it is losing the ability to breath.
Oh or some make a MacGuyver chamber with dry ice in a small container with holes inside a larger tub with the rats in it.