Not to keep beating a dead horse, but the use of CO2 as a type of euthanasia isn't really the most humane way to go about it anyway. I'm not trying to judge anyone because I do and have used CO2 in the form of dry ice to euthanize. With that being said, when using CO2 the animals do not "go to sleep" they actually suffocate while still conscious. CO2 has a greater affinity for hemoglobin than O2 and the animals die of acidosis and hypoxia. The idea of them "going to sleep" only applies to CO exposure. I only say this because I initially mixed this up. When this was brought up to me however I had to think about it again and that individual was correct....lol a health sciences major mixing the 2 up.