Carbon monoxide has no smell. It is completely odorless, colorless, and tasteless, which is exactly why it’s so dangerous. If you’re noticing an unusual smell in your house, it isn’t carbon monoxide. But that fact should make you more concerned about CO, not less, because you cannot rely on your senses to detect it.
Why You Can’t Smell Carbon Monoxide
Carbon monoxide is a gas produced by incomplete combustion of fuels like natural gas, wood, gasoline, and propane. Its molecules are small and nonreactive with the odor receptors in your nose. There is no chemical workaround for this. Unlike natural gas, which utility companies treat with a sulfur-based additive called mercaptan to give it that distinctive rotten-egg smell, no one adds an odorant to carbon monoxide. CO forms as a byproduct inside furnaces, water heaters, generators, car engines, and stoves. By the time it enters your air, there’s no practical way to tag it with a scent.
This is the core problem. Each year in the United States, more than 400 people die from accidental carbon monoxide poisoning unrelated to fires, over 100,000 visit an emergency department, and more than 14,000 are hospitalized. Many of those cases happen because residents had no idea the gas was building up around them.
What You Might Actually Be Smelling
If something smells off in your house, the source is likely something other than CO. A rotten-egg odor usually points to a natural gas leak, which is its own emergency. A burning or metallic smell near a furnace could indicate overheating components, melting wires, or dust burning off heat exchangers. An exhaust-like smell might mean combustion gases are backdrafting into your home from a blocked flue or chimney, and while those gases can contain carbon monoxide, the smell itself comes from other compounds in the exhaust mixture, not the CO.
None of these smells should be ignored. But the absence of a smell doesn’t mean you’re safe.
How Carbon Monoxide Harms You
CO enters your lungs and binds to hemoglobin, the protein in red blood cells that normally carries oxygen. It latches on far more tightly than oxygen does, effectively locking oxygen out. As CO builds up in your bloodstream, your organs and brain get less and less of the oxygen they need.
The symptoms depend on concentration and exposure time. At lower levels (around 35 parts per million), you might develop a headache and dizziness after six to eight hours of constant exposure. At 200 ppm, headaches and impaired judgment set in within two to three hours. At 400 ppm, the situation becomes life-threatening after three hours. At 800 ppm, you can lose consciousness within 45 minutes. At extremely high concentrations (6,400 ppm and above), death can occur in under 20 minutes.
The early symptoms, headache, nausea, dizziness, and fatigue, mimic the flu or general tiredness. People often don’t connect them to CO exposure, especially during winter when furnaces run constantly and windows stay shut. A useful clue: if everyone in the household feels sick at the same time, and symptoms improve when you leave the house, carbon monoxide should be high on your list of suspects.
Carbon Monoxide Detectors Are Your Only Warning
Because your body cannot detect CO before it starts poisoning you, a working detector is the only reliable early warning system. Most residential detectors use electrochemical sensors: carbon monoxide enters a small chamber, triggers a chemical reaction in an electrolyte solution, and generates an electrical current proportional to the CO concentration. When that concentration crosses a dangerous threshold, the alarm sounds. Some models use metal oxide sensors instead, which work by measuring changes in electrical resistance on a chip when CO is present.
Place detectors outside every sleeping area in your home. If you have a fuel-burning appliance (furnace, gas stove, fireplace, water heater) or an attached garage, detectors are especially critical near those areas. Follow the manufacturer’s instructions for mounting height, since recommendations vary by model. Test them monthly and replace them according to the manufacturer’s timeline, typically every five to seven years, because sensors degrade over time.
What to Do When an Alarm Goes Off
If your carbon monoxide detector sounds, get everyone out of the house immediately. Don’t stop to open windows or track down the source. Go outside or to a neighbor’s home, and call emergency services from there. Do not re-enter the house until responders have inspected and cleared it.
If extreme weather makes going outside dangerous and neighbors aren’t nearby, the U.S. Consumer Product Safety Commission recommends staying in one room with a door or window open to the outside. Make sure no vehicles are idling in the garage and no fuel-burning appliances are running in that room. Open all windows in the room, close doors to the rest of the house, and turn on an exhaust fan if one is available.
If anyone is unconscious, having convulsions, or unable to move on their own, call 911 immediately and mention possible carbon monoxide poisoning so responders arrive prepared.
Common Sources Inside a Home
Any device that burns fuel can produce carbon monoxide, but certain situations raise the risk significantly:
- Furnaces and boilers with cracked heat exchangers, blocked vents, or poor maintenance
- Gas stoves and ovens used for heating (they’re not designed for it)
- Portable generators run inside a garage, basement, or near open windows
- Cars left idling in an attached garage, even with the garage door open
- Fireplaces and wood stoves with blocked or poorly drafting chimneys
- Gas-powered tools like pressure washers or chainsaws used in enclosed spaces
Annual inspection of fuel-burning appliances by a qualified technician is one of the most effective ways to prevent CO buildup. Chimneys and vents should be checked for blockages before each heating season, and generators should never be operated indoors or in any partially enclosed area.