Carbon Monoxide Poisoning From a Space Heater

It is a cold January night and your furnace has 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 move the needle.

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, and whether the model carries an oxygen-depletion sensor (ODS), which ANSI Z21.11.2 has required on unvented gas units since its 1983 edition.

How the ANSI Z21.11.2 Standard Forces Indoor Gas Heaters to Carry an Oxygen Sensor

The most consequential equipment rule for unvented gas heaters in US homes is rarely visible on the box. ANSI Z21.11.2 governs unvented gas-fired room heaters. The 2016 edition is the twenty-eighth, and the standard lists its own predecessors: 2013, 2011, 2007, 2000, 1996, 1992, 1989, 1983, 1978, 1974, 1971, 1967 and 1962. The date that matters here is 1983, not the 1980 most summaries give, including an earlier version of this page. CPSC proposed a federal ODS mandate in 1980 and said at the time that "the ODS never has been used on American heaters"; the mandatory federal standard took effect in December 1981; and in May 1983 the Commission voted to begin revoking it because the ANSI and AGA voluntary standard by then "would be as effective". The 1983 edition is the one that lines up with that. The sensor shuts off the gas supply to both the main burner and the pilot when the oxygen level in the surrounding air drops to 18 percent, or higher if the manufacturer specifies. The standard applies to heaters "having input ratings up to and including 40,000 Btu/hr", and two limits inside that are worth knowing before you buy: an unvented heater suitable for a bedroom is rated 10,000 Btu/hr or less, and one suitable for a bathroom 6,000 Btu/hr or less. It limits CO output to 200 parts per million on an air-free basis. That ceiling is half what ANSI Z21.47 allows a vented central furnace, which may run to 400 ppm air-free, and the reason for the gap is where the exhaust ends up: a furnace sends its 400 ppm up a flue, while an unvented heater releases everything it makes into the room you are sitting in. If you buy a new unvented gas heater from a US retailer, the ODS is built in and is the reason the unit can be sold for indoor use at all.

The ODS does not announce itself. From the user's side it presents as a heater that keeps going out for no reason anyone can see, in a room that feels fine. That is the device working: it is reporting the room rather than reporting itself. The responses people reach for are the wrong ones, cleaning the pilot assembly, booking a service call for a fault the heater does not have, or replacing the unit with an older one that does not keep doing this. What the shutdown calls for is more air in the room and a CO alarm in it, because the conditions that starve a pilot are the same ones that degrade combustion at the main burner.

Two practical implications:

• Older heaters predating the 1983 edition (still circulating in resale, rentals, vacation properties, and stored backup gear) lack the ODS and were sold under permissive earlier rules. Date-check any used heater before lighting it: the rating plate is on the back or inside the cabinet, and it carries the manufacturer and the model number you need to check the unit against the maker or the certification body. A heater whose plate is missing, painted over or illegible cannot be dated at all, and that is the answer rather than an obstacle to it. Do not light it.
• The ODS protects against oxygen depletion, not against every CO failure mode. A heater with poor combustion can still emit unsafe CO before the room oxygen drops to 18 percent. The ODS is one layer; a CO alarm in the same room is the second.

Per-Fuel CO Yield in Plain Terms

Not all fuel-burning heaters produce CO at the same rate, even when working correctly. Combustion completeness depends on burner design, fuel quality, and ventilation. A rough ranking from lowest to highest CO yield in normal operation:

• Properly tuned natural gas or propane (vented, with adequate combustion air): low CO output; most of the carbon ends up as CO2 and water vapor, exhausted through the flue.
• Unvented gas heater with intact ODS: low to moderate CO output during normal operation; the 200 ppm air-free ceiling exists because some CO leakage into the room is inevitable.
• Kerosene heater (properly maintained, recommended fuel grade): moderate CO output, and all of it stays in the room. What moves the number is the fuel rather than the heat setting. CPSC tells owners to burn water-clear 1-K grade and nothing else, because heavier grades gum the wick, and diesel or number 2 heating oil, which look close enough to a buyer at a filling station, clog it and produce dangerous levels of carbon monoxide.
• Heater used with wrong fuel or contaminated fuel: high CO output; incomplete combustion drives CO sharply upward and is the failure mode behind many residential incidents.
• Charcoal grill, hibachi, or smoldering biomass indoors: extremely high CO output and a recurring cause of fatal incidents. Never use any of these in a living space, garage, or covered porch.

CPSC Fatality Data Worth Knowing

The Consumer Product Safety Commission tracks non-fire CO deaths from consumer products. In its most recent published annual estimate (2020 data), 211 total deaths were associated with consumer products, of which 33 were tied to portable heaters specifically. Portable heaters were the second-largest single-product category that year, behind portable generators at 92. The 2020 figure is consistent with the broader pattern: over the past decade, fuel-burning heating equipment has accounted for roughly 30 to 50 CO deaths per year in the US, with the variance driven largely by winter-storm activity and outage-prompted use of equipment not designed for indoor heating.

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.

Two Legislatures Looked at a Space Heater and Reached Opposite Answers

We hold carbon monoxide law for all 51 US jurisdictions, and the space heater is one of the few appliances two of them have addressed by name in opposite directions. North Carolina wrote it in. Its statute defines the fuel-burning source it cares about by listing the appliances, and the legislature's list runs through space heaters, wall and ceiling heaters, ranges, ovens, stoves, furnaces, fireplaces, water heaters and clothes dryers, burning natural or liquefied petroleum gas, fuel oil, kerosene, wood or coal for heating, cooking, drying or decorative purposes. A space heater is the requirement's own example of what it is for.

Michigan wrote it out, in the section covering places people sleep transiently. MCL 125.1504d(1) requires the owner or operator of residential occupancies where occupants are primarily transient, boarding houses, hotels and motels named in the text, to put one operational carbon monoxide device at each "source point". A source point is an area holding a mechanism providing a common source of heat from a fossil-fuel-burning furnace, boiler or water heater, and the definition expressly excludes a place where the only fuel-burning thing present is a wood or fossil-fuel-burning fireplace or a wood or fossil-fuel-burning space heater. So a lobby with a decorative gas fireplace is not a source point, a boiler room is, and a room heated by a space heater and nothing else does not create the duty at all. Two further limits are worth knowing before anyone relies on that: the section reaches only buildings newly constructed on or after 1 December 2009, and the device count is per source point rather than per room.

The practical reading is not that Michigan is careless. Its rule puts the alarm at the plant that serves the whole building, which is a defensible design for a hotel with a central boiler and a poor one for the room with the portable heater in it. What it means for you is that the appliance most likely to be running in front of you is, in that state's transient-lodging section, the one appliance that does not oblige anyone to fit an alarm near it. The alarm in that room is yours to bring, which is the same conclusion this page reaches from the combustion side.

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
• CPSC: Non-Fire CO Deaths Associated with Consumer Products, 2020 Annual Estimates
• MCL 125.1504d, carbon monoxide devices in transient residential occupancies, for the source-point definition and its fireplace and space-heater exclusion
• North Carolina CO detector laws, for the statutory appliance list that names space heaters
• CPSC: Winter heating equipment safety warning (2023)
• ANSI Z21.11.2-2016, Gas-Fired Room Heaters Volume II, Unvented Room Heaters (ODS requirement)
• CPSC, Kerosene Heater Safety, for the water-clear 1-K instruction and what heavier grades, diesel and number 2 heating oil do to a wick
• CDC: Carbon Monoxide Poisoning Prevention
• NFPA: Carbon Monoxide Safety

Last updated: August 12, 2026