Page 668 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Toxic Gases and Vapors Chapter | 48 633

VetBooks.ir studies of the acute (exposure durations between 1 and most commonly produced by incomplete hydrocarbon com-
bustion. A component of CO poisoning is almost always
120 min) lethality effects of poison gases (specifically gas
present in cases of smoke inhalation injury (Alarie, 2002;
weapons in the case of Fritz Haber, notably phosgene,
methylchloroformate, cyanide gas, chloracetone, xylybro- Jones, 2003; Fitzgerald et al., 2006). CO is also produced
mide, and chlorine) that exposure to a low concentration by the mixed function oxidase-mediated biotransformation
of a poisonous gas for a long time often had the same of methylene chloride (dichloromethane), a common sol-
effect (death) as exposure to a high concentration for a vent component present in paint strippers and degreasers
short time. They discovered that, in general, a simple (Weaver, 2004). The combustion of methylene chloride
mathematical relationship applied: C 3 t 5 k, where C is produces phosgene. CO is also produced endogenously as a
the concentration of the poisonous gas, t is the time of byproduct of erythropoiesis.
exposure, and k (the “toxic load” required to produce the CO commonly causes poisoning and high-mortality in
effect) is a constant (Haber, 1924; Lohs, 1990; Witschi, humans in the United States (Sadovnikoff et al., 1992;
1999, 2000). If different concentrations and times of Meredith, 1993; McGuigan, 1999; Hampson and Stock,
exposure are used, this implies that C 1 3 t 1 5 C 2 3 t 2 . 2006; Harduar-Morano and Watkins, 2011). Epidemics of
This relationship, in theory, can also be used to extrapo- CO poisoning in humans and animals are notoriously
late concentration values between short-term and long- associated with the occurrence of storms, cold snaps or
term exposures, i.e., other severe weather, particularly when such events are
accompanied by a loss of electrical power and/or the fail-
C 1 3 t 1
C 2 5 ure of heating systems. Veterinarians, farm workers, and
t 2
animals are at risk of exposure to CO in intensive animal
In modern risk assessment, the ten Berge modification production units that are heated by hydrocarbon combus-
n
of Haber’s law is commonly used: C 3 t 5 k (ten Berge tion. Both acute lethal and sublethal CO poisonings are
et al., 1986). The exponential n is a regression coefficient well-known problems in intensive pig operations, particu-
for the exposure concentration exposure duration rela- larly those relying on gas heating systems (Boller, 1976;
tionships for the relevant effect. In general, the value of n Keller, 1976; Wood, 1979; Stuart and Oehme, 1982;
lies between 1 and 3. If suitable data are not available to Dominick and Carson, 1983; Morris et al., 1985a,b;
derive n, a default value of n 5 1 is used for extrapolating Pejsak et al., 2008). CO in engine fumes may also reduce
from shorter to longer exposure durations and a default egg hatchability (Swarbrick, 1989).
value of n 5 3 is used for extrapolating from longer to CO has also been investigated as a veterinary
shorter exposure durations. Using the ten Berg modifica- euthanasia agent, and has some utility when mass
tion, the Haber’s law equation becomes: euthanasia of poultry is necessary (Moreland, 1974;
Simonsen et al., 1981; Chalifoux and Dallaire,
n
C 3 t 1
1
C 2 5 1983; Lambooy et al., 1985; Enggaard Hansen et al.,
t 2
1991; Kingston et al., 2005; Gerritzen et al., 2006).
However, note that there are many cases in which However, there are substantial operator safety concerns
Haber’s law and the ten Berge-modified Haber’s law do with the use of CO for this purpose. CO treatment has
not accurately describe the dose time relationships for also been used for improving the color of muscle foods
the toxicological effects of gases. The use of these simple (Hamling et al., 2008; Mantilla et al., 2008; Jeong and
relationships may seriously over- or under-estimate the Claus, 2010, 2011). Again, strict safety standards are
degree of toxicological effects, particularly when there necessary to protect human workers in such
are large extrapolations in terms of the time of exposure circumstances.
(Weller et al., 1999; Miller et al., 2000; Hoyle et al.,
2010). High-quality data for the specific duration of expo-
sure of interest are often preferable to the use of Haber’s Toxic Dose
law or the ten Berge modification. At physiological equilibrium, an atmospheric CO level of
50 ppm produces a carboxyhemoglobin (COHb) level of
8% in humans, which is the basis for the US
SPECIFIC TOXIC GASES
Occupational Safety and Health Administration PEL 8-h
Carbon Monoxide time-weighted average level of 50 ppm (Weaver, 2004).
Reduction in cognitive performance occurs in humans
Overview, Uses, and Sources of Exposure exposed to levels as low as 17 ppm for 1.5 2.5 h (COHb
CO is colorless, odorless and virtually undetectable without level of 2%). Situations that result in lower alveolar oxy-
the use of gas detection technologies, hence its reputation gen partial pressure (e.g., high altitudes), increased alveo-
as a “silent killer” (Weaver, 2004). CO is ubiquitous and lar ventilation (e.g., higher metabolic rates and increased