U.S. Animal Protection Position on Carbon Monoxide



Gassing pound animals with carbon monoxide began when the American SPCA took
over the New York City animal control contract in 1895. The use of gas stopped
what was then considered the inhumane practice of drowning strays in the Hudson
River. Carbon monoxide gassing subsequently gained acceptance across the U.S.
and became one of the more commonly used methods of euthanasia in animal control
facilities. However, the method of carbon monoxide gassing, while inexpensive
and effective, was never evaluated as a humane method by veterinarians.
Therefore, the physiological and psychological effects of this method on
individual animals were a matter of speculation.

AVMA Guidelines On Euthanasia

In 1972, at the request of the American Veterinary Medical Association (AVMA)
Council on Research, the Executive Board of the AVMA convened a Panel on
Euthanasia. The task assigned to this Panel was to define euthanasia as it
applies in veterinary medicine; survey any veterinary and scientific literature
on the various methods of euthanasia; and provide guidance on agents and methods
to veterinarians who (presumably) would be performing or directing these
procedures. Included in this mandate was to comment on the possible effects of
each particular method on the physiology and behavior of animals. The Panel was
not directed to obtain was firsthand (practice) knowledge of the individual
methods, or distinguish methods which were more humane than others.
Additionally, the Panel did not distinguish humane methods that could easily be
performed by non-veterinarians in animal control settings. The Panel on
Euthanasia issued its report on March 1, 1972 in the AVMA Journal.

The Panel on Euthanasia was reconvened in 1978, 1986, 1993, 2000 and 2007.
Following the 2007 session, the title of the final document was changed to the
2007 AVMA Guidelines on Euthanasia. While more references were cited,
clarifying language was used and summary tables were provided, the Guidelines
still did not provide a comparative assessment of humane methods that might be
used by non-veterinarians in animal control settings. These Guidelines also
neglected to discuss newer veterinary drugs that facilitated a multiple modality
approach to euthanasia.

When the General Considerations in the AVMA Guidelines are compared with the
discussions on the use of inhalational methods of carbon monoxide for
euthanasia, several glaring dichotomies become evident. Among the 12 General
Considerations, seven do not substantially conform to the letter and intent of
the Guidelines. Specifically, the (1) ability to induce loss of consciousness
and death without causing anxiety, apprehension, distress or pain (2) the time
required to induce loss of consciousness, (3) reliability, (4) safety for
personnel, (7) emotional effect on observers or operators, (10) compatibility
with species, age and health status and (11) ability to maintain equipment in
proper working order are stringent requirements that are difficult to manage and
are highly variable with the inhalational use of carbon monoxide. The following
point-by-point discussion of the problems associated with the General
Considerations is relevant.

Discussion Of AVMA Guidelines

The ability of inhalational agents to be used for euthanasia is considered
humane “if the animal is rapidly exposed to a high concentration of the agent.”
However, the AVMA Guidelines recognize that carbon monoxide is explosive in
concentrations above 10% in air, which also increases the risk of acute toxicity
in humans. Based upon this presumption of hazard, the Guidelines recommend
“commercially prepared gas” at concentration of approximately 6% be used.

The ability of carbon dioxide to cause rapid loss of consciousness without
anxiety, apprehension, distress or pain is questionable based upon the few
scientific studies performed in animals and the anecdotal information from human
exposure. In a carefully controlled study using 6% carbon monoxide in dogs, the
actual time to loss of consciousness could not be determined. But it was
recorded that abnormal brain waves which occurred 20-25 seconds before loss of
consciousness was associated with agitation and vocalization. A subsequent
study by the same investigators demonstrated that the pre-euthanasia
administration of tranquilizers reduced, but did not eliminate agitation and
vocalization. Other scientific studies demonstrate that cats can have tremors
and convulsions prior to loss of consciousness and pigs demonstrate
pre-narcosis agitation and vocalization. An important consideration in
assessing the acute effects of carbon monoxide exposure of animals are the
symptoms reported by humans who recover from carbon monoxide toxicosis. Among
the commonly reported symptoms of “early carbon monoxide toxicosis are headache,
dizziness, and weakness. As concentrations of (carbon monoxide)
carboxyhemoglobin increase, these signs may be followed by decreased visual
acuity, tinnitus, nausea, progressive depression, confusion, and collapse.
Because carbon monoxide stimulates motor centers in the brain, loss of
consciousness may be accompanied by convulsions and muscular spasms.” It is
clear from these descriptions that use of carbon monoxide does not prevent
anxiety, apprehension, or distress, let alone pain (headache, convulsions and
muscle spasms) before narcosis is attained.

