Carbon monoxide is released whenever carbon-based fuels burn without enough oxygen to fully combust. This process, called incomplete combustion, produces CO instead of carbon dioxide. It happens in engines, furnaces, grills, wildfires, industrial plants, and even inside your own body. Understanding the specific sources helps explain why this colorless, odorless gas shows up in so many poisoning cases each year.
Why Incomplete Combustion Produces CO
Any material containing carbon (gasoline, wood, natural gas, coal, propane, charcoal) can release carbon monoxide. The key factor is oxygen supply. When plenty of oxygen is available and temperatures are high enough, carbon atoms bond with two oxygen atoms to form carbon dioxide (CO₂), a relatively harmless gas at normal concentrations. When oxygen is limited, temperatures are low, or the burn time is too short, carbon atoms pick up only one oxygen atom, forming carbon monoxide (CO).
This is why a well-tuned gas furnace with proper ventilation produces very little CO, while the same furnace with a cracked heat exchanger or blocked flue can fill a home with dangerous levels. The chemistry is simple: starve a fire of air, and it shifts from producing CO₂ to producing CO.
Household Appliances and Heating Equipment
The most common indoor sources of carbon monoxide are fuel-burning appliances that are either malfunctioning, poorly maintained, or inadequately vented. According to the American Lung Association, these include gas furnaces, gas ranges and ovens, gas water heaters, gas clothes dryers, fireplaces, wood stoves, coal and oil furnaces, space heaters (oil or kerosene), and gas-powered tools like lawn mowers. Any of these can release CO into living spaces if their exhaust systems are blocked, their burners are dirty, or they’re operated in enclosed areas without ventilation.
A properly functioning gas stove in a ventilated kitchen produces trace amounts of CO that dissipate quickly. The danger comes from accumulation: a furnace with a cracked heat exchanger running overnight, a fireplace with a closed damper, or a space heater in a sealed bedroom. Because CO has no smell or color, levels can climb for hours before anyone notices symptoms.
Vehicles and Engines
Internal combustion engines are among the most potent sources of carbon monoxide. Gasoline engines produce CO as a routine byproduct of burning fuel, and the concentrations in raw exhaust are staggering. NIOSH testing of boat engines found that a standard gasoline engine without a catalytic converter produced exhaust concentrations between 10,000 and 14,000 ppm of CO at idle. Even with a catalytic converter, exhaust readings reached around 800 ppm once the engine warmed up.
To put those numbers in perspective, OSHA data shows that 1,600 ppm causes confusion, staggering, and death in under two hours. At 12,800 ppm, a person can lose consciousness after two or three breaths and die within minutes. This is why running a car, generator, or any gasoline engine in a garage or enclosed space is so dangerous. Even partially enclosed areas are risky. NIOSH measurements found CO levels of 500 to 1,000 ppm at the back of idling boats, and concentrations exceeded the “immediately dangerous to life” threshold of 1,200 ppm at slow speeds.
Charcoal and Grills
Charcoal is a particularly deceptive source of carbon monoxide because it continues releasing large quantities of CO long after visible flames have gone out. Glowing embers look harmless, but testing by the German Federal Institute for Risk Assessment found that just 800 grams of charcoal (less than two pounds) produced CO concentrations above 3,000 ppm in an enclosed room within two hours of the coals beginning to glow. At that level, a person would experience headache, dizziness, and nausea within minutes and could lose consciousness in 10 to 15 minutes.
The risk compounds over time because CO doesn’t break down quickly indoors. Each breath adds more CO to the bloodstream on top of what’s already been absorbed. This is why bringing a charcoal grill, hibachi, or camp stove indoors for warmth or cooking, something that happens most often during power outages, is one of the leading causes of mass CO poisoning events.
Industrial and Commercial Processes
On a larger scale, several major industries produce carbon monoxide as a byproduct or even intentionally as a chemical building block. Coal gasification, steam reforming of natural gas, and partial oxidation of hydrocarbons all generate CO. Steel manufacturing is a significant source, as the blast furnace process relies on carbon reacting with iron ore at high temperatures, producing CO in the process.
CO is also a valuable industrial feedstock. Chemical manufacturers use it to produce acetic acid, formic acid, and precursors for plastics like polyurethane and polycarbonate. In these settings, the gas is contained and handled with strict safety protocols, but leaks and confined-space exposures remain a serious occupational hazard.
Natural and Biological Sources
Wildfires, volcanic eruptions, and natural gas seeps all release carbon monoxide into the atmosphere. Forest fires can produce enough CO to measurably raise concentrations across entire regions, and satellite instruments routinely track these plumes.
Your own body also produces small amounts of CO. When red blood cells reach the end of their roughly 120-day lifespan, your body breaks down the heme molecule (the iron-containing part of hemoglobin) using an enzyme called heme oxygenase. This process releases tiny quantities of carbon monoxide along with iron and a pigment that eventually becomes bilirubin. Far from being just metabolic waste, this internally produced CO plays a role in regulating blood vessel tone, blood flow to the liver, and certain signaling processes in the brain. Researchers identified a neurotransmitter function for this endogenous CO as far back as 1993. The amounts are minuscule and pose no health risk.
CO Levels and What They Do to You
The health effects of carbon monoxide depend on concentration and exposure time. OSHA’s data provides a clear picture of the progression:
- 35 ppm: Headache and dizziness after 6 to 8 hours of constant exposure.
- 100 ppm: Slight headache within 2 to 3 hours.
- 200 ppm: Headache, impaired judgment, and irritability within 2 to 3 hours.
- 400 ppm: Frontal headache and nausea within 1 to 2 hours. Life-threatening after 3 hours.
- 800 ppm: Dizziness, nausea, and convulsions within 45 minutes. Possible death within 2 hours.
- 1,600 ppm: Confusion, staggering, and death in under 2 hours.
- 3,200 ppm: Unconsciousness in 10 to 15 minutes. Death within 30 minutes.
- 12,800 ppm: Unconsciousness after 2 to 3 breaths. Death in under 3 minutes.
CO poisons by binding to hemoglobin in red blood cells about 200 times more readily than oxygen does. Once attached, it blocks oxygen from reaching tissues. The early symptoms (headache, dizziness, nausea) mimic the flu, which is one reason CO poisoning is so often missed. At high concentrations, people lose consciousness without warning.
What CO Detectors Actually Detect
Most household carbon monoxide alarms are designed to alert you to sustained, dangerous exposures rather than brief, low-level spikes. Detectors certified under UL 2034 standards will not sound an alarm at concentrations below 70 ppm, and they won’t trigger at 30 ppm even after 30 continuous days. Most units alarm somewhere in the 30 to 70 ppm range depending on how long the level persists.
This means your detector is calibrated to catch the kinds of slow, steady leaks that come from a failing furnace or a blocked chimney. It won’t necessarily catch a brief puff of CO from a gas stove or a car starting in the garage for a few seconds. Placing detectors on every level of your home, particularly near bedrooms and attached garages, gives you the best coverage for the sources most likely to cause sustained indoor buildup.