Carbon monoxide

CAS RN: 630-08-0

Toxicity Summary

It is a product of the incomplete combustion of carbon-containing fuels and is also produced by natural processes or by biotransformation of halomethanes within the human body. With external exposures to additional carbon monoxide, subtle effects can begin to occur, and exposure to higher levels can result in death. The health effects of carbon monoxide are largely the result of the formation of carboxyhemoglobin (COHb), which impairs the oxygen carrying capacity of the blood ... During typical daily activities, people encounter carbon monoxide in a variety of microenvironments - while travelling in motor vehicles, working at their jobs, visiting urban locations associated with combustion sources, or cooking or heating with domestic gas, charcoal or wood fires - as well as in tobacco smoke. ... Studies of human exposure have shown that motor vehicle exhaust is the most important source for regularly encountered elevated carbon monoxide levels ... The workplace is another important setting for carbon monoxide exposures ... Certain industrial processes can expose workers to carbon monoxide produced directly or as a byproduct ... Carbon monoxide is absorbed through the lungs, and the concentration of carboxyhemoglobin will depend ... mainly on the concentrations of inspired carbon monoxide and oxygen ... and will also depend on the duration of exposure, pulmonary ventilation, and the concentration of carboxyhemoglobin originally present ... In addition to its reaction with hemoglobin, carbon monoxide combines with myoglobin, cytochromes, and metalloenzymes such as cytochromoe c oxidase and cytochrome P-450 ... The binding of carbon monoxide to hemoglobin, producing carboxyhemoglobin and decreasing the oxygen carrying capacity of blood, appears to be the principal mechanism of action underlying the induction of toxic effects of low-level carbon monoxide exposures. The precise mechanisms by which toxic effects are induced ... are not understood fully but likely include the induction of a hypoxic state in many tissues of diverse organ systems ... A unique feature of carbon monoxide exposure, therefore, is that the blood carboxyhemoglobin level represents a useful biological marker of the dose that the individual has received ... The formation of carboxyhemoglobin is a reversible process; however, because of the tight binding of carbon monoxide to hemoglobin, the elimination half-time is quite long, ranging from 2 to 6.5 hr ... The level of carboxyhemoglobin in the blood may be determined directly by blood analysis or indirectly by measuring carbon monoxide in exhaled breath ... Decreased oxygen uptake and the resultant decreased work capacity under maximal exercise conditions have clearly been shown to occur ... However, of greater concern at more typical ambient carbon monoxide exposure levels are certain cardiovascular effects (i.e., aggravation of angina symptoms during exercise) likely to occur in a smaller, but sizeable, segment of the general population. This group, chronic angina patients, is currently viewed as the most sensitive risk group for carbon monoxide exposure effects ... The adverse health consequences of low level carbon monoxide exposure to patients with ischemic heart disease are very difficult to predict in the at-risk population of individuals with heart disease ... At high carbon monoxide concentrations, excessive increases in hemoglobin and hematocrit may impose an additional workload on the heart and compromise blood flow to the tissues ... It is unlikely that carbon monoxide has any direct effects on lung tissue except for extremely high concentrations associated with carbon monoxide poisoning ... Occupational or accidental exposure to the products of combustion and pyrolysis, particularly indoors, may lead to acute decrements in lung function if the carboxyhemoglobin levels are high. It is difficult, however, to separate the potential effects of carbon monoxide from those due to other respiratory irritants in the smoke and exhaust ... Of special note are those individuals who are taking drugs with primary or secondary depressant effects that would be expected to exacerbate carbon monoxide-related neurobehavorial decrements. Other groups at possible increased risk for carbon monoxide-induced neurobehavorial effects are the aged and ill ... Under normal circumstances, the brain can increase blood flow or tissue oxygen extraction to compensate for the hypoxia caused by exposure to carbon monoxide ...
