Any appliance that burns fuel can emit carbon monoxide. This includes gas furnaces, gas stoves, wood-burning fireplaces, kerosene space heaters, portable generators, gas water heaters, gas dryers, and charcoal grills. The risk increases sharply when these appliances are poorly maintained, improperly vented, or used in enclosed spaces without adequate airflow.
Carbon monoxide forms during incomplete combustion. When a fuel like natural gas, propane, wood, or kerosene burns without enough oxygen, the carbon in the fuel only partially oxidizes. Instead of producing carbon dioxide (the relatively harmless byproduct of complete combustion), the reaction stops one step short and produces carbon monoxide. This is why ventilation matters so much: a well-tuned burner with plenty of air supply produces very little CO, while a dirty, oxygen-starved one can produce dangerous amounts.
Gas Furnaces and Boilers
Your home heating system is one of the most common sources of carbon monoxide in residential settings. Gas and oil furnaces contain a component called a heat exchanger, which separates the combustion chamber (where fuel burns) from the air that circulates through your home. When that heat exchanger cracks, combustion gases, including carbon monoxide, can leak directly into your living space. Cracks often develop from years of thermal expansion and contraction, and they’re invisible without professional inspection.
ENERGY STAR recommends annual professional maintenance that includes checking all gas or oil connections, gas pressure, burner combustion, and the condition of the heat exchanger. A dirty burner or cracked heat exchanger causes improper combustion and reduces both safety and efficiency. If your furnace is more than 15 to 20 years old, these inspections become especially important.
Gas Stoves and Ovens
Gas stoves produce carbon monoxide every time they’re in use, though typically at low levels when the burners are properly adjusted. The concern grows in homes without effective ventilation. The EPA recommends using a range hood that vents to the outdoors while cooking to reduce exposure to combustion byproducts. If you don’t have a vented range hood, opening a nearby window while cooking helps dilute pollutants. The American Public Health Association notes that residents can also reduce exposure by shifting some cooking to electric appliances like microwaves, electric kettles, or toaster ovens.
Fireplaces and Wood Stoves
Wood-burning fireplaces and stoves produce significant amounts of carbon monoxide as part of normal operation. Under proper conditions, combustion gases rise through the chimney and exit the home. Problems start when something blocks that path. Creosote, a tar-like substance that accumulates inside chimneys over time, can obstruct the flue and force smoke and carbon monoxide back into the living space. Bird nests, debris, and structural damage to the chimney can cause the same backdrafting effect.
Downdrafts are another issue. If the air pressure inside your home is lower than outside (common in tightly sealed modern homes, especially when exhaust fans are running), air can flow down the chimney instead of up, pushing CO into the room. Annual chimney cleaning and inspection are the most effective ways to prevent these problems.
Unvented Space Heaters
Kerosene and gas space heaters that don’t vent to the outdoors release all of their combustion byproducts directly into the room. The U.S. Consumer Product Safety Commission confirms that kerosene heaters produce low levels of carbon monoxide and nitrogen dioxide during normal operation, and that exposure to even these low levels can be harmful to people with respiratory or circulatory conditions.
If you use an unvented heater, keep a door open to the rest of the house to allow air circulation. In a closed room, or in a newer home with tight construction, open an outside window about an inch to bring in fresh air. These heaters should never be your primary heat source in a sealed space.
Portable Generators
Portable generators are one of the deadliest sources of residential carbon monoxide poisoning, largely because people run them indoors or in attached garages during power outages. A generator’s gasoline engine produces high concentrations of CO, enough to reach life-threatening levels in an enclosed garage within minutes.
The Texas Department of Insurance recommends placing portable generators at least 20 feet from any door, window, or vent. Never run one inside a garage, basement, or crawl space, even with the door open. The exhaust output is simply too high for any indoor ventilation to handle safely.
Other Sources in and Around the Home
Several less obvious appliances also produce carbon monoxide:
- Gas water heaters burn fuel continuously or on demand and rely on proper venting through a flue pipe. A disconnected or corroded vent pipe can send CO into the home.
- Gas dryers require venting to the outdoors. A blocked or crushed dryer vent traps combustion gases inside.
- Charcoal and propane grills produce large amounts of CO and should never be used indoors or in enclosed patios.
- Vehicles idling in an attached garage allow exhaust to seep into the house, even with the garage door open.
- Gas-powered lawn equipment like pressure washers and lawnmowers can produce dangerous CO levels if used in or near enclosed spaces.
How Much CO Is Dangerous
Carbon monoxide concentration is measured in parts per million (ppm). The EPA’s outdoor air quality standard caps safe exposure at 9 ppm over eight hours and 35 ppm over one hour. At higher concentrations, the timeline to serious harm shortens quickly. Exposure to around 150 ppm for four hours or more can cause irreversible health effects. Concentrations above 150 ppm sustained over eight hours can be life-threatening, and levels at 1,200 ppm are classified as immediately dangerous to life.
Most home CO detectors are designed to alarm at 70 ppm after one to four hours, or at higher concentrations more quickly. Symptoms of CO poisoning, including headache, dizziness, nausea, and confusion, can begin at levels well below what triggers a standard alarm, particularly in children, elderly adults, and people with heart or lung conditions.
Detector Placement and Maintenance
The EPA recommends installing a carbon monoxide detector on every floor of your home, with priority given to areas near bedrooms so the alarm can wake you at night. If you’re only installing one, place it near the sleeping areas. Follow the manufacturer’s instructions for mounting height, as recommendations vary by model. CO detectors have a limited lifespan, typically five to seven years, and should be replaced when they expire. Test them monthly using the built-in test button.
Reducing Your Risk
The common thread across every CO source is combustion plus confinement. Any fuel-burning appliance that is poorly maintained, inadequately vented, or used in a space without enough fresh air becomes a potential hazard. A few practical steps cover most of the risk:
- Schedule annual inspections for your furnace, boiler, water heater, and chimney before heating season.
- Use vented range hoods when cooking with gas, or open a window if you don’t have one.
- Never use outdoor equipment indoors, including generators, grills, and camp stoves.
- Keep vents and flues clear of debris, snow, or animal nests, especially after storms.
- Don’t idle your car in an attached garage, even with the door raised.