Carbon monoxide (CO) forms whenever something burns without enough oxygen to complete the reaction. Instead of carbon fully combining with oxygen to make carbon dioxide (CO₂), the limited oxygen supply leaves carbon only partially oxidized, producing CO, a colorless, odorless, and poisonous gas. This process, called incomplete combustion, is by far the most common way CO enters the world, but it’s not the only one. Your own body produces small amounts of it, and industry manufactures it on purpose.
Incomplete Combustion: The Core Chemistry
Any fuel that contains carbon, whether it’s natural gas, gasoline, wood, or coal, can produce CO when it burns. In a perfect combustion reaction, every carbon atom pairs with two oxygen atoms to form CO₂. But combustion is rarely perfect. When the oxygen supply runs short, hydrogen atoms in the fuel grab oxygen first, and the carbon atoms get whatever is left over. The result is CO instead of CO₂.
Several conditions push combustion toward CO production: a restricted air supply, low flame temperatures, short burn times that don’t let the reaction finish, and poor mixing between fuel and air. Any of these can leave carbon stranded with only one oxygen atom attached.
Common Household Sources
Inside a home, CO typically comes from appliances and equipment that burn fuel. The U.S. Environmental Protection Agency identifies these as the main indoor sources:
- Gas stoves and ovens, especially older models or those with misadjusted burners
- Furnaces and boilers, particularly when worn, poorly maintained, or connected to blocked or leaking flues
- Gas water heaters that back-draft exhaust into the living space
- Unvented kerosene and gas space heaters, which release combustion byproducts directly into the room
- Wood stoves and fireplaces with poor draft or leaking chimneys
- Generators and gasoline-powered equipment run indoors or in attached garages
- Car exhaust from vehicles idling in attached garages
- Tobacco smoke
The common thread is a combustion device that either lacks enough ventilation or isn’t functioning properly. A furnace with a cracked heat exchanger, a chimney blocked by debris, or a gas range burning in a small kitchen with no exhaust fan can all push indoor CO levels into dangerous territory.
Vehicle Engines and CO
Internal combustion engines are one of the largest sources of CO worldwide. Inside an engine cylinder, fuel and air mix and ignite in fractions of a second. That speed means combustion is almost never complete, and some carbon exits the exhaust as CO rather than CO₂.
The amount of CO an engine produces depends heavily on the air-to-fuel ratio. When the mixture is “rich” (more fuel relative to air), there isn’t enough oxygen to finish the job, and CO levels spike. At higher loads, the excess air ratio drops and CO can climb. Temperature also plays a role: cooler combustion generally produces more CO because the reaction doesn’t proceed as completely. Modern vehicles use catalytic converters to oxidize CO into CO₂ in the exhaust stream, which dramatically cuts tailpipe emissions. Older vehicles or those with failing catalytic converters release far more.
Wildfires and Other Large-Scale Combustion
Wildfires are a massive natural source of CO. Burning vegetation rarely has ideal airflow, so huge volumes of carbon monoxide pour into the atmosphere alongside smoke and particulates. Power plants, incinerators, and industrial boilers contribute as well, though modern pollution controls reduce their output significantly compared to uncontrolled burning.
CO also forms through chemical reactions in the atmosphere itself. Sunlight drives photochemical reactions that break down methane and other volatile organic compounds, producing CO as a byproduct. Organic molecules in surface water and soil undergo similar transformations. These atmospheric and environmental pathways are slower and more diffuse than combustion, but they contribute meaningfully to the global CO budget.
How Your Body Makes CO
Your body produces small amounts of carbon monoxide as part of normal metabolism. When red blood cells reach the end of their roughly 120-day lifespan, they’re broken down, primarily in the spleen. An enzyme called heme oxygenase splits the heme molecule (the iron-containing part of hemoglobin) into three products: a bile pigment called biliverdin, free iron, and carbon monoxide.
This isn’t a malfunction. The CO produced internally appears to serve signaling roles in the body, similar to nitric oxide. The quantities are tiny compared to what you’d inhale from a malfunctioning furnace, but it means CO is always present in your blood at trace levels. Most tissues express the enzyme at low levels under normal conditions, with the spleen producing the most because that’s where old red blood cells are recycled.
Industrial CO Production on Purpose
Industry doesn’t just produce CO as waste. It deliberately manufactures carbon monoxide as a key ingredient in chemical production. The primary method is steam reforming of natural gas (methane), a process that has been used commercially since the early 1930s. At temperatures between 800°C and 1,000°C and pressures of 20 to 40 bar, methane reacts with steam to produce synthesis gas, or “syngas,” a mixture of hydrogen and carbon monoxide.
Syngas is the starting material for manufacturing a wide range of chemicals, including methanol, acetic acid, and synthetic fuels. Producers can adjust the hydrogen-to-CO ratio depending on what they need. An alternative approach, partial oxidation of methane, reacts methane with a limited supply of pure oxygen to yield syngas with a different composition. Coal can also serve as the feedstock, though building a coal-based syngas plant costs roughly three times more than a natural gas-based facility. Biomass, heavy oil, and naphtha are other possible starting materials, making syngas production flexible across different fuel sources.
Why the Same Reaction Produces CO or CO₂
Whether combustion yields CO or CO₂ comes down to oxygen availability. Picture a campfire: the center of a dense log pile, where airflow is poor, generates more CO. The outer edges, where flames have free access to air, burn more completely and produce mostly CO₂. Scale that up to a furnace, an engine, or a wildfire, and the same principle applies. Anything that restricts oxygen, shortens reaction time, or lowers temperature tips the balance toward CO.
This is why proper ventilation and equipment maintenance matter so much for safety. A well-tuned gas furnace with an open flue produces very little CO. The same furnace with a cracked heat exchanger and a partially blocked chimney can fill a home with lethal concentrations overnight. The chemistry hasn’t changed. Only the oxygen supply has.