Is Carbon Monoxide Flammable?

Yes, carbon monoxide (CO) is a flammable gas that can burn or explode once it makes up a large share of the air in a space and meets a spark, flame, or hot surface.

The gas turns toxic at far lower levels, however. So, poisoning usually becomes a danger long before fire does.

WestAir supplies carbon monoxide across California and Arizona.

What Is the Carbon Monoxide Flammable Range?

The carbon monoxide flammable range runs from 12.5% to 74% by volume in air, according to the National Institute for Occupational Safety and Health (NIOSH).

Below the lower limit, the mixture holds too little fuel for a flame to spread. Above the upper limit, it holds too little oxygen.

CO’s lower limit sits well above the limits of common fuel gases. Propane can ignite at 2.1% and methane at 5%, while CO needs roughly one-eighth of the air in a space to be replaced before it will burn.

CO’s upper limit is unusually high. Propane stops burning above 9.5% and methane above 15%. Hydrogen, at 4% to 75%, is one of the few common gases with a range as wide as CO’s.

How Does Carbon Monoxide Burn?

Carbon monoxide burns by reacting with oxygen (O₂) to form carbon dioxide (CO₂), releasing heat and a pale blue flame.

Each CO molecule picks up one more oxygen atom in the reaction, written as 2CO + O₂ → 2CO₂.

A hot surface can ignite CO without any spark. Its autoignition temperature is about 605°C (1121°F).

In industry, CO often burns as part of a blend. It’s a main fuel component of synthesis gas (syngas), a mix of hydrogen and CO made by gasifying coal or biomass or by reforming natural gas. Hydrogen has a lower flammable limit than CO, so syngas can ignite at lower concentrations than pure CO.

Is Carbon Monoxide Explosive?

Carbon monoxide can explode when it collects in an enclosed space at a burnable concentration and meets a spark, flame, or hot surface, and its compressed gas cylinders carry a separate risk of rupturing when heated.

Inside a room, tank, or duct, burning gas expands faster than it can vent. Pressure builds until walls, doors, or equipment give way.

The U.S. Department of Transportation’s Emergency Response Guidebook (ERG) warns that cylinders exposed to fire can vent toxic, flammable gas through their pressure relief devices, and that heated containers can explode. Ruptured cylinders can also rocket across a room.

For a leak that’s already burning, the ERG advises against putting out the flame unless the gas supply can be shut off. The flame consumes escaping CO. Extinguishing it would let a toxic, ignitable cloud spread and possibly reignite.

Why Does Carbon Monoxide Poison Long Before It Can Burn?

Carbon monoxide is immediately dangerous to life or health (IDLH) at 1,200 parts per million (ppm), while its lower flammable limit equals 125,000 ppm, so the gas reaches life-threatening levels at roughly one-hundredth of the concentration needed to burn.

Inhaled CO binds to hemoglobin, the oxygen-carrying protein in red blood cells, about 200 to 250 times more strongly than oxygen does. Think of a parking garage where CO grabs the spaces and is very slow to pull out, leaving oxygen nowhere to park. The heart and brain then run short of the oxygen they need.

U.S. workplace exposure limits for CO are a small fraction of its IDLH. The Occupational Safety and Health Administration (OSHA) caps average exposure at 50 ppm over an 8-hour shift. NIOSH recommends 35 ppm averaged over a workday of up to 10 hours, with a 200 ppm ceiling.

Where Do Carbon Monoxide Fire Hazards Show Up?

Carbon monoxide reaches burnable concentrations mainly in three settings: chemical plants running concentrated CO or syngas, steel mills recovering furnace gas as fuel, and cylinder storage areas or gas lines carrying pure CO.

• Chemical plants: many industrial uses of carbon monoxide, such as making methanol and acetic acid, send high-concentration CO or syngas through reactors and piping.
• Steel mills: blast furnace gas typically holds about 20% to 26% CO, and basic oxygen furnace gas around 63%. Because most of blast furnace gas is nitrogen and CO₂, which don’t burn, leaked gas has to make up at least about 30% of its mixture with air before it can ignite.
• Cylinder storage and gas lines: a failed regulator, valve, or fitting releases undiluted CO.

How Do You Detect a Carbon Monoxide Leak?

You detect a carbon monoxide leak with fixed gas monitors in storage and process areas and portable meters worn by workers, because CO is colorless, odorless, and tasteless.

Early exposure symptoms such as headache, dizziness, and nausea are vague and easy to blame on something else.

CO monitors measure the gas in ppm, the same unit used for its exposure limits. Toxic alarm set points sit far below CO’s lower flammable limit, so a toxic alarm warns of a leak long before the gas could approach flammable levels.

CO weighs about 97% as much as air, so it spreads through a room rather than pooling near the floor like propane or collecting at the ceiling like hydrogen. Place fixed monitors near potential leak points such as cylinder connections, and follow the manufacturer’s instructions for mounting height.

Personal CO meters are useful for welders, maintenance crews, and lab staff who enter furnaces, tanks, or gas cabinets where fixed monitors can’t reach. Where CO is one of several hazards, a multi-gas meter covers them in a single device.

How Should Carbon Monoxide Cylinders Be Stored and Handled?

Carbon monoxide cylinders require ventilated storage away from oxidizers and heat, the correct valve connection, cylinder materials suited to the gas, and routine leak checks.

The U.S. Department of Transportation (DOT) classifies compressed CO as a Division 2.3 poison gas with flammable gas as a secondary hazard, so cylinders ship with both labels.

Store CO cylinders upright and secured in a cool, well-ventilated area or a ventilated gas cabinet, away from ignition sources. The National Fire Protection Association’s compressed gas code, NFPA 55, calls for keeping flammable gas cylinders at least 20 feet from oxygen and other oxidizing gases, or separating them with a noncombustible barrier that has at least a half-hour fire-resistance rating and blocks the line of sight between the cylinders.

CO cylinders use the Compressed Gas Association (CGA) 350 valve connection. It has a left-hand thread, shared with other flammable gases such as hydrogen and methane, which keeps flammable gas regulators from connecting to oxygen equipment. Never use an adapter to force a mismatched connection.

At high pressure, CO can react with iron and nickel to form small amounts of metal carbonyls—toxic compounds that contaminate the gas. Some specialty gas suppliers recommend aluminum cylinders for high-purity CO to limit that reaction.

After connecting a regulator, leak-check every fitting with a compatible leak-detection solution or a handheld CO detector, and close the cylinder valve whenever the cylinder isn’t in use.

Treat Carbon Monoxide as a Toxic Gas First

Carbon monoxide can burn, but only after a leak has already reached lethal concentrations. Ppm-level monitoring, ventilated storage, and the correct cylinder hardware guard against both hazards at the same time.

Lawrence Haynes

Lawrence is the Marketing Director at WestAir Gases & Equipment in San Diego, CA