It also depends on the species and flow rate. IIRC rodents handle CO2 ok but rabbits are very aversive. From https://research.utexas.edu/wp-content/uploads/sit

It also depends on the species and flow rate. IIRC rodents handle CO2 ok but rabbits are very aversive. From https://research.utexas.edu/wp-content/uploads/sites/3/2015/10/guideline03.pdf https://research.utexas.edu/wp-content/uploads/sites/3/2015/...: The 2013 edition of the American Veterinary Medical Association Guidelines for the Euthanasia of Animals concludes that CO2 euthanasia is humane only when rodents are exposed to a gradually rising concentration that will cause narcosis (loss of consciousness due to the depression of cerebral activity) before the sensitive respiratory tissues are exposed to high concentrations. An optimal flow rate for CO2 euthanasia systems should displace 10%-30% of the chamber (or cage) volume per minute The full AVMA guidelines are here: https://www.avma.org/KB/Policies/Documents/euthanasia.pdf https://www.avma.org/KB/Policies/Documents/euthanasia.pdf