I am currently breeding mice for my baby ball and 5ft corn. i did a lot of research on pre killing them for freezing or same day feeding. the co2 chamber was in fact the most humane and plenty of reasonable proof to support it. the website i found was actually a design for euthanizing small pets up to two pounds for pet owners that did not have the finances to have them put down at the vet. carbon dioxide or co2 levels at less than roughly ten percent eliminates pain, increasing the percentage makes it an anesthetic (sp?) at this point the animal is on cloud nine. high concentrations kill the animal. and i will say high concentrations do cause respiratory, throat, and nasal irritation but by that time the animal is beyond feeling these side effects. for my gas chamber i used co2 released from mixing baking soda and vinegar. i think the cartridges are too pressurized and stress and could possibly cause harm to mice before the gas begins to work. this is my system and what i believe to be the most humane way to pre kill mice.