What is the difference between CO2 and carbon monoxide?

By Daniel Clark | Published: October 29, 2019

Carbon dioxide (CO2) and carbon monoxide (CO) are mentioned often, and sometimes get confused with one another due to their similar names. This is an understandable conflation, but they are two completely difference substances. Nevertheless, there are a few things they have in common. Both are:

  • Combustion byproducts
  • Common compounds in the atmosphere
  • Tasteless and odorless
  • Deadly under the right circumstances

The confusing similarity in their names is a consequence of the naming conventions in chemistry. Chemically, monoxide has one oxygen atom compared with two in a dioxide molecule, giving the naming distinction. But that doesn't mean they're basically the same thing. In regards to safety, there are several important differences. Carbon dioxide, for starters, is far more common and in vastly greater amounts than monoxide. Monoxide is far more toxic but less common. Normal air contains about 0.04% carbon dioxide and only traces of CO; concentrations that pose no direct hazard to life under normal circumstances. In fact, CO2 is a necessary part of the process of photosynthesis in plants without which they couldn't survive.

Carbon Monoxide (CO) has the ominous moniker of “The Silent Killer." In concentrations greater than the occupational exposure limit of 25ppm, it has deleterious central nervous system effects and can be fatal to humans. It has no smell, no color and no taste, and can exert its effects with little warning, potentially causing harm even where concentration of oxygen remains a

t the normal 20.9%. In still air, CO will tend to rise, as it is slightly lighter than the surrounding air, however their closeness in density means that any turbulence at all causes it to mix.

(Learn more in Carbon Monoxide: The Silent Killer.)

The deadly mechanism of CO is to bond to hemoglobin (the oxygen transporting protein in red blood cells) following exposure, forming carboxyhemoglobin and preventing the normal binding of oxygen. If the exposure is sufficiently high, there will be no hemoglobin available for oxygen to bind to, and the exposed person's cells will die as a result. This is a process called chemical asphyxiation.

(Learn 3 Ways to Manage Carbon Monoxide Risks at Loading Docks.)

Not to say carbon dioxide is benign, but in terms of toxicity it has nothing on its cousin. CO2 has an IDLH (Immediately Dangerous to Life and Health) concentration at 40,000 ppm with a short term exposure limit of 30,000 ppm. Contrast that with monoxide's ceiling limit of 200 ppm and you start to get an idea of the relative, acute hazard each presents. CO2 is far less toxic than CO and yet it deserves consideration for the simple fact that it is abundant, so much so that under some conditions it can displace the breathable air in a confined space posing a “simple asphyxiation” hazard. As CO2 is about 60% heavier than air, it will tend to sink, meaning it particularly collects in low lying areas and underground spaces that are poorly ventilated.

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Written by Daniel Clark | Safety and Quality Management System Specialist

Daniel Clark

Daniel Clark is the founder and President of Clark Health and Safety Ltd., providing safety and quality consultation across various industries in Calgary, Alberta. Daniel has a Bachelor of Science degree, certification in health and safety, certificates in both CAD design and CNC, auditing certifications and the designation of Canadian Registered Safety Professional. Being raised and practicing in Calgary, the heart of Canada’s energy industry, most of Daniel’s career has been energy related. He has performed safety and quality roles from field supervision to office-based administration and management. Daniel’s consulting business has worked with organizations offering engineering services, restoration, pipeline, environmental, manufacturing and food service.

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