Carbon Monoxide

Carbon monoxide (CO) is a gas commonly associated with incomplete combustion. Significant amounts of carbon monoxide are produced by heating equipment, industrial processes, cigarettes, accidental fires and the internal combustion engines. Before the adoption of natural gas, which contains only 16% carbon monoxide, blast furnace gas was used, which was 25% carbon monoxide. About 90% of victims of fires die from carbon monoxide poisoning since the gas is the most common cause of fatal gassing. Whenever there are fire explosions, the atmosphere is contaminated by carbon monoxide. This occurs both during and after the fire. The gas has various adverse effects both to human health and to the environment. This essay describes carbon monoxide gas and how it bonds with haemoglobin to cause carbon monoxide poisoning in humans.

Carbon Monoxide Definition

Carbon monoxide is a gas that has no colour, taste, and odour. The gas does not have any irritating effect. CO is non-combustible even though it is produced due to incomplete combustion (Hanley & Patel, 2017). Joseph Priestley first identified the gas in the 18th century (Hess, 2017). Many chemists have used it in extracting metals from their ores (Hess, 2017). The physical properties of the gas account for its dangerous nature. Two gases are formed whenever oxygen combines with carbon. In adequate supply of air, carbon dioxide is formed. When there is limited supply of air, only half as much oxygen combines with carbon to form CO (Hess, 2017). The reaction is represented by Equation i. In limited supply of oxygen, two molecules of carbon combine with one molecule of oxygen in the presence of heat to produce two molecules of carbon monoxide (Hanley & Patel, 2017).

2C(s)+O_2 (g)→2CO(g)——i

The gas also forms as a pollutant during the burning of hydrocarbon fuels including diesel, natural gas and petrol. The combustion efficiency determines the amount of carbon monoxide produced as shown in Equation ii. During the burning of natural gas (methane), two molecules of methane combine with three molecules of oxygen to form two molecules of carbon monoxide and one molecule of water in the form of steam (Hess, 2017).

2CH_4 (g)+3O_2 (g)→2CO(g)+H_2 O(g)——ii

Carbon monoxide supports combustion and burns with a pale blue flame to produce carbon dioxide. As can be seen in equation iii, when burning, two molecules of CO react with one molecule of oxygen to produce two molecules of carbon dioxide.

2CO(g)+O_2 (g)→2CO_2 (g)——iii

CO can be produced in the laboratory by first obtaining carbon dioxide. The most common means of acquiring the carbon dioxide gas necessary for the laboratory production of carbon monoxide include the use dry ice or obtaining it from a CO2 cylinder (Hanley & Patel, 2017). In the absence of the two, neutralization reactions can be used between carbonates and acids, or hydrogen carbonates and acids to generate carbon dioxide (see equations iv and v)

2HCl(aq)+CaCO_3 (s)→CaCl_2 (aq)+H_2 O(l)+CO_2 (g)——iv
HCl(aq)+NaHCO_3 (s)→NaCl(aq)+H_2 O(l)+CO_2 (g)——v

The carbon dioxide is then made to pass over heated charcoal or coal to produce carbon monoxide. The carbon in charcoal or coal reduces the carbon dioxide to form two molecules of CO as shown in equation vi.

〖CO〗_2 (g)+C(s)→2CO(g)——iii

The trace carbon dioxide that remain after the reaction are removed by passing the produced gas through an aqueous solution of sodium hydroxide, which reacts with CO2 to produce sodium carbonate and water (Hanley & Patel, 2017).

The gas is very poisonous because it has a high affinity to haemoglobin (Hanley & Patel, 2017). The affinity between haemoglobin and carbon monoxide was discovered in 1970 by Claude Bernard, thereby explaining why it is poisonous (Hess, 2017). His findings explained that the gas disrupts the transportation of oxygen from the lungs to tissues by combining with haemoglobin to form carboxy-haemoglobin, which is a relatively stable compound and only dissociates slowly (Hess, 2017). Inspired by the issues that carbon monoxide was causing in British coal mines, Haldane explored the link between the gas and haemoglobin and explained in 1895 that carboxy-haemoglobin forms in an equilibrium reaction (Hess, 2017). The reaction depends on the partial pressure of oxygen and CO within an inspired gas. Haldane did his research by passing carbon monoxide through a bottle in which there is a mouse. His findings were that mice are less resistant to the gas as compared to humans (Hess, 2017).

