September 21, 2001

Nelson Garnett, D.V.M.
Director, Office of Laboratory Animal Welfare
National Institutes of Health
RKL I, Suite 1050, MSC 7982
6705 Rockledge Dr.
Bethesda, MD 20892-7982

Dear Dr. Garnett,

The HSUS has conducted an extensive review of the available literature on the use of carbon dioxide for euthanasia and anesthesia, a copy of which is attached. We have revised the paper in order to make the distinction between The HSUS's recommendations in regards to the spirit of PHS Policy and actual PHS Policy requirements, as you have requested.

The use of CO₂ for euthanasia and anesthesia is very common, especially in research on rats and mice, yet its use does not appear to meet the requirements of PHS Policy. As you know, Government Principle #4 states "Proper use of animals, including the avoidance or minimization or discomfort, distress and pain when consistent with sound scientific practices, is imperative. Unless the contrary is established, investigators should consider that procedures that cause pain or distress in human beings may cause pain or distress in other animals." As you will see in the attached review, CO₂ indisputably causes pain in humans, which is the basis of Government Principle #4.

In addition to the human evidence, there are conflicts within the literature as to whether or not CO₂ causes pain and distress in animals. However, CO₂ has actually been used in research to elicit a painful or acute stress response in animals. Conflicting evidence is found not only within the literature, but within the currently practicing laboratory animal medicine community as well. For example, some people working in the field of animal research say that a chamber that has been prefilled with carbon dioxide causes extreme distress to the animals, while others say that gradual induction of the gas causes distress. The various studies and opinions at the very least raise warning flags.

We are requesting that OLAW send a letter to all IACUCs that have an assurance on file with OLAW, and urge them to consider alternatives to the use of carbon dioxide by itself for euthanasia and anesthesia (such as the use of a pre-anesthetic prior to CO₂ induction instead of CO₂ alone), based on PHS Policy requirements. Research institutions should be alerted to the negative effects that CO₂ causes in humans and can potentially cause in animals.

As you well know, improved animal welfare will lead to improved science. This particularly holds for widespread practices that potentially cause pain and distress in research animals. The HSUS believes that the use of CO₂ merits attention in this regard. Thank you for your time and we look forward to hearing from you.

Sincerely,

Andrew Rowan, D. Phil.
Senior Vice President for Research, Education and International Affairs
The Humane Society of the United States