Carbon Monoxide Poisoning From a Space Heater

It is a cold January night and your furnace just died. You pull an old kerosene heater out of the closet, fill it up, and set it running in the living room with the windows shut tight. By morning your family wakes up with headaches and nausea — and the only reason they wake up at all is that someone left the bedroom door open. Fuel-burning space heaters are a real source of carbon monoxide, and the risk rises sharply when ventilation drops and people sleep near them. This page explains which heaters can produce CO, what the warning signs look like, what to do if you suspect exposure, and the prevention steps that actually reduce risk.

Quick Safety Summary

• Electric space heaters do not produce CO — they do not burn fuel.
• Fuel-burning heaters (propane, kerosene, natural gas) can produce CO, especially unvented models in closed rooms.
• Symptoms often mimic flu: headache, dizziness, nausea. Multiple people sick at once is a red flag.
• Never use outdoor-rated heaters, grills, or generators to heat indoor spaces.
• Install CO alarms near sleeping areas — they are your backup when you cannot smell the danger.

Not All Space Heaters Are the Same

The term "space heater" covers a wide range of devices. The critical distinction for CO risk is whether the unit burns fuel:

When a Space Heater Can Cause CO Poisoning

The risk is highest when several factors combine:

• Using an unvented fuel-burning heater in a closed room — windows sealed, doors shut.
• Operating a heater that is not designed or approved for indoor use (common with outdoor heaters or improvised setups during outages).
• Blocked air inlets or outlets, dirty burners, or poor combustion causing higher CO output.
• Using the wrong fuel type or contaminated fuel.
• Running the heater overnight while sleeping — the highest-risk scenario because early symptoms are missed.

Symptoms of CO Exposure From a Heater

CO symptoms often resemble flu or exhaustion: headache, dizziness, weakness, nausea, chest discomfort, and confusion. Severe exposure can lead to fainting or seizures. A practical red flag: multiple people in the same room feel sick, symptoms improve outdoors, or a CO alarm activates.

Because CO is odorless, you may not notice any unusual smell even when levels are dangerous. Combustion byproducts (soot, fumes) may or may not be noticeable — they are not a reliable indicator of CO.

What to Do If You Suspect CO From a Heater

1. Turn the heater off if you can do it quickly without delaying evacuation.
2. Get everyone outside to fresh air immediately — including pets.
3. Call emergency services if anyone has symptoms or a CO alarm sounded.
4. Do not re-enter until it is confirmed safe.
5. Have the heater inspected by a qualified technician before using it again.

For a full emergency checklist: what to do if a CO alarm goes off.

Prevention: The Steps That Matter Most

• Prefer electric heat indoors when possible — no combustion means no CO.
• If you use a fuel-burning heater, ensure it is explicitly rated for indoor use and follow the manual exactly.
• Maintain ventilation per manufacturer guidance — "cracking a window" may be recommended for some devices, but it is not a guarantee of safety.
• Install working CO alarms near sleeping areas and on every occupied level. See how many CO detectors do I need.
• Never use outdoor heaters, grills, or generators to heat indoor spaces.
• Stop using a heater that causes soot, unusual flames, persistent odors, or repeated symptoms — have it inspected or replaced.
• Replace CO alarms that are past their end-of-life date — an expired sensor cannot protect you.

Warning Signs That a Heater Is Not Burning Cleanly

A fuel-burning heater that is producing excessive CO often shows visible or behavioral clues — but not always. Watch for:

• Yellow or orange flames instead of steady blue (on gas models) — this can indicate incomplete combustion.
• Soot deposits on walls, ceilings, or the heater itself.
• A persistent burning or chemical smell — while CO itself is odorless, combustion byproducts that accompany it may be noticeable.
• Condensation on windows in the room where the heater runs — this signals poor ventilation and trapped combustion gases.
• Pilot light that keeps going out — can indicate an airflow or gas supply issue.

If you notice any of these signs, stop using the heater, ventilate the room, and have the unit inspected by a qualified technician before further use. None of these signs guarantee CO is present — but they indicate conditions that make CO production more likely.

Power Outages: When Heater Risk Spikes

During winter power outages, people often turn to fuel-burning heaters they would not normally use — outdoor propane units brought inside, camping stoves, or old kerosene heaters pulled from storage. This is when CO incidents spike. If the power is out and you need supplemental heat, use only heaters specifically rated for indoor use, maintain ventilation, and make sure your battery-powered CO alarms are functional. For generator-related risks during outages, see generator in a garage.

Apartments and Shared Buildings

In apartments, CO may originate from your unit or from shared systems — boilers, water heaters, or even a neighbor's unvented heater. If your CO alarm sounds, treat it as real even if your heater seems off: exhaust can travel through walls, utility chases, and ventilation paths. For apartment-specific guidance, see CO leak in an apartment.

Sources & References

• CPSC: Carbon Monoxide Information Center
• CDC: Carbon Monoxide Poisoning Prevention
• NFPA: Carbon Monoxide Safety

Last updated: February 14, 2026