Both oxygen (O2) and carbon monoxide have high affinities to molecules that contain iron. However, CO binds about 210 times more effectively with the iron-containing haemoglobin than O2. Since 21% of the air is made up of oxygen, half of the haemoglobin is eventually combined by only 0.1% carbon monoxide in the air. It takes between four and five hours for the level of carboxy-haemoglobin in the blood to decline from the time of the formation of the substance and the time exposure ends (Hess, 2017). The decline is exponential by 50%. As a result of the exponential properties, the adverse effects of the gas can be cumulative. This implies that it is possible for one to be poisoned by the intermittent exposure that the person experienced in the course of the day, including through tobacco smoking (Crocker et al., 2017).

Carboxy-haemoglobin does not have the ability to carry oxygen. Therefore, a level of 50% implies that the blood’s capacity to carry oxygen is minimized by about half (Hess, 2017). As a consequence, an individual’s ability for performing maximum exercises is highly minimized. Once the capacity of the blood to carry oxygen has been reduced, the body compensates for the decline by making cardiac output more abundant. This explains why the heart beats more strongly and more quickly during the first stages of carbon monoxide poisoning (Crocker et al., 2017).

During exposure to carbon monoxide, myoglobin molecules in the muscles are also affected. The affinity of the molecule to CO is 60 times more than that of oxygen. The compensatory mechanism of the body ends up failing and the heart muscle contractility also reduces (Hess, 2017). This may be very fatal when it causes intense tissue hypoxia (Crocker et al., 2017). The hypoxic effect is further compounded by the fact that the presence of carboxy-haemoglobin reduces the oxygen that normal haemoglobin holds. A further reduction in the oxygen in the tissues increases the ability of carbon monoxide to bind with other molecules containing iron, especially the enzymes cytochrome A3 and cytochrome P450. Cigarette smokers have raised levels of carboxy-haemoglobin in their blood since they inhale carbon monoxide through the smoke (Crocker et al., 2017).

Beneficial Uses of Carbon Monoxide

Carbon monoxide has also been used for some beneficial purposes. Due to its intense affinity for haemoglobin, low concentrations of the gas have been used as markers in the measurement of the lung surface area available for oxygen transfer, and the speed of blood that flows through the lungs. As was discovered by Sjostrand in 1951, CO is also a product of the metabolism of the body. The haem produced by the senescent red blood cells is broken down by the enzyme haem oxygenase to produce bile salts and carbon monoxide. The bile salts are eliminated through liver excretion while the gas released constitutes the blood with a 0.2-1.0% concentration of carboxy-haemoglobin, which is normal (Crocker et al., 2017).
Previously, it was thought that the carbon monoxide that is endogenous is just a by-product (De La Torre et al., 2017). However, Verma later discovered that specific regions in the brain have haem oxygenase (De La Torre et al., 2017). The scholar suggested that the produced CO functions as a neurotransmitter. The enzyme guanylyl cyclase is activated by carbon monoxide, and it in turn regulates the levels of cyclic GMP within the cells. The GMP controls cellular activity. The enzyme haem oxygenase has also been discovered on the walls of blood vessels, where they produce carbon monoxide for the initiation of vasodilation (De La Torre et al., 2017). Scholars have linked endogenous CO with memory, sense of smell, control of levels of hormones in the blood, and cerebellar functions (De La Torre et al., 2017). The gas is also responsible for the development of smooth muscle tone.

