CO and CO2 – What’s the difference?

carbon monoxide and carbon dioxide gas cylinders

Carbon monoxide and carbon dioxide are two similarly sounding gases with different properties. So what's the difference?

The differences between carbon monoxide (CO) and carbon dioxide (CO2) are important, but the gases are often confused. While they may sound the same they are completely different gases with different sources, chemical properties and dangers.

The media often adds to this confusion because of their inability to properly identify the two gases. Countless stories are written about CO dangers from CO2 leaks. Other stories are written about the dangers of CO2 and global climate change. A search online for “CO2 detector” will provide results for “CO detector.” This confusion leads some to assume the gases are both equally bad and dangerous. They are not.

That’s why it is helpful to understand the similarities and differences between CO and CO2.

Table of Contents

CO and CO2 – What’s the Same?
CO and CO2 – What’s the Difference?
About Carbon Monoxide
CO Facts
CO Recommended Limits
About Carbon Dioxide
CO2 Facts
CO2 Recommended Limits
Parts per Million vs. Percentage Gas


CO and CO2 – What’s the same?

  • Both are made from carbon and oxygen molecules.
  • Both are colorless, tasteless and odorless gases
  • Both are in the air we breath (albeit in different concentrations)
  • Both are released during combustion or fire
  • Both are important industrial gases
  • Both are potentially deadly

While they both have the word "carbon" in their name, -monoxide (mono in Greek means 1) refers to the bond between a single carbon molecule and a single oxygen molecule while -dioxide (di in Greek means 2) refers to the bond between a single carbon molecule and two oxygen molecules, (oxide means a simple compound of oxygen). In other words, CO is C+O while CO2 is O+C+O.

Both carbon dioxide and carbon monoxide are colorless, odorless and tasteless gases. However, some describe the odor of high levels of CO2 as “acidic” or “bitter.”

While both CO and CO2 are potentially deadly, this happens at vastly different concentrations. While 35 ppm (0.4%) of CO is quickly life threatening, it takes more than 30,000 ppm (3%) of CO2 to reach the same risk level.

Compressed carbon dioxide and carbon monoxide are both important industrial gases. For example, CO2 is used to carbonate beverages and to increase plant growth in indoor greenhouses. CO is used during the manufacturing of iron and nickel as well as the production of methanol. In spite of their molecular similarity, they both behave very differently when interacting with other molecules.

CO and CO2 – What’s the difference?

co co2 difference

The most important difference is that carbon dioxide is a common, naturally occurring gas required for plant and animal life. CO is not common. It is a byproduct of the burning of fossil fuels such as oil, coal, and gas. 

CO poisoning occurs when carbon monoxide builds up in your bloodstream. Your body replaces the oxygen in your red blood cells with carbon monoxide. leading to serious tissue damage. CO2 poisoning occurs when the lungs cannot take in enough oxygen.

CO2 does not undergo oxidation reactions and is a non-flammable gas. CO undergoes oxidation reactions and is therefore a flammable gas.

CO2 has a molar mass of about 44g/mol. CO2 has a molar mass of about 28g/mol.

About Carbon Monoxide

The most dangerous gas most of us will ever face is carbon monoxide (CO). Not normally occurring in nature, this gas is the result of oxygen-starved combustion from improperly ventilated fuel-burning motors and appliances like:

  • Oil and gas furnaces
  • Gas water heaters or gas ovens
  • Gas or kerosene space heaters
  • Fire places and wood stoves
  • Portable generators
  • Older autos without catalytic converters 

Too much carbon monoxide in an unventilated space is deadly. In fact, carbon monoxide poisoning is the most common type of fatal poisoning worldwide. This is why many new homes are built with CO detectors in addition to smoke detectors. 

For protection against CO poisoning CO2Meter offers CO safety Monitors.

CO Facts

  • CO is almost entirely a man-made gas that is not normally found in the earth's atmosphere. 
  • CO is produced at dangerous levels by oxygen-starved combustion in improperly ventilated fuel-burning appliances such as generators, oil and gas furnaces, gas water heaters, gas ovens, gas or kerosene space heaters, fireplaces, and stoves
  • The highest CO emissions are produced by internal combustion engines without a catalytic converter.
  • CO can be a flammable gas in higher concentrations (sometimes referred to as C1D1 or C2D2 environments). Devices to measure carbon monoxide in these concentrations are often designed to be explosion-proof.
  • CO is the most common type of fatal poisoning in the world.

