Though both carbon monoxide (CO) and carbon dioxide (CO2) contain the same elements; carbon and oxygen, they are quite different. The general difference between the two is the number of carbon and oxygen atoms. Carbon monoxide contains one carbon and one oxygen whereas carbon dioxide contains one carbon and two oxygen atoms.

Our atmosphere contains 0.041% of carbon dioxide naturally. CO2 also gets into the atmosphere because of the breathing of humans and many other living organisms. Plants use carbon dioxide from the atmosphere to make food in the form of sugars during photosynthesis. On the other hand, carbon monoxide is a toxic gas. It results in suffocation (CO poisoning) as it builds up. Incomplete combustion (limited supply of O2) of coal and other fossil fuels results in the formation of carbon monoxide.

Carbon Monoxide vs. Carbon Dioxide: The Chemistry of CO and CO2

Carbon dioxide vs Carbon monoxide

Carbon dioxide Carbon monoxide
Carbon dioxide is composed of carbon attached to two oxygen atoms Carbon monoxide is composed of carbon attached to one oxygen atom.
The molecular formula of carbon dioxide is CO2
The molecular formula of carbon monoxide is CO
It occurs naturally in the atmosphere It can form in natural processes
It is formed as a product of breathing, by humans and other organisms, fermentation, and complete combustion of fossil fuels, etc It is produced because of the incomplete combustion of fossil fuels
Carbon dioxide is toxic if present more than a certain limit Carbon monoxide is extremely toxic and fatal. A small amount of CO can cause suffocation (CO poisoning)
It is a non-flammable gas It is flammable
COdoes not undergo oxidation reactions
CO undergoes oxidation reactions
Carbon and oxygen share a covalent bond in CO2
Carbon and oxygen share covalent as well as a coordinate covalent bonds
Molecular mass of CO2 is 44 g/mol
Molecular mass of CO is 28 g/mol
Its bond length [C=O] is 116.3 pm Its bond length [C≡O] is 112.8 pm

Similarities between CO and CO2

Carbon monoxide and carbon dioxide, despite many differences, also have similarities:

  • Both CO2 and CO are odorless, colorless, and tasteless gases.
  • Both contain only carbon and oxygen atoms.
  • Elevated levels of both gases can cause health problems and even death.
  • Both are released during combustion.

Major differences between carbon dioxide and carbon monoxide

Some major differences between carbon dioxide and carbon monoxide have been discussed in this section in detail.


Both natural and human sources release carbon dioxide in the air. Natural sources include decomposition, oceanic release, and respiration. Human activities that lead to increased CO2 levels in the atmosphere are cement production, deforestation as well as the burning of natural gas and coal, etc.

There is no natural source of carbon monoxide as it is produced as a result of human activities which involve the combustion of fossil fuels. The greatest sources are cars, trucks, and other vehicles or machinery. A variety of items in our homes such as gas heaters, leaking chimneys, furnaces, and gas stoves also release CO into the air.


Carbon dioxide is used by plants in the process of photosynthesis to make food. This means increased carbon dioxide in our atmosphere can be controlled by plantations. The more the plants, the greater will be the intake of carbon dioxide.

CO2 is also used as dry ice (a refrigerant), in fire extinguishers, for inflating life rafts and life jackets, blasting coal, foaming rubber and plastics, promoting the growth of plants in greenhouses, and in carbonated beverages.

On the other hand, CO is primarily used in industrial processes, such as, in the production of large quantities of aldehydes in a process known as oxo-synthesis. Carbon monoxide is also used as a reducing agent to convert the naturally occurring oxide of metal to pure metal, For example, extraction of iron (Fe) from the blast furnace.


Thermal combustion is the most common source of both carbon dioxide and carbon monoxide.

Carbon dioxide is produced when methane burns in oxygen. The reaction is as follows:

CH4 + 2O2 → CO2 + 2H2O

It is also produced by the decomposition of limestone (CaCO3).

CaCO3 → CaO + CO2

Carbon dioxide is a by-product of the fermentation of sugar.

C6H12O6 → 2CO2 + 2C2H5OH

The major source of production of carbon monoxide for industrial use is producer gas which consists of CO and nitrogen gas in higher quantities. The producer gas is usually obtained from coal.

Another source of carbon monoxide is “water gas”. It is a mixture of hydrogen and carbon monoxide produced in a reaction between steam and carbon:

H2O + C → H2 + CO   (ΔH = +131 kJ/mol)

In the laboratory, a more convenient method is used to produce CO; by the dehydration of formic acid or oxalic acid in the presence of another acid, such as sulfuric acid.

Permissible exposure limit (PEL)

Carbon dioxide poisoning is rare as its higher concentration can be life-threatening. This can happen in a confined space.

OSHA (Occupational Safety and Health Administration) has set a CO2 permissible exposure limit (PEL) of 5,000 ppm over an eight-hour period and 30,000 ppm over a 10-minute period.

On the other hand, carbon monoxide is far more dangerous than carbon dioxide. The current OSHA permissible exposure limit for carbon monoxide is 50 ppm averaged over eight hours which is much lower than the PEL for carbon dioxide. The exposure level of CO considered immediately dangerous to life and health is 1500 ppm.

Key Takeaways

Carbon Monoxide vs. Carbon Dioxide: The Chemistry of CO and CO2

Additional Articles

Concepts Berg

What is the difference between carbon dioxide and carbon monoxide sensing?

The detection of the two gases differs because CO2 detectors commonly use infrared sensors, while CO detectors use electrochemical sensors.

How much carbon monoxide is dangerous?

The OSHA PEL for CO is 50 parts per million (ppm).

How to prevent CO poisoning in your home?

Proper ventilation is needed to avoid the accumulation of CO in homes. This is a must-to-do when using home appliances that use domestic gas.

Which is worse? carbon monoxide or carbon dioxide.

Carbon monoxide is more dangerous than carbon dioxide. A low level of CO2 is not harmful whereas CO is fatal even in small quantities.

Do cars emit carbon monoxide or carbon dioxide?

Mostly, cars emit carbon dioxide. However, combustion may not be perfect every time. Therefore, carbon monoxide emission is also there.

Does CO or CO2 support combustion?

Carbon dioxide doesn’t support combustion and that’s why it is used in fire extinguishers. Carbon monoxide supports combustion and burns with a pale blue flame.

Why is carbon dioxide sometimes called a greenhouse gas?

Carbon dioxide, like other greenhouse gases, absorbs heat radiations coming from the sun. This results in the rise in temperature in the earth’s atmosphere also known as the greenhouse effect.