Carbon Monoxide Poisoning

The carbon monoxide poisoning symptoms are manifested in relation to the concentration of the gas one has inhaled. Even though the onset of poisoning is slow, the affected person may pass out abruptly. Headache, with or in some cases without nausea is also experienced by most of the victims, which is related to the vasodilation effect of CO (Hess, 2017). After a while, the individual may feel lethargy and drowsiness and feel breathless on exertion may ensue. Due to cardiac hypoxia, one may feel chest pain at any stage. When one is lethargic and drowsy, he/she may not be able to think clearly and attempt to escape since his or her cerebral function is already affected. If not rescued, the individual goes into a coma and then the next thing is death. Victims of carbon poisoning tend to have lips that are unnaturally bright red (Crocker et al., 2017).

In case there is someone around to rescue the victim of carbon monoxide poisoning, the individual is treated by removing him/her from the contaminated atmosphere to a carbon-monoxide-free environment and administering 100% oxygen to the individual. Recovery is sped up by use of hyperbaric oxygen, which has been discovered to also minimize the long-term neurological complications (Huang et al., 2017; Buckley et al., 2011). There are still a lot that have not yet been understood about the positive and negative effects of carbon monoxide on health (Crocker et al., 2017).

How Carbon Monoxide Bonds to Haemoglobin

Porphyrin-iron (II) system makes up the most important part of haemoglobin. The iron (II) ion in the centre is coordinated from five directions and has a water atom that is weakly bound in the sixth coordination slot. In some instances, the water atom is displaced by a distal histidine. The makeup of the system makes it easy for the carbon monoxide molecules to diffuse and bind with it. This is done through the displacement of the water and histidine. It is important to note that CO is bound 100,000 times better than oxygen by isolated haem (Hess, 2017).

There are four haem groups in each haemoglobin molecule, which can bind reversibly with oxygen molecules. Once oxygen binds to any of the sites, it causes the protein to undergo a conformational change. The change makes it possible for oxygen to bind to the other sites. CO bind to the same sites as oxygen, but more readily and tightly (Hess, 2017). Oxygen-bearing haemoglobin would in normal circumstances release the gas in areas where there is a lower concentration of oxygen with ease. However, when carbon monoxide binds to haemoglobin, the release is slower and with difficulty. As a consequence, the carbon monoxide-bound haemoglobin molecules accumulate with continued exposure to the gas. This reduces the number of available haemoglobin for binding and delivering oxygen. The result is a gradual suffocation of the individual (Hess, 2017).


Carbon monoxide is a colourless, odourless and tasteless gas which is a product of incomplete burning. In limited supply of oxygen, two carbon atoms react with only one molecule of oxygen to produce two molecules of carbon monoxide. CO has been found to have some beneficial uses including its function as a neurotransmitter and regulation of skin tone. Carbon monoxide is also a very poisonous gas. It has a higher affinity to the haemoglobin in the red blood cells than oxygen. It binds with the iron sites in haemoglobin to form deoxy-haemoglobin, which dissociates slower than oxy-haemoglobin formed by oxygen when it binds with haemoglobin. This reduces the number of haemoglobin available for the transportation of oxygen to the body tissues.

As tissues become deprived of oxygen, the heart tries to compensate by beating faster and the victim ends up becoming breathless. Headache and nausea ensues, and the person may end up going into a stroke and eventually die. There are certain risky behaviour that place individuals at high risks of being poisoned by carbon monoxide, one being the smoking of tobacco. The body also produces CO that are used for various beneficial functions. There is need for further studies to identify and understand the beneficial biological functions of carbon monoxide.


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De La Torre et al. (2017). Ex vivo tracking of endogenous CO with a ruthenium (II) complex. J Am Chem Soc.
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Hess, D.R. (2017). Inhaled Carbon Monoxide: From Toxin to Therapy. Respir Care, 62(10): 1333-42.
Huang et al. (2017). Hyperbaric Oxygen Therapy Is Associated With Lower Short- and Long-Term Mortality in Patients With Carbon Monoxide Poisoning. Chest, 152(5): 943-953.