CO Recommended Limits

  • Symptoms of mild CO poisoning include headaches, dizziness, and violent vomiting at concentrations less than 100 ppm
  • 0.1 ppm is the current average CO level on the planet
  • 9-50 ppm is the standard maximum limit for an 8-hour workday.
  • 200-400ppm will result in physical symptoms followed by unconsciousness and death within hours.
  • Concentrations above 800 ppm can be life-threatening in minutes.

About Carbon Dioxide

Carbon dioxide (CO2) is colorless, odorless and tasteless. It is a naturally occurring gas in earths atmosphere naturally produced by the decomposition of plant and animal life as well as respiration, which takes in oxygen and exhales CO2. Plants and trees depend on CO2 for life (they take in CO2 and give out oxygen). 

While not as deadly as carbon monoxide, carbon dioxide can affect your health both directly and indirectly. The direct effect is simple: too much carbon dioxide in an enclosed space – for example, in a submarine – can suffocate you long before the oxygen runs out.

Think this can’t happen to you? Actually, dozens of people die every year as the result of leaky CO2 storage tanks attached to soda machines in bars and restaurants or in unventilated keg coolers when a beer line is left open. Others die in dry ice (frozen carbon dioxide) storage lockers used for temporary food storage.

For protection from CO2 in enclosed spaces, GasLab offers CO2 safety Alarms.

CO2 Facts

  • CO2 is a common gas in the atmosphere and is required for plant life
  • CO2 is a natural byproduct of human and animal respiration, fermentation, chemical reactions, and the decomposition of plant and animal life.
  • In the atmosphere CO2 measures approximately 400 ppm (parts per million).
  • CO2 is non-flammable, with no explosive properties
  • CO2 poisoning is rare; however scuba divers have to watch out for it (the bends)
  • Leaking pressurized CO2 tanks in enclosed areas can be dangerous for occupants - both from high levels of CO2 and from lower levels of oxygen (O2 displacement / Asphyxiation).

CO2 Recommended Limits

  • 410 ppm is the current average CO2 level on the planet
  • ASHRAE recommends a 1,000 ppm limit for office buildings and classrooms to ensure overall health and performance
  • OSHA limits workplace exposure levels to 5,000 ppm time-weighted average (over 8 hours)
  • Drowsiness can occur at 10,000 ppm (1%) – common in closed cars or auditoriums
  • Symptoms of mild CO2 poisoning include headaches and dizziness at concentrations less than 30,000 ppm (3%)
  • At 40,000 ppm (4%) CO2 can be life-threatening

Understanding PPM - parts per million

While large gas concentrations in a volume of air are measured in percentages, small volumes are measured in parts-per-million or parts per million (ppm) by volume (ppmv). 

When measuring small volumes, the range of concentrations is from 0 to 1,000,000, which equals 0-100%. Every 10,000 ppm equals 1% concentration.  For example, instead of saying "1% gas by volume," scientists will say "10,000 ppm." This is because 10,000 / 1,000,000 = 1%.

Why use ppm? This is because it is easier to write that the CO2 level in a room has risen from 400 ppm to 859 ppm than to explain that the CO2 level has risen from 0.04% to 0.0859%. However, both are correct. Conversely, when measuring gases above 10,000ppm it is simpler to write 1%.

Read more about parts-per-million here.


In conclusion, regardless of what industry you work in, leaks and overexposure to both gases can occur around you each and every day. Recently publicized fatalities involving both CO2 and CO have refocused attention on the need to accurately and effectively detect and monitor for the presence of gases.

Understanding the gases and being able to prevent potential injuries and hazards from occurring is the best preventive first step you can take.

For additional information on CO or CO2 solutions, contact our technical sales team. We will be happy to assist you and help educate you on the difference between the gases, what makes them hazardous and what devices can better assist in eliminating potential injuries from occurring.

For more information, speak to a CO2Meter specialist at or (877) 678-4259.

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