Carbon monoxide comes from the incomplete burning of anything that contains carbon. When a fuel like wood, gas, charcoal, or oil burns without enough oxygen, the carbon in that fuel only partially oxidizes, producing carbon monoxide (CO) instead of the relatively harmless carbon dioxide (CO₂). This happens in engines, furnaces, fireplaces, grills, wildfires, and even certain chemical reactions in the atmosphere. Understanding the specific sources helps you identify risks in your own home and surroundings.
Why Incomplete Combustion Produces CO
Every carbon-based fuel, whether it’s natural gas, gasoline, wood, or coal, is meant to combine with oxygen when it burns. If there’s plenty of oxygen available, the carbon atoms fully bond with oxygen to form CO₂ and water vapor. That’s complete combustion. But when the oxygen supply is restricted, or when the flame gets cooled too quickly by a nearby surface, the carbon atoms only pick up one oxygen atom instead of two. The result is carbon monoxide.
Several conditions push combustion toward producing CO: a cramped or poorly ventilated space that limits airflow, a clogged burner or dirty filter that chokes the flame, or simply running a fuel-burning device in an enclosed area. The chemistry is straightforward. Less oxygen means more CO. This is why virtually every source of carbon monoxide traces back to something burning in less-than-ideal conditions.
Common Household Sources
Inside your home, the most common CO sources are fuel-burning appliances. Furnaces, gas water heaters, gas stoves and ovens, wood-burning fireplaces, kerosene space heaters, and clothes dryers that run on gas all produce some carbon monoxide during normal operation. When they’re properly vented and maintained, the CO exits your home through flues and exhaust systems. Problems start when those systems malfunction, get blocked, or are used in ways they weren’t designed for.
The Consumer Product Safety Commission reports that more than 200 people in the United States die every year from CO produced by non-automotive consumer products, including malfunctioning furnaces, ranges, water heaters, room heaters, portable generators, and fireplaces. One overlooked risk: covering the bottom of a gas oven with aluminum foil blocks the combustion airflow and can cause CO to build up. Using a gas stove, oven, or clothes dryer to heat your home is another common and dangerous mistake, since these appliances aren’t designed to run continuously in that role and aren’t vented to handle it.
Portable Generators and Engines
Gasoline-powered portable generators are one of the deadliest CO sources during power outages. They produce high concentrations of carbon monoxide and are often placed too close to homes, in garages, or even indoors. A running generator can flood an enclosed space with lethal CO levels within minutes.
All internal combustion engines produce carbon monoxide, though the amount varies by fuel type. Gasoline engines generally emit the most CO, followed by propane and diesel engines. A gasoline-powered forklift operating indoors, for instance, requires roughly 16,000 cubic feet per minute of ventilation to keep CO at safe levels. Diesel-powered vehicles require less ventilation per unit of power but still produce meaningful amounts. Cars idling in attached garages, even with the garage door partially open, can push dangerous levels of CO into the home.
Charcoal, Grills, and Camping Stoves
Burning charcoal is an especially potent CO source. Research measuring CO generation from burning charcoal found it produces 137 to 185 milliliters of carbon monoxide per minute per kilowatt of heat output. To prevent poisoning, you’d need roughly 41 to 56 cubic meters of fresh air per hour for every kilowatt the charcoal is putting out. Most rooms in a house don’t come close to that level of ventilation. This is why using a charcoal grill, hibachi, or charcoal-burning camping stove indoors, in a tent, or in a garage is extremely dangerous, even with windows cracked.
Vehicles and Traffic
Motor vehicles are one of the largest overall sources of carbon monoxide. Exhaust from cars, trucks, buses, and motorcycles releases CO directly into the air, particularly during cold starts when engines run richer (meaning they use more fuel relative to oxygen). Urban areas with heavy traffic tend to have higher ambient CO levels, especially in tunnels, parking garages, and along congested roadways. Catalytic converters in modern vehicles have dramatically reduced per-vehicle emissions compared to older models, but the sheer number of vehicles on the road keeps transportation as a major contributor.
Industrial and Commercial Sources
Heavy industry produces large volumes of carbon monoxide. Steel production using blast furnaces generates CO as a byproduct of converting iron ore. Coal gasification, steam reforming of natural gas, and partial oxidation of hydrocarbons are all carbon-intensive industrial processes that release significant CO. Power plants burning fossil fuels and waste incinerators also contribute. In the chemical sector, CO is actually a valuable raw material used to manufacture acetic acid, formic acid, and precursors for plastics like polyurethane and polycarbonate.
Natural and Outdoor Sources
Not all carbon monoxide comes from human activity. Wildfires are a major natural source, burning vast stretches of vegetation under oxygen-limited conditions and releasing CO into the atmosphere on a massive scale. Volcanic eruptions also emit carbon monoxide along with other gases. Beyond direct combustion, CO forms through photochemical reactions in the atmosphere when sunlight breaks down methane and other volatile organic compounds. Organic molecules in surface waters and soils contribute to this atmospheric production as well. These natural sources mean that some baseline level of CO always exists in outdoor air, though concentrations are typically very low in areas far from human activity.
Safe Exposure Limits
The EPA’s national air quality standard sets a limit of 9 parts per million (ppm) averaged over eight hours and 35 ppm averaged over one hour. The World Health Organization uses similar thresholds: 9 ppm over eight hours and 26 ppm over one hour. These limits are designed to protect the general public, including people with heart disease who are more vulnerable to CO’s effects. For context, a well-functioning gas appliance in a properly ventilated home typically produces CO levels well below these thresholds, while a charcoal grill burning indoors can blow past them in minutes.
Where to Place CO Detectors
Because carbon monoxide is slightly lighter than air and tends to rise with warm air from combustion sources, detectors should be placed on a wall about five feet above the floor. Ceiling placement also works. Every floor of your home needs its own detector. If you’re only getting one, put it near the bedrooms and make sure the alarm is loud enough to wake you up. Don’t place a detector directly next to or above a fireplace or any flame-producing appliance, as this can cause false readings.
Detectors are particularly important if your home has any gas appliances, a fireplace, or an attached garage. CO is colorless and odorless, so without a detector, dangerous concentrations can build up long before anyone notices symptoms like headache, dizziness, or nausea.