Yes, fire produces carbon monoxide (CO) as a byproduct of incomplete combustion. Any time a carbon-based fuel burns without enough oxygen to fully convert to carbon dioxide, carbon monoxide forms instead. This includes wood fires, charcoal grills, gas stoves, candles, and virtually every other flame fueled by organic material. The less oxygen available, the more CO a fire generates.
Why Fire Produces Carbon Monoxide
When a fuel containing carbon burns with plenty of oxygen, the carbon atoms combine fully with oxygen to create carbon dioxide (CO₂), which is relatively harmless at normal concentrations. But combustion is rarely perfect. In the real world, parts of a fire are always starved for oxygen, whether because of how the fuel is stacked, how air circulates around the flame, or how hot the fire burns. In those oxygen-poor zones, carbon atoms pick up only one oxygen atom instead of two, producing carbon monoxide.
This is why smoldering fires are especially dangerous. A roaring, well-ventilated fire burns more completely and produces less CO. A slow, smoldering fire, like embers in a fireplace or a dampened campfire, is much less efficient and generates significantly more carbon monoxide per unit of fuel burned.
How Much CO Different Fuels Produce
Not all fuels release the same amount of carbon monoxide. Research from Cranfield University measured CO emissions from wood and charcoal under controlled conditions, and the differences are striking. Wood sawdust produced between roughly 99 and 258 milligrams of CO per gram of fuel. Charcoal produced considerably more, ranging from about 99 to 495 milligrams per gram, with hazel charcoal hitting the highest recorded value at 495 mg/g at elevated temperatures.
The reason charcoal emits more CO is that it’s already been partially burned during its manufacturing process. What remains is nearly pure carbon, and when that carbon smolders without adequate airflow, it converts heavily to CO rather than CO₂. This is why using a charcoal grill indoors or in a garage is one of the most common causes of carbon monoxide poisoning.
Gas fuels like propane and natural gas tend to burn more cleanly than solid fuels because the gas mixes more evenly with air. But they still produce CO, especially when burners are dirty, improperly adjusted, or operating in a space with limited ventilation. A yellow or orange flame on a gas appliance is a visual warning that combustion is incomplete. A properly burning gas flame should be blue.
How Fast CO Builds Up Indoors
Carbon monoxide is colorless and odorless, which makes it impossible to detect without an alarm. What makes it particularly dangerous is how quickly it accumulates in enclosed spaces. According to the CDC’s National Institute for Occupational Safety and Health (NIOSH), CO can build to dangerous or fatal concentrations within minutes, even in areas that appear well ventilated. In some scenarios, dangerous levels can accumulate within seconds.
NIOSH sets the “immediately dangerous to life and health” threshold at 1,200 parts per million (ppm). At that concentration, a 30-minute exposure produces enough CO in the bloodstream to cause headaches, and higher levels quickly become incapacitating. Exposure to 1,500 to 2,000 ppm for one hour is considered dangerous. At those levels, CO displaces oxygen in your red blood cells, and the effects escalate in a predictable pattern:
- Mild exposure (10-20% blood saturation): Slight headache, possibly no noticeable symptoms at all.
- Moderate exposure (around 35% saturation): Impaired coordination and difficulty with physical tasks.
- Severe exposure (40%+ saturation): Mental confusion, disorientation, inability to move or escape.
The insidious part is that symptoms start mild and worsen gradually. Many people mistake early CO poisoning for a common headache or fatigue, which means they stay in the contaminated space longer and breathe in more.
Visual Signs of Incomplete Combustion
While you can’t see or smell carbon monoxide itself, there are visible clues that a fire or appliance is producing elevated levels. The Minnesota Department of Health identifies several warning signs to watch for in your home:
- Yellow or orange flames on gas appliances instead of a clean blue flame
- Soot streaks around fuel-burning appliances or fallen soot in a fireplace
- No upward draft in your chimney, meaning exhaust gases aren’t venting properly
- Excess moisture or condensation on windows and walls near the appliance
- Rust on flue pipes or discolored bricks at the top of a chimney
Any of these signs suggest that combustion gases, including CO, are not being properly vented outside.
Workplace and Residential Exposure Limits
Regulatory agencies set specific limits on how much carbon monoxide is considered safe over time. OSHA’s workplace limit is 50 ppm averaged over an eight-hour workday. NIOSH recommends a stricter limit of 35 ppm over eight hours, with a ceiling of 200 ppm that should never be exceeded even briefly. For context, outdoor air in most cities contains less than 1 ppm.
Residential CO alarms are designed around these thresholds. Alarms meeting the UL 2034 standard are required to sound before CO reaches levels that would impair your ability to react and evacuate. They won’t alarm at very low concentrations, which is by design to avoid false alarms from brief, harmless spikes. But they will trigger well before levels become life-threatening, provided the alarm is properly placed and has working batteries.
Common Situations That Create Risk
Most carbon monoxide incidents follow a pattern: a fire or combustion device operating in a space without adequate ventilation. Some of the most frequent scenarios include running a generator inside a garage or basement, using a charcoal grill in an enclosed patio or tent, operating a gas stove for heat with windows closed, and letting a car idle in an attached garage. Fireplaces with blocked or poorly maintained chimneys are another common source, especially during winter when dampers get closed accidentally or creosote buildup restricts airflow.
Portable fuel-burning heaters, particularly kerosene models, are high-risk because they’re often used in bedrooms or small spaces during power outages. Even a well-functioning heater in a sealed room will gradually deplete oxygen and shift combustion toward CO production. Opening a window a few inches is not a reliable safeguard, since CO can build faster than the opening can ventilate.
If you use any fuel-burning device indoors, a working CO alarm on every level of your home is the single most important protection. CO alarms are inexpensive, widely available, and the only reliable way to detect a gas your senses cannot.