Laboratory Animal CO2 Euthanasia: Welfare Concerns and Alternatives

Carbon dioxide is the most widely used euthanasia method for laboratory rodents but causes distress before loss of consciousness, prompting development of more humane alternatives.

Key Facts

CO2 is used to euthanize the majority of laboratory mice and rats in North America
Studies show rodents find CO2 aversive and experience distress before loss of consciousness
Animals placed directly in high CO2 concentrations may experience pain from mucous membrane irritation
Gradual fill methods reduce but do not eliminate aversion compared to inert gas methods
Isoflurane overdose and injectable agents provide more humane euthanasia but have practical limitations
Regulatory guidance in some jurisdictions still recommends CO2 despite evidence of welfare concerns

Welfare Considerations

CO2 euthanasia presents a welfare challenge that has been systematically understudied relative to its prevalence. Rodents exposed to CO2 show avoidance behavior and physiological stress indicators before losing consciousness, suggesting the experience is aversive. The mucous membranes of the respiratory tract are irritated by CO2, potentially causing pain in the period before unconsciousness. Inert gas methods using argon or nitrogen do not share these properties but have been less widely adopted due to cost and availability. The scale of CO2 euthanasia — tens of millions of animals annually — makes even modest improvements in humaneness significant in aggregate welfare terms.

What You Can Do

Advocate for research institutions to transition from CO2 to more humane euthanasia methods
Support regulatory guidance updates that reflect current welfare evidence on CO2 euthanasia
Donate to organizations working on validation of humane alternatives to CO2 for laboratory rodents
Encourage institutional animal care committees to require welfare justification for CO2 use
Support research into the comparative welfare costs of different euthanasia methods for rodents