Carbon monoxide

What is it?	A colorless, odorless gas that can be dangerous.
Where is it found?	In car exhaust, faulty heaters, and fires.
Why is it dangerous?	It can make you very sick or even harm you seriously.
How to stay safe?	Make sure heaters and cars are well-maintained and have good ventilation.

Carbon monoxide (CO) is a poisonous gas that you cannot see, smell, or taste. It is made of one carbon atom and one oxygen atom connected together. This gas is used in many ways to help make drugs, fragrances, and fuels.

The most common way carbon monoxide forms is when things that burn, like fuels, do not burn completely. It can come from tobacco smoke, stoves that are not working right, or heating systems that are broken. Because it is very dangerous, carbon monoxide is a big problem for indoor air quality.

Even though it can be harmful in large amounts, carbon monoxide also has important jobs in living things. Our bodies make tiny amounts of it, which can help send messages in the brain. But when too much builds up, it can become very dangerous and cause serious harm.

Physical and chemical properties

Carbon monoxide is the simplest oxocarbon. It has one carbon atom and one oxygen atom connected by a strong triple bond. Because it is lighter than air, it can float and move easily.

The bond between carbon and oxygen in carbon monoxide is very strong. This makes the gas stable but also dangerous. The way the atoms share their electrons gives carbon monoxide a small charge difference, with carbon being a little negative and oxygen a little positive. This affects how carbon monoxide behaves when it meets other molecules.

Bonding and dipole moment

The most important resonance form of carbon monoxide is −C≡O+. An important minor resonance contributor is the non-octet carbenic structure :C=O.

The strong bond between carbon and oxygen can be seen in how the molecule vibrates. This vibration happens often, showing how strong the bond is.

Carbon and oxygen share their electrons in a special way to form this triple bond. Because oxygen pulls more on the shared electrons, the molecule has a small charge difference, with carbon slightly negative and oxygen slightly positive. This makes the molecule uneven, with oxygen having more electrons than carbon.

Bond polarity and oxidation state

Even though oxygen usually pulls electrons more strongly, in carbon monoxide the carbon end is slightly negative. The bonds between carbon and oxygen are polar covalent bonds, meaning the electrons are not shared equally.

In terms of oxidation state, carbon in carbon monoxide is considered to have a state of +2. This is worked out by thinking that all the bonding electrons belong to the more electronegative oxygen atom. Only the electrons that are not shared belong to carbon.

Thermal and physical properties of carbon monoxide (CO) at atmospheric pressure
Temperature (°C)	Temperature (K)	Density (kg/m³)	Specific heat (J/g °C)	Dynamic viscosity (cg/m s)	Kinematic viscosity (cm²/s)	Thermal conductivity (cW/m °C)	Thermal diffusivity (cm²/s)	Prandtl number
−73.15	200	1.6888	1.045	1.27	0.0752	1.7	0.0963	0.781
−53.15	220	1.5341	1.044	1.37	0.0893	1.9	0.119	0.753
−33.15	240	1.4055	1.043	1.47	0.105	2.06	0.141	0.744
−13.15	260	1.2967	1.043	1.57	0.121	2.21	0.163	0.741
6.85	280	1.2038	1.042	1.66	0.138	2.36	0.188	0.733
26.85	300	1.1233	1.043	1.75	0.156	2.5	0.213	0.73
46.85	320	1.0529	1.043	1.84	0.175	2.63	0.239	0.73
66.85	340	0.9909	1.044	1.93	0.195	2.78	0.269	0.725
86.85	360	0.9357	1.045	2.02	0.216	2.91	0.298	0.725
106.85	380	0.8864	1.047	2.1	0.237	3.05	0.329	0.729
126.85	400	0.8421	1.049	2.18	0.259	3.18	0.36	0.719
176.85	450	0.7483	1.055	2.37	0.317	3.5	0.443	0.714
226.85	500	0.67352	1.065	2.54	0.377	3.81	0.531	0.71
276.85	550	0.61226	1.076	2.71	0.443	4.11	0.624	0.71
326.85	600	0.56126	1.088	2.86	0.51	4.4	0.721	0.707
376.85	650	0.51806	1.101	3.01	0.581	4.7	0.824	0.705
426.85	700	0.48102	1.114	3.15	0.655	5	0.933	0.702
476.85	750	0.44899	1.127	3.29	0.733	5.28	1.04	0.702
526.85	800	0.42095	1.14	3.43	0.815	5.55	1.16	0.705

Occurrence

Carbon monoxide can be found in many places, usually in very small amounts. It is created when plants break down.