... Studies in several laboratory animal species provide strong evidence that maternal carbon monoxide exposures ... produce reductions of birth weight, cardiomegaly, delays in behavorial development and disruptions in cognitive function ... Laboratory animal studies suggest that enzyme metabolism of xenobiotic compounds may be affected by carbon monoxide exposure ... The decreases in xenobiotic metabolism shown with carbon monoxide exposure might be important to individuals receiving treatment with drugs ... Tissues of highly active oxygen metabolism, such as heart, brain, liver, kidney, and muscle, may be particularly sensitive to carbon monoxide poisoning. There are reports ... of effects on liver, kidney, bone and the immune capacity of the lung and spleen. It is generally agreed that the severe tissue damage occurring during acute carbon monoxide poisoning is due to one of more of the following: (1) ischemia resulting from the formation of carboxyhemoglogin, (2) inhibition of oxygen release from oxyhemoglobin, (3) inhibition of oxygen release from oxyhemoglobin, (3) inhibition of cellular cytochrome function (e.g., cytochrome oxidases) and (4) metabolic acidosis ... Whereas certain data also suggest that perinatal effects (e.g., reduced birth weight, slowed post-natal developments, sudden infant death syndrome) are associated with carbon monoxide exposure, insufficient evidence exists by which to either qualitatively confirm such an association in humans or establish any pertinent exposure-effect relationships ... There remains little direct information on the possible enhancement of carbon monoxide toxicity by concomitant drug use or abuse ... The greatest evidence for a potentially important interaction of carbon monoxide comes from studies with alcohol in both laboratory animals and humans, where at least additive effects have been obtained. The significance of this is augmented by the high probable incidence of combined alcohol use and carbon monoxide exposure ... Besides being a source of carbon monoxide for smokers as well as non-smokers, tobacco smoke is also a source of other chemicals with which environmental carbon monoxide could interact ... On the basis of known effects described, patients with reproducible exercise-induced ischemia appear to be the best established as a sensitive group within the general population that is at increased risk for experiencing health effects of concern (i.e., decreased exercise duration due to exacerbation of cardiovascular symptoms) at ambient or near-ambient carbon monoxide concentrations ... Decrements in exercise duration in the healthy population would therefore be of concern mainly to competing athletes, rather than to ordinary people carrying out the common activities of daily life. It can be hypothesized, however, from both clinical and theoretical work and from experimental research on laboratory animals, that certain other groups in the population may be at probable risk from exposure to carbon monoxide. Identifiable probable risk groups can be categorized by gender differences; by age ...; by genetic variations ...; by pre-existing diseases ...; or by the use of medications, recreational drugs or alterations in environment ... Unfortunately, little empirical evidence is currently available by which to specify health effects associated with ambient or near-ambient carbon monoxide exposure to these probable risk groups ...
... Carbon monoxide is responsible for a large percentage of the accidental poisonings and deaths reported throughout the world each year ... Outdoors, concentrations of carbon monoxide are highest near street intersections, in congested traffic, near exhaust gases from internal combustion engines and from industrial sources, and in poorly ventilated areas such as parking garages and tunnels. Indoors, carbon monoxide concentrations are highest in workplaces or in homes that have faulty or poorly vented combustion appliances or downdrafts or backdrafts. The symptoms and signs of acute carbon monoxide poisoning correlate poorly with the level of carboxyhemoglobin measured at the time of arrival at the hospital ... Neurological symptoms of carbon monoxide poisoning can ocur, such as headache, dizziness, weakness, nausea, confusion, disorientation and visual disturbances. Exertional dyspnea, increases in pulse and respiratory rates and syncope are observed with continuous exposure ... When carboxyhemoglobin levels are higher than 50%, convulsions and cardiopulmonary arrest may occur. Complications occur frequently in carbon monoxide poisoning (immediate death, myocardial impairment, hypotension, arrhythmias, pulmonary edema). Perhaps the most insidious effect of carbon monoxide poisoning is the delayed development of neuropyschiatric impairment ... and the neurobehavioral consequences, especially in children. Carbon monoxide poisoning during pregnancy results in high risk for the mother, by increasing the short-term complications rate and for the fetus by causing fetal death, developmental disorders, and cerebral anoxic lesions. Furthermore, the severity of fetal intoxication cannot be assessed by the maternal rate. Carbon monoxide poisoning occurs frequently, has severe consequences, including immediate death, involves complications and late sequelae and is often overlooked ...
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