The Humane Society of the United States (HSUS) recommends the injection of sodium pentobarbital, prepared specifically for use as a euthanasia product, as the preferred agent for the euthanasia of dogs and cats. This method, when properly performed, has been found to be the most humane, safest, least stressful, and most professional choice by The HSUS, the American Veterinary Medical Association, the American Society for the Prevention of Cruelty to Animals, American Humane Association, and the National Animal Control Association. However, not every animal care and control agency has access to sodium pentobarbital and the majority of these agencies use carbon monoxide as an alternative to euthanize animals.
For those states where shelters cannot legally obtain, use, and administer sodium pentobarbital, The HSUS considers the use of CO a conditionally acceptable method of euthanasia for some animals when delivered by a commercially manufactured and properly equipped chamber. However, the many limitations of CO make the method less practical, considerably slower, and more expensive than lethal injection.
It is always unacceptable to use CO for the euthanasia of dogs and cats who are:
1. Geriatric;
2. Under the age of four months;
3. Sick or injured;
4. (Obviously) pregnant.
Old, sick, or injured animals may have poor blood pressure or weak hearts, which may delay the effects of CO, causing them to experience distress prior to unconsciousness. Animals under the age of four months may not have the lung capacity to inhale enough CO to be effective. In pregnant animals, it is likely the mother will die from exposure to CO before the unborn puppies/kittens. Consequently, it is possible the puppies/kittens may die as a result of the mother’s death (by suffocating to death) rather than from exposure to CO. The HSUS condemns using CO for animals meeting the above criteria.
Because of these limitations, there must always be an acceptable backup method available, namely sodium pentobarbital. In most cases, agencies without access to sodium pentobarbital will need to contract with a local veterinarian to have sodium pentobarbital administered to these animals. Agencies without legal accessibility to sodium pentobarbital should explore the possibility of partnering with local veterinarians to perform routine euthanasia as well.
In order to use carbon monoxide in an appropriate and acceptable fashion, the following minimum requirements and conditions must be met:
The Equipment
The chamber must be a commercially manufactured unit designed specifically for carbon monoxide euthanasia and be properly maintained. It should be stationed and utilized in a well-ventilated area and exhausted to the outside. If it is designed to euthanize more than one animal at a time, it must be equipped with independent sections or cages to separate incompatible or frightened animals. The interior of the chamber must be well-lit, and equipped with view-ports, a regulator (which maintains the gas concentration), and flow-meter (which measures the gas concentration). Only bottled commercial-grade gas must be used; engine or chemically-generated gas is not acceptable due to impurities and heat which are painful and inhumane. The chamber should achieve a minimum of 6% concentration of gas (the concentration should never exceed 10%, a level at which CO may become explosive) within 20 seconds, the animals must be unconscious within 45-60 seconds, and death must occur within two to four minutes. Monitoring equipment (such as an alarm) must be used during operation and, after each use the chamber must be cleaned thoroughly.
The Euthanasia Process
Different species of animals must never be placed in the chamber together. The chamber must never be overcrowded. Animals should have enough room to sit or lie down comfortably. A euthanasia technician should be present during the entire cycle in the event there is an equipment malfunction or other problem. The sound of the gas entering the chamber may create fear in some animals, especially cats. They may become restless, frightened, and/or aggressive, further stressing other animals. Since the gas does not render the animals unconscious immediately, animals may appear to go through a period of resistance. Dogs may vocalize or howl and become rigid. There is uncertainty whether or not the animal is conscious at this stage of the process.
Animals must be left in the chamber and exposed to the gas for a minimum of thirty minutes to ensure death. The chamber must be fully exhausted before animals are removed. Positive verification of death must be determined for each animal prior to disposal. There are four signs that a technician should check to verify death: lack of respiration, lack of eye reflexes, lack of heartbeat, and presence of rigor mortis. Only the fourth sign, rigor mortis, is a certain sign of death. In rare instances, an animal may appear to lack respiration, eye reflexes, or a heartbeat, but still be alive.
Staff Safety
In 2000, an animal shelter employee in Tennessee died from carbon monoxide exposure while using CO to euthanize animals. Staff must be fully notified of potential health risks involved with using CO. Carbon monoxide is a hazardous substance: it is highly toxic and has no odor, no color, and no taste. It is the leading cause of accidental poisoning in the United States and repeated exposure to CO, even at low levels, can result in many long-term effects including (but not limited to): impaired memory, breathing difficulties, muscle weakness, heart irregularity, and brain damage. Exposure among pregnant women can cause low birth weight in their infants. The use of a CO chamber should be closely monitored, and in compliance with Occupational Safety and Health Association (OSHA) requirements. According to OSHA, the current permissible exposure limit for CO is 50 parts per million parts of air per cubic meter. Although some people believe that using CO is safer because staff does not have to individually handle and/or restrain animals, loading animals in the chamber does require staff to handle animals, including those who are scared, aggressive, feral, or fractious. These animals may require prior sedation, tranquilization, or anesthesia to insure employee safety.
One of the most critical responsibilities of those in the animal care and control field is to provide the most humane death possible for companion animals when euthanasia is necessary. The HSUS strongly recommends that all animal care and control agencies evaluate current euthanasia procedures frequently, ensure that animals are being properly handled, and verify that employees are competent, compassionate, and properly trained.
For more information on euthanasia methods, policies, and procedures, see The HSUS Statement on Euthanasia for Dogs and Cats.
Last Updated 8/23/05
