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Products of Combustion: Fire Gases, Flame, Heat & Smoke
What a fire leaves behind, which of it hurts you, and why smoke is both a poison and a fuel.
- The four products of combustion are fire gases, flame, heat and smoke.
- Carbon monoxide binds hemoglobin about 200 to 250 times more strongly than oxygen. Hydrogen cyanide blocks cells from using it.
- Smoke is fuel as well as poison. It can ignite when hot and mixed with air.
- Smoke is toxic whatever its color, and its toxins collect far from the fire.
- Wear SCBA through overhaul and decontaminate after every fire.
Nothing disappears: the four products of combustion
When a material burns, none of its matter is destroyed. It is transformed. The mass of the products equals the mass of the fuel and the oxygen consumed, which is why a house fire fills a neighborhood with smoke. The four products of combustion are fire gases, flame, heat and smoke.
Fire gases
Every fire makes a different cocktail depending on what is burning and how much oxygen it gets. The ones that hurt people most:
| Gas | Where it comes from | Why it matters |
|---|---|---|
| Carbon monoxide (CO) | Incomplete burning of any carbon-containing material | Colorless, odorless and tasteless. Hemoglobin binds CO about 200 to 250 times more strongly than oxygen, starving the body of oxygen |
| Carbon dioxide (CO₂) | Burning of carbon-containing fuels | Less toxic but speeds up breathing, which increases the intake of other toxic gases; can displace oxygen at high concentrations |
| Hydrogen cyanide (HCN) | Burning of nitrogen-containing materials such as wool, silk, nylon and polyurethane foam | Chemical asphyxiant that blocks cells from using oxygen; contributes to incapacitation |
| Hydrogen chloride (HCl) | Burning of PVC, found in pipe, wire insulation and siding | Severe irritant that turns acidic on moist tissue in the eyes and airway |
| Acrolein | Burning of wood, paper, fats and some plastics | Powerful irritant of the eyes and respiratory tract |
| Sulfur dioxide | Burning of sulfur-containing materials such as rubber | Irritates the eyes, nose and lungs |
Smoke inhalation, not flame, kills most fire victims. Toxic gases often collect in places far from the fire, so people die in rooms and corridors the flames never reached.
Smoke: particles, gases and fuel
Smoke is a mixture of gases, tiny solid particles (soot) and liquid droplets. Some materials produce more of it than others. Liquid fuels, oils, tar, paint, rubber and many plastics typically give off dense, black smoke in large quantities.
Two points matter on the fireground. First, smoke is fuel. It contains unburned, flammable products of combustion and can ignite when hot enough and mixed with air. That is why a smoke-filled room can flash over, and why a ventilation-limited room can backdraft. Second, you cannot judge toxicity by looking. Smoke is toxic whether it is thick or thin, and the color of the smoke is not a reliable measure of the poisons in it.
Heat
Heat spreads fire, but it also injures. Direct heat causes burns. Breathing very hot gases can injure the airway. Working in full gear in a hot environment causes heat exhaustion, dehydration and heat stroke. This is why departments use rehab and rotate crews, and why your own body is part of your size-up.
Flame
Flame is the visible, glowing part of a burning gas. When burning gas mixes with the right amount of oxygen, the flame gets hotter and less luminous because the carbon burns more completely. Some fires produce heat and smoke with no flame at all, as in smoldering. A few fuels burn with a flame that is almost invisible in daylight. Hydrogen is one, and a thermal imaging camera helps in these cases.
Protecting yourself from the products of combustion
- Wear your SCBA from arrival through overhaul. Smoldering debris and overhaul produce carbon monoxide and other toxic products even after the flames are out.
- Clean up. Occupational exposure as a firefighter was classified in 2022 by the International Agency for Research on Cancer as carcinogenic to humans. Gross decontamination at the scene, cleaning gear and washing skin as soon as possible reduce what you carry home.
- Treat smoke as toxic every time, whatever its color, and treat rehab as part of the operation, not an afterthought.
MGM Grand, Las Vegas (1980): when smoke travels farther than fire
The fire at the MGM Grand began in a restaurant and mostly burned the casino floor on the ground level. It killed 85 people, and most of them died of smoke inhalation and carbon monoxide, many on the upper floors of the hotel tower, far from the flames.
Investigators from the Clark County Fire Department and NFPA found that smoke traveled upward through unprotected vertical openings, through poorly enclosed stairs, through elevator shafts and through the building’s heating and ventilation system. The casino and the restaurant where the fire started were not sprinklered, and the fire grew quickly on the available fuels. NFPA’s lead investigator said that with sprinklers it would have been a one- or two-sprinkler fire.
The lesson: smoke is the fire’s fastest-moving product. Where it can go, toxic gases go, and the fire does not have to be nearby.
Fireground talk
- Overhaul
- Searching for and extinguishing hidden fire after knockdown. Stay on air.
- Gross decon
- Removing the bulk of soot and contaminants from gear and skin at the scene.
- Rehab
- Rest, hydration and medical monitoring for crews after hard work.
- Off-gassing
- Gear and equipment continuing to release contaminants after a fire.
Practice it in the simulators
Study questions
What are the four products of combustion?
Fire gases, flame, heat and smoke.
Why is carbon monoxide so dangerous?
It is colorless and odorless, and hemoglobin binds it about 200 to 250 times more strongly than oxygen, so the blood carries less oxygen to the body.
Why is smoke called fuel?
It contains unburned, flammable products of combustion. When hot and mixed with enough air it can ignite, as in flashover and backdraft.
Can you tell how toxic smoke is by its color or thickness?
No. Smoke is toxic regardless of its density or color, so firefighters should treat all smoke as dangerous and stay on air.
Sources and further reading
- Carbon Monoxide, National Research Council — National Academies Press
- Lessons from the Past: MGM Grand Fire — Firehouse
- MGM Grand Hotel Fire Anniversary (Clark County Fire Department findings) — FireRescue Magazine
- IARC Monographs Volume 132: Occupational Exposure as a Firefighter — International Agency for Research on Cancer, 2022
- NFPA 1851, Standard on Selection, Care, and Maintenance of Protective Ensembles for Structural Fire Fighting — NFPA (print standard)
About this guide. The Fire Behavior & Chemistry series was originally informed by LBFD Training Division recruit training material. This edition has been independently rewritten and expanded using the sources listed on each page, and its diagrams were drawn for this site. Free Fire Simulator is not affiliated with or endorsed by LBFD. Spotted an error or have a better source? Use the Contact Us button.