Steel wool burns. Steel beams don't. Both are steel. That contradiction is the whole answer to whether steel is flammable, and it explains something firefighters think about on every commercial building fire.
Solid steel is not flammable. It will not ignite in an ordinary fire and it melts at about 2,500°F, far hotter than a house fire. But steel does oxidize (rust) and that reaction gives off heat. When steel is shredded into very fine strands, as in steel wool, the surface area is so large that the oxidation happens fast enough to glow and burn. And long before steel melts, heat makes it weak: at 1,100°F it has lost about half its strength.
Why Steel Wool Burns
Burning is fast oxidation. A steel beam oxidizes so slowly we call it rust. Steel wool is the same iron with millions of times more surface exposed to oxygen, so a spark or a 9-volt battery is enough to start a reaction that races through the strands at over 1,000°F. It is the same reason flour dust explodes while a bag of flour just sits there. We cover it in is steel wool flammable.
What Fire Does to Steel Structures
This is what matters on the fireground. Unprotected steel loses strength as it heats:
| Steel temperature | Strength remaining | What it means |
|---|---|---|
| Up to ~600°F | Nearly full | Fine |
| ~1,000°F | About 60% | Sagging begins under load |
| ~1,100°F | About 50% | The common design failure point |
| ~1,500°F | Under 20% | Collapse likely |
| ~2,500°F | Melts | Rarely reached in a building fire |
A well-ventilated room fire reaches 1,100 to 1,500°F at the ceiling within minutes. That is why steel in commercial buildings is wrapped in fireproofing (spray-on, gypsum, or concrete), why unprotected steel bar joists in big-box stores and warehouses are a known collapse hazard, and why firefighters are trained to be cautious about lightweight steel roofs. Steel also expands as it heats, which can push walls out before it fails.
Does Steel Give Off Anything Toxic in a Fire?
Bare steel does not. Coatings do: galvanized steel releases zinc oxide fumes when heated (the cause of "metal fume fever" in welders), and painted or plastic-coated steel gives off whatever the coating produces.
The Bottom Line
Steel is one of the least flammable materials in a building, and one of the most dangerous when it gets hot. It doesn't burn; it bends, then it fails.
Related reading: can fire melt metal and is concrete flammable.