Carbon monoxide is also present in Earth’s atmosphere in tiny amounts. It comes from human activities and natural events like volcanoes, forest fires, and burning fossil fuels. It can also come from the ocean and activity deep within the Earth. Carbon monoxide affects the atmosphere by influencing other gases, such as methane, and it eventually turns into carbon dioxide. In space, carbon monoxide is common and helps scientists study stars and clouds of gas. It has been found around stars, on planets like Venus, and even in comets.

**Composition of dry atmosphere, by volume**
Concentration (ppmv)	Source
0.1	Natural atmosphere level (MOPITT)
0.5–5	Average level in homes
5–15	Near properly adjusted gas stoves in homes, modern vehicle exhaust emissions
17	Atmosphere of Venus
100–200	Exhaust from automobiles in the Mexico City central area in 1975
700	Atmosphere of Mars
	Car exhaust fumes after passing through catalytic converter
5,000	Exhaust from a home wood fire
30,000–100,000	Undiluted warm car exhaust without a catalytic converter
Parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, see volume (thermodynamics))

Pollution and health effects

Urban pollution

Carbon monoxide can be found in some cities. It mainly comes from vehicle exhaust and other engines that burn fuels like gasoline, wood, or charcoal.

Role in ground level ozone formation

Main article: Ground level ozone

Carbon monoxide helps create a type of air pollution called ground-level ozone. It mixes with other chemicals in the air and sunlight to form this ozone, which can be harmful to breathe.

Indoor air pollution

Main article: Indoor air quality

Carbon monoxide can also be dangerous inside homes. It can come from things like smoking, faulty stoves, or heaters that burn fuels like gas or wood.

Mining

Miners call carbon monoxide “whitedamp” because you can’t see it. It can build up in mines with poor air flow. It comes from engines, explosives, or even coal that heats up slowly.

Health effects

Breathing in too much carbon monoxide can be very dangerous. It can make people very sick and cause problems with thinking and behavior. Even smaller amounts over time can make people feel tired, give them headaches, or cause other health problems.

Chemistry

Carbon monoxide is important in chemistry. It can work with many different materials. It often connects with metals to form special compounds called metal carbonyls. For example, iron can combine with carbon monoxide to make iron pentacarbonyl. This is a liquid that stays stable in air. Nickel can also mix with carbon monoxide to create nickel carbonyl.

In other types of chemistry, carbon monoxide helps change oils into acids. It can also turn certain materials into benzaldehyde. It can mix with hydrogen gas and oils to make aldehydes, using special metals to help.

Production

Carbon monoxide is made when things burn without enough oxygen. This can happen in engines or stoves, especially in closed spaces. It is also made in some chemical processes.

Factories have many ways to make carbon monoxide. One way is by burning carbon with air at high temperatures to make a mix called producer gas. Another way is by mixing steam with carbon to make water gas, which contains hydrogen and carbon monoxide. Carbon monoxide can also be made by special reactions with carbon dioxide or by heating metal ores with carbon.

Use

Carbon monoxide is an industrial gas used to make many everyday products. It helps create important chemicals like aldehydes, which are used to make detergents. It is also used to make phosgene, a compound that helps produce materials such as polycarbonates and polyurethanes.

In metalworking, carbon monoxide helps turn ores into pure metals by removing oxygen. It plays a key role in processes like making steel in blast furnaces. Scientists have also suggested using carbon monoxide as a possible fuel for future missions on Mars, where it could be made from the planet's carbon dioxide air. Additionally, carbon monoxide is used in special laboratory techniques to study the surfaces of metal electrodes.

Biological and physiological properties

Physiology

Main article: Gasotransmitter

Medicine

Many scientists are studying carbon monoxide to see if it can help treat diseases. They think it might be useful for problems like tissue damage during surgery, organ rejection after transplants, and some serious infections. Researchers are working on safe ways to give carbon monoxide to patients and are testing its effects.

Microbiology

Tiny organisms in our bodies, called microbiota, can also use carbon monoxide as a signal. Some bacteria make carbon monoxide and use it as food, while others change it into methane gas.

Food science

Carbon monoxide is used in some packaging for fresh meat in the United States to keep it looking red and fresh. It combines with a substance in meat to keep the color bright red for longer. This method was approved by the U.S. Food and Drug Administration in 2002, but many other countries do not allow it.

History

Humans have had a long relationship with carbon monoxide since they first learned to control fire around 800,000 BC. Early people learned that fire could be dangerous when brought inside their homes. As people began working with metals around 6,000 BC during the Bronze Age, they also faced dangers from carbon monoxide.

Ancient civilizations told stories about fire, like the tale of Prometheus in Greek mythology. Aristotle noticed that burning coals made harmful fumes, and Galen thought the air could change in ways that might cause harm. Later, scientists like Joseph Priestley and Carl Wilhelm Scheele studied carbon monoxide in laboratories. Today, carbon monoxide is used in important ways to make fuels, chemicals, and even vitamins.