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, flammable gas that you cannot see, smell, or taste. It is made of one carbon atom and one oxygen atom connected very tightly. This simple gas plays important roles in many industrial processes, helping make drugs, fragrances, and fuels.

The most common way carbon monoxide forms is through incomplete combustion of things that burn, like fuels. It can come from tobacco smoke, malfunctioning stoves, or heating systems that do not work properly. Because it is so dangerous, carbon monoxide is one of the most serious threats to indoor air quality.

Even though it is harmful in large amounts, carbon monoxide also has important roles in living organisms. Our own bodies make small amounts of it, which can act like a messenger in the brain. However, when too much builds up, it can become very dangerous and even cause serious harm.

Physical and chemical properties

Carbon monoxide is the simplest oxocarbon. It is made of one carbon atom and one oxygen atom connected by a strong triple bond. Because of its weight, it is slightly lighter than the air around us.

The bond between carbon and oxygen in carbon monoxide is very strong. This makes the gas stable but also very poisonous. The way the atoms share their electrons gives carbon monoxide a slight charge difference, with carbon being a little negative and oxygen a little positive. This also means that carbon monoxide can act differently depending on what other molecules it interacts with.

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 in carbon monoxide can be seen in how often the molecule vibrates. This vibration happens at a high frequency, showing just how strong the bond is.

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

Bond polarity and oxidation state

Even though oxygen is more electronegative, meaning it usually pulls electrons more strongly, the charge difference in carbon monoxide is such that the carbon end ends up 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 figured out by thinking of all the bonding electrons as belonging 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 concentrations in Northern Hemisphere spring as measured with the MOPITT instrument

Carbon monoxide can be found in many places, usually in very small amounts. It is created when plants break down, making about 60 million tons each year.

Carbon monoxide is also present in Earth’s atmosphere in tiny amounts, around 80 parts per billion. It comes from human activities and natural events like volcanoes, forests fires, and burning fossil fuels. It can also come from the ocean and geological 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. Sometimes, we can even see large amounts of carbon monoxide from space above cities.

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. In wealthier countries, it’s often from broken or poorly kept appliances. In places with less money, burning things like wood for cooking or heating can release carbon monoxide.

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, cause problems with thinking and behavior, and in severe cases, can lead to losing consciousness or even death. 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 and can work with many different materials. It often connects with metals to form special kinds of compounds called metal carbonyls. For example, iron can combine with carbon monoxide to make iron pentacarbonyl, which 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 or turns certain materials into benzaldehyde. It can also mix with hydrogen gas and oils to make aldehydes, using special metals to help the process.

Production

Carbon monoxide is most often made when things burn without enough oxygen. This can happen in engines or stoves, especially if they are in a closed space. It is also made during some chemical processes and needs to be cleaned up.

There are many ways to make carbon monoxide in factories. 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 around the world 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 in clinical trials.

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 likely learned that carbon monoxide could be harmful when they brought fire 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 to explain fire, such as the tale of Prometheus in Greek mythology. Aristotle noted that burning coals produced harmful fumes, and Galen thought that the air changed in a way that could cause harm when breathed in. In more recent history, scientists like Joseph Priestley and Carl Wilhelm Scheele were able to create and study carbon monoxide in laboratories. Today, carbon monoxide is used in important industrial processes to make fuels, chemicals, and even vitamins.