Carbon monoxide (CO) comes from the incomplete burning of anything that contains carbon. When a fuel like gas, wood, oil, or charcoal burns without enough oxygen to fully combust, it produces CO instead of carbon dioxide (CO2). This colorless, odorless gas shows up in homes, vehicles, workplaces, and outdoor air from a wide range of sources.
How Carbon Monoxide Forms
Complete combustion converts carbon in fuel into carbon dioxide and water. But when the oxygen supply is restricted, the carbon doesn’t fully oxidize. Instead of CO2, you get CO. This is why enclosed or poorly ventilated spaces are particularly dangerous: the limited airflow starves the flame of oxygen and shifts the chemical reaction toward CO production.
Any material containing carbon can produce CO under these conditions. Natural gas, propane, gasoline, diesel, wood, charcoal, and coal all generate it when they burn incompletely. The less oxygen available relative to the fuel, the more CO is produced.
Household Sources
Most residential CO comes from fuel-burning appliances. Furnaces, gas ranges, water heaters, room heaters, and fireplaces all produce some CO during normal operation. When these appliances malfunction or aren’t properly vented, CO can accumulate indoors to dangerous levels. A cracked heat exchanger in a furnace or a blocked chimney flue are classic examples.
Charcoal grills and portable generators are especially hazardous when used in enclosed areas because they produce large volumes of CO and are often brought into garages, basements, or tents where ventilation is poor. Gas ovens and clothes dryers should never be used to heat a home, since they aren’t designed to safely vent combustion gases into living spaces for extended periods.
Vehicles and Engines
Internal combustion engines are major CO producers. Cars, trucks, lawn mowers, power washers, and boats all generate it in their exhaust. A running car in a closed garage can raise CO to lethal levels within minutes. Even outdoors, idling vehicles near open windows or intake vents can push CO into a building.
Catalytic converters in modern vehicles reduce CO output significantly, but older cars, poorly maintained engines, and small gas-powered equipment like generators and chainsaws still release substantial amounts.
Industrial and Commercial Sources
Industry produces CO both as a byproduct and on purpose. Steel manufacturing, fuel combustion in boilers and incinerators, and highway vehicle exhaust are major contributors to ambient CO levels. Power plants burning coal or natural gas release it as well.
On the intentional side, CO is manufactured at industrial scale through the partial oxidation of natural gas or the gasification of coal and coke. Most of it is used immediately on-site for chemical synthesis, including the production of acetic acid, polycarbonates, and synthetic fuels. It also serves as a reducing agent in metal refining, including nickel recovery. Because so much CO is consumed at the point of production, exact global volumes are hard to pin down.
Why Carbon Monoxide Is Dangerous
CO is harmful because it hijacks the way your blood carries oxygen. When you inhale it, CO crosses into your bloodstream and binds to hemoglobin, the protein in red blood cells that normally transports oxygen. The problem is that CO binds to hemoglobin with about 200 times the affinity of oxygen, so even small amounts can lock onto a large share of your hemoglobin and prevent it from picking up oxygen. The result is that your tissues slowly starve of oxygen even though you’re still breathing.
Low-level exposure is notoriously hard to recognize. Symptoms include headache, fatigue, difficulty concentrating, nausea, and flu-like illness. Because these are so nonspecific, many people don’t connect them to CO until the exposure worsens or someone else in the household develops the same complaints. At higher concentrations, confusion, loss of consciousness, and death can follow.
How Much Is Too Much
The EPA’s national air quality standard limits CO to 9 parts per million (ppm) averaged over 8 hours, and 35 ppm over 1 hour. The World Health Organization sets similar thresholds: 9 ppm for 8 hours, 26 ppm for 1 hour, and up to 87 ppm for exposures lasting no more than 15 minutes. These limits are designed to protect the general population, including people with heart disease who are more vulnerable to oxygen deprivation.
For context, a well-functioning gas stove in a ventilated kitchen might produce 5 to 15 ppm of CO nearby. A car idling in a closed garage can push levels above 400 ppm in under 10 minutes.
Detecting CO in Your Home
Because you can’t see or smell carbon monoxide, a detector is the only reliable way to know it’s present. Install one on every floor of your home, placing it on a wall about 5 feet above the floor or on the ceiling. CO is slightly lighter than air and tends to rise with warm air, so this height catches it effectively. If you’re starting with a single detector, put it near the bedrooms and make sure the alarm is loud enough to wake you.
Keep detectors away from fireplaces and flame-producing appliances, where brief, normal spikes in CO could trigger false alarms. Beyond detectors, the most effective prevention is annual inspection of all fuel-burning appliances and never running generators, grills, or vehicles in enclosed spaces.