What is the difference between carbon monoxide and carbon dioxide?

When referring to carbon monoxide (CO) and carbon dioxide (CO2), people often confuse or interchange the two. For the most part, people are aware that they are two different gases, but which one is the good one and which is the bad one, or is it even correct to classify them that way? Before getting into how and where carbon monoxide and carbon dioxide affect people and the environment and how to test for them, let’s get a basic understanding of where they come from.

Both gases are a combination of carbon and oxygen.  When combustion of carbon is complete, which requires the presence of sufficient oxygen (plenty of air) the result is mainly carbon dioxide. Keep in mind that combustion refers to “the chemical combination of a substance with oxygen” and does not always, but sometimes, involve fire and flames. When combustion of carbon is incomplete, which happens when there is a limited supply of air, only half as much oxygen adds to the carbon (one oxygen atom, CO versus two oxygen atoms, CO2) and instead carbon monoxide is formed.

Where does carbon dioxide come from?

carbon dioxide in the workplace

Carbon dioxide is produced during some fermentation processes such as wine making.

Natural sources account for the majority of CO2 released into the atmosphere. These include oceans, animal (and human) and plant respiration, decomposition of organic matter, forest fires, and emissions from volcanic eruptions. [i]  While the lesser of CO2 emissions are anthropogenic (stemming from human activity), “87% of all human-produced carbon dioxide emissions come from the burning of fossil fuels like coal, natural gas and oil”. [ii]

Burning of these fuels releases energy that is turned into heat, electricity, or power for transportation. The remainder of these emissions results from “clearing of forests and other land use changes, as well as some industrial processes such as cement manufacturing”. [iii] Once CO2, a largely non-reactive gas, is released, it is rapidly mixed throughout the atmosphere.

Where does carbon monoxide come from?

Carbon monoxide, unlike carbon dioxide, does not occur naturally in the atmosphere. The “incomplete” combustion of coal, natural gas, and oil is a known environmental source of carbon monoxide. Low levels of oxygen and low temperatures result in the formation of higher percentages of CO in the combustion mixture.

CO2 and CO Production and Emissions at Home and in the Workplace

CO2 – Industrial plants that produce hydrogen or ammonia from natural gas, coal, or large-volume fermentation operations where plant products are made into ethanol, are some of the largest commercial producers of carbon dioxide. Carbon dioxide has many applications in the food and beverages industry, including carbonating drinks and conserving wine. In agriculture CO2 can be used to enhance plant growth, with differing opinions on its advantages and disadvantages. In solid form, CO2 is what we know as “dry ice” and is commonly used for transportation of frozen or chilled foods and medical or pharmaceutical materials.

CO – While carbon monoxide is most commonly an unwanted by-product, packaged carbon monoxide is used in a variety of industries including metal fabrication, in the manufacture of chemicals, for the reduction of ores in the manufacture of metal carbonyls, in the pharmaceutical industry and in electronic and semiconductor applications.

carbon monoxide firefighter

Firefighters are often called in because of carbon monoxide in the home.

Dangerous levels of carbon monoxide can be produced around your home or office from any fuel burning appliance including gas furnaces (non-electric), gas stoves, gas dryers, gas water heaters, fireplaces and automobiles.

In industrial settings, the internal combustion engine is the chief source of workplace exposure to carbon monoxide. Many furnaces and ovens also produce large amounts of the gas, especially when they are not properly maintained. Truck drivers, forklift operators, or people working near this type of equipment run the hazard of exposure. Workers near or within enclosed areas such as manholes, garages, tunnels, loading docks, warehouses, vehicle repair shops, and splicing vehicles are also at risk.

Health problems associated with carbon monoxide and carbon dioxide

CO2 – While carbon dioxide poisoning is rare, a high concentration of it in a confined space can be toxic. Excess carbon dioxide uses up space in the air instead of oxygen creating an environment for asphyxiation. Symptoms of mild CO2 poisoning include headaches and dizziness at concentrations less than 30,000 ppm. At 80,000 ppm CO2 can be life-threatening.  As a are reference , OSHA (Occupational Safety and Health Administration) has set a CO2 exposure limit of 5,000ppm over an eight-hour period and 30,000 ppm over a 10-minute period.

CO – The National Conference of State Legislatures (NCSL) indicates that “over 10,000 people are poisoned by carbon monoxide needing medical treatment each year; and over 500 people in the U.S. die annually from carbon monoxide poisoning”.  Approximately half of the deaths from unintentional CO Poisonings result from the inhalation of smoke from fires. Other significant causes are vehicle exhausts and deaths in industrial or commercial settings.

Carbon monoxide will chemically bind to the parts of your blood that are supposed to pick up and carry oxygen molecules, so it chemically blocks your system from getting oxygen. High blood level concentrations of CO will prevent sufficient amounts of oxygen from reaching vital organs. The current OSHA permissible exposure limit (PEL) for carbon monoxide is 50 ppm averaged over eight hours. The CO exposure level considered as being immediately dangerous to life and health is 1500 ppm. Because carbon monoxide, also referred to as the ‘Silent Killer’, is colorless, odorless, tasteless, and non-irritating, the early signs of poisoning are difficult to detect.

Gas Detection

It may be worth mentioning that when it comes to choosing a gas detector in the workplace, a single-gas carbon monoxide detector will not measure carbon dioxide levels, nor does it work the other way around. Not only are the sensors specific for each gas, but detector placement also plays a role. Carbon dioxide is heavier than air and gas detectors should be placed lower, near the ground, while carbon monoxide is slightly lighter than air and detectors can be placed higher up. There are a number of options when it comes to the best gas detector, including single-gas, multi-gas, portable and area monitors. The most important factor in choosing the right instrument is that you know and understand the environment and properties of the gas or gases for which it is intended.


[i] https://www.netl.doe.gov/research/coal/carbon-storage/carbon-storage-faqs/what-are-the-primary-sources-of-co2
[ii] https://whatsyourimpact.org/greenhouse-gases/carbon-dioxide-emissions
[iii] https://whatsyourimpact.org/greenhouse-gases/carbon-dioxide-emissions


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