Yes, fire produces carbon monoxide. Every fire does, to some degree. When flames burn fuel without enough oxygen to fully convert it to carbon dioxide and water, the result is carbon monoxide, a colorless, odorless gas that can be lethal in enclosed spaces. The amount produced depends on the fuel, the oxygen supply, and the temperature of the fire.
How Fire Creates Carbon Monoxide
Carbon monoxide forms through incomplete combustion. In a perfect burn, carbon in the fuel combines with oxygen to produce carbon dioxide. But fires rarely burn perfectly. When oxygen is limited, or temperatures are too low, or the fuel doesn’t spend enough time in the flame, carbon only partially oxidizes. Instead of carbon dioxide, you get carbon monoxide.
This incomplete reaction releases only about 52% of the total energy stored in the fuel. So a fire producing a lot of carbon monoxide is also a fire that’s burning inefficiently, wasting heat and generating soot. Three conditions push combustion toward carbon monoxide production: low temperature, limited oxygen, and short residence time (how long the fuel stays in the hottest part of the flame).
Oxygen concentration has a dramatic effect. Research on combustion reactions shows that at normal atmospheric oxygen levels (around 21%), most carbon burns completely. Drop that to 7% oxygen, and the rate of carbon monoxide production becomes dominant, especially at higher temperatures. This is exactly what happens when a fire burns in a closed room and gradually consumes the available air. The fire doesn’t go out right away. It shifts from producing mostly carbon dioxide to producing dangerous amounts of carbon monoxide.
Which Fuels Produce the Most
All carbon-based fuels produce carbon monoxide when burned, but some are worse than others. Charcoal is a particularly heavy carbon monoxide emitter because it burns as a glowing solid rather than with open flames, and much of the combustion happens right at the surface where oxygen access is inconsistent. Studies measuring emissions from charcoal combustion in cookstoves found that carbon monoxide concentrations exceeded 35 ppm, the limit the EPA considers safe for one hour of human exposure.
Wood fires produce carbon monoxide as well, particularly during the smoldering phase when flames die down and the wood chars. Gas appliances generate less carbon monoxide when properly adjusted, but malfunctioning or unvented gas heaters can produce significant amounts. Vent-free gas space heaters are tested against an industry standard of 200 ppm of carbon monoxide (measured in air-free conditions), and even compliant units emit a measurable fraction of that limit.
Gasoline and diesel engines, candles, kerosene lamps, and propane stoves all produce carbon monoxide too. Anything that burns carbon-containing fuel in a confined or oxygen-limited setting will generate it.
How to Spot Incomplete Combustion
You can’t smell or see carbon monoxide itself, but you can often spot the conditions that produce it. Flame color is one of the most reliable visual cues. A blue flame indicates efficient, complete combustion. An orange or yellow flame signals incomplete combustion, meaning carbon monoxide production is likely elevated. This applies to gas stoves, furnaces, water heaters, and any appliance with a visible flame.
Other warning signs include:
- Soot streaks around fuel-burning appliances or inside a fireplace
- Excess moisture and condensation on windows, walls, or cold surfaces near the appliance
- Rusting on flue pipes or vent connections
- No upward draft in a chimney, meaning exhaust gases (including carbon monoxide) are staying in the room
- Damaged or discolored bricks at the top of a chimney
Why Carbon Monoxide Is So Dangerous
Carbon monoxide binds to hemoglobin in your blood roughly 200 times more effectively than oxygen does. Once it latches on, it blocks your red blood cells from carrying oxygen to your organs and brain. The effects build gradually, which is part of what makes it so dangerous. You can be slowly poisoned without realizing the source.
The earliest symptoms are a headache, dizziness, and weakness. As exposure continues, nausea, vomiting, chest pain, and confusion develop. Severe poisoning causes loss of consciousness, heart rhythm problems, and death. According to the CDC, these symptoms can mimic the flu or food poisoning, which leads many people to miss the real cause, especially during winter when windows are closed and heaters are running.
Workplace safety standards set by OSHA cap exposure at 50 ppm averaged over an eight-hour shift. Workers must be removed from the area entirely if levels reach 100 ppm. For context, a charcoal grill or a poorly vented fireplace can easily push indoor concentrations past 35 ppm in a closed room.
Protecting Yourself Indoors
Carbon monoxide detectors are the single most important safeguard. Because the gas is slightly lighter than air and tends to rise with warm currents, the EPA recommends placing detectors on a wall about 5 feet above the floor, or on the ceiling. Don’t mount one directly next to or above a fireplace or stove. Every floor of your home needs its own detector, and if you’re only getting one, place it near the bedrooms where it can wake you at night.
Ventilation is the other critical factor. Never use a charcoal grill, portable gas stove, or generator indoors. Keep fireplace dampers open when a fire is burning and for some time afterward. Have fuel-burning furnaces and water heaters inspected annually, and pay attention to flame color. If a gas appliance that normally burns blue starts producing orange or yellow flames, it needs servicing. The appliance is still burning fuel, but it’s now generating substantially more carbon monoxide with every minute of operation.