While the reliability of carbon monoxide to produce death in animals is not
questioned, the capability to produce humane death is clearly questionable in
the scientific literature and in anecdotal information received from observers
and operators of gassing chambers. Both sources report the clinical signs
described in the discussion above prior to the loss of consciousness among many

Safety of personnel is clearly a problem with carbon monoxide. Above a 10%
concentration, this gas is potentially explosive. The Material Safety Data
Sheet (MSDS) describes this agent as extremely hazardous for personnel because
it is highly toxic and difficult to detect. Acute exposure to concentrations in
concentrations as low as 0.2% of inspired air can produce signs of carbon
monoxide toxicosis. Chronic exposure to very low concentrations of carbon
monoxide as little as 0.05% of inspired air may be a health hazard in
cardiovascular disease and have teratogenic effects. Gas chambers must be
surrounded by an efficient exhaust or ventilation system to prevent accidental
exposure of humans.

There is a high incidence of negative emotional effect on observers and
operators associated with the use of carbon monoxide gas used for euthanasia.
Many operators report that they simply “turn on the gas and walk away,” which
does not fulfill the AVMA Guideline to observe animals until they lose
consciousness and ensure that individual animals are dead (euthanized). The
only study that indicated

The ability to determine the compatibility of carbon monoxide euthanasia with
animal species, different ages of animals and the health status of individual
animal requires knowledge of taxonomy, neonatology and some ability to perform a
clinical evaluation of individual animals. Carbon monoxide is contraindicated
in reptiles, amphibians, diving birds and mammals that have a great capacity for
holding their breath and/or living with anaerobic metabolism. Use of carbon
monoxide in neonatal animals less than 16 weeks of age is contraindicated
because of residual fetal hemoglobin in their blood. And lastly, animals with
impaired respiratory function may not be able to assimilate carbon monoxide
rapidly through respiration.

A carbon monoxide chamber requires a special site, ventilation and construction.
Operation requires thorough maintenance, strict operating procedures and a
stringent requirement for training operators on maintenance, procedures and
hazards. These requirements under the AVMA Guidelines impose difficult and in
some circumstances conflicting limitations on reliably performing humane
euthanasia with carbon monoxide. The AVMA Guidelines state the chamber should
(preferably) be located outside, must constructed from the highest quality
materials, must be cleanable and should allow for separation of individual
animals. If a chamber is located inside a building, there is a requirement for
a carbon monoxide monitoring system. A not well understood feature of these
chambers is that they have to be safely vented to the outside in a manner that
allows displacement of air inside the chamber with the carbon monoxide gas.
They cannot be air tight to function properly. The Guidelines also require a
chamber to be well lit and have view a port to allow the direct observation of
animals. All lights and mechanical apparatus must be explosion proof because
of the combustibility of carbon monoxide. Operation procedures require thorough
cleaning between uses to minimize odors (pheromones) that would cause anxiety,
apprehension and distress in animals subsequently euthanatized. Loading of the
chamber permits only animals of the same species in a manner that precludes
overcrowding. In introducing the carbon monoxide gas into the chamber, the flow
rate should be adequate to rapidly achieve a uniform concentration of at least
6% carbon monoxide inside the chamber, but should be so rapid as to produce
noise that causes agitation. And finally, all personnel using carbon monoxide
chambers must be instructed thoroughly on all the above Guidelines and must
understand the hazards and limitations. It is also mandatory that carbon
dioxide be used in compliance with all Federal (NIOSH/OSHA, EPA) and state
regulations. Based upon the above considerations, the operation of a carbon
monoxide chamber to cause humane euthanasia by anyone other than a veterinarian
who could judge a malfunction of the apparatus, or an untoward physiological or
behavior problem with an individual animal, is not reasonable.

United States Animal Protection
Dr. Robert Hopkins Dawn Taylor Bechtold



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