Carbon monoxide (CO) is produced whenever fuel burns without enough oxygen to complete the combustion process. Instead of creating carbon dioxide (the relatively harmless gas you exhale), the reaction stalls partway, releasing carbon monoxide, a colorless, odorless gas that can poison or kill in enclosed spaces. Each year in the United States, more than 400 people die from unintentional CO poisoning unrelated to fires, over 100,000 visit an emergency department, and more than 14,000 are hospitalized.
How Carbon Monoxide Forms
Every fire needs fuel, heat, and oxygen. When oxygen supply is plentiful, burning a carbon-based fuel (natural gas, wood, gasoline, propane, charcoal) converts the carbon fully into carbon dioxide and water vapor. When the oxygen supply is restricted, the temperature is too low, or the fuel doesn’t spend enough time burning, the carbon only partially oxidizes. The result is carbon monoxide instead of carbon dioxide.
This is called incomplete combustion, and it happens more easily than most people realize. A well-tuned gas furnace produces very little CO. The same furnace with a dirty burner, cracked heat exchanger, or blocked exhaust vent can produce dangerous amounts. Any situation that starves a flame of air pushes the chemistry toward CO production.
Common Sources in the Home
According to the Consumer Product Safety Commission, any fuel-burning appliance in a home is a potential CO source. When these appliances are maintained and properly vented, they produce very little CO. Problems start when something malfunctions or when equipment is used in ways it wasn’t designed for. The most common residential sources include:
- Gas furnaces and boilers with cracked heat exchangers or blocked flues
- Water heaters that aren’t drafting exhaust properly
- Gas stoves and ovens, especially when used for heating a room
- Fireplaces with clogged or poorly maintained chimneys
- Portable space heaters burning kerosene or propane indoors
- Attached garages where a car, lawn mower, or generator idles
- Charcoal grills or camp stoves brought inside a home, tent, or garage
Tobacco smoke is another indoor source that often goes unmentioned. In homes with heavy smoking and poor ventilation, CO levels from cigarettes alone can become measurable.
Generators and Small Engines
Portable generators are one of the deadliest CO sources during power outages. Small internal combustion engines, including those in generators, pressure washers, and propane-powered forklifts, can emit dangerous or lethal concentrations of carbon monoxide when used in poorly ventilated spaces. A single generator running in a basement, crawlspace, or attached garage can fill a home with fatal levels of CO within minutes. The same applies to running a boat engine in an enclosed dock area or using a motorcycle indoors for any reason.
Why Poor Ventilation Makes It Deadly
Carbon monoxide by itself dissipates quickly outdoors. The danger comes when CO accumulates in an enclosed or semi-enclosed space faster than fresh air can dilute it. Specific conditions that allow buildup include clogged chimneys or exhaust vents, rooms sealed tightly for winter with a fuel-burning heater inside, garages connected to living spaces, and kitchens with unvented gas appliances.
The World Health Organization has noted that the highest non-accidental indoor CO levels occur in residential garages and in kitchens where cooking happens over open flames with no exhaust ventilation. Even a small, consistent leak from a faulty appliance can raise CO concentrations over hours to a level that causes chronic symptoms people may not connect to poisoning.
How CO Harms the Body
When you breathe in carbon monoxide, it crosses from your lungs into your bloodstream almost instantly. There it latches onto hemoglobin, the protein in red blood cells that normally carries oxygen, with an affinity roughly 200 times stronger than oxygen’s. This forms a compound called carboxyhemoglobin, which effectively locks oxygen out. Your blood keeps circulating, but it delivers far less oxygen to your brain, heart, and muscles.
The result is a slow suffocation from the inside out, even though you’re still breathing. Because CO is odorless and the early symptoms mimic common illnesses, many people don’t realize what’s happening until they’re too impaired to help themselves.
Symptoms at Different Exposure Levels
The most common early symptoms of CO poisoning are headache, dizziness, weakness, and nausea. These often get mistaken for the flu, a hangover, or food poisoning, especially in winter when windows are closed and furnaces are running. A key clue: if multiple people in the same building develop headaches and nausea at the same time, and if the symptoms improve when you leave the building, CO exposure is a strong possibility.
As exposure continues or concentrations rise, symptoms progress to vomiting, chest pain, confusion, difficulty walking, and impaired memory. Severe poisoning causes loss of consciousness, seizures, heart rhythm disturbances, and death. At concentrations around 400 parts per million (ppm), a healthy adult can tolerate a 60-minute exposure with serious discomfort. At 1,500 to 2,000 ppm, just one hour of exposure becomes life-threatening.
For context, the workplace safety limit set by OSHA is 50 ppm averaged over an eight-hour shift, and NIOSH recommends a ceiling of 200 ppm that should never be exceeded even briefly.
Who Faces the Greatest Risk
CO affects everyone, but some people are more vulnerable at lower concentrations. Infants, elderly adults, people with chronic heart disease, and those with anemia already have compromised oxygen delivery. For them, even moderate CO levels can trigger dangerous symptoms faster. Pregnant women face an additional risk because CO crosses the placenta and can harm fetal development even when the mother’s symptoms seem mild.
Detecting and Preventing CO Buildup
Because you cannot see, smell, or taste carbon monoxide, battery-operated or plug-in CO detectors are the only reliable early warning system. The EPA recommends placing a detector on every floor of your home, with at least one near sleeping areas loud enough to wake you. If the alarm sounds, leave the building immediately and call emergency services from outside.
Prevention comes down to two things: proper maintenance and adequate ventilation. Have fuel-burning appliances inspected annually by a qualified technician. Never run a generator, grill, or vehicle engine in an attached garage, basement, or any enclosed space, even with the garage door open. Don’t use a gas oven or range to heat your home. Keep chimneys and flues clear. If you use a fireplace, make sure the damper is fully open before lighting a fire and stays open until the fire is completely out.