Watch a campfire for a few minutes and you'll see how smoke works. A bed of hot coals barely smokes at all. Toss on a green stick or a handful of damp leaves, and smoke pours off it. Then the new wood catches, the flames come up, and the smoke drops off. That tells you most of what you need to know. Smoke is mostly fuel that got hot but didn't finish burning.
What's in a piece of wood
Wood is made of four basic things:
- Water. Green wood is heavy with it, and even dry firewood has some.
- Volatile compounds, like sap and resins, that turn into gas when they get hot. Those gases are what actually burn in the flame.
- Carbon, which is what's left glowing in the coals.
- Ash, the minerals that won't burn at all.
Where the smoke comes from
When heat hits wood, it drives the water off first as steam. Then it cooks the volatile compounds out as gas. If there's enough heat and enough oxygen, that gas catches and burns in the flame, and you don't see much smoke. If there isn't, because the wood is wet, the fire is short on air, or it hasn't caught yet, a lot of that gas goes up unburned. It cools, condenses into tiny droplets, and carries bits of soot along with it. That cloud is smoke.
So a fire that's burning hot and clean makes less smoke, and a fire that's starved for air, smoldering, or burning something wet makes more.
Why a building fire smokes so much
A house or a business isn't full of dry firewood. It's full of plastics, foam, carpet, and finishes, and those put out thick, dark smoke when they burn. Inside a room, a fire also uses up the oxygen around it, so it burns dirtier the longer it goes.
That smoke is more than soot. It carries carbon monoxide and other toxic gases from everything that's burning. Carbon monoxide has no smell, and a few breaths of heavy smoke can leave you confused and slow right when you need to think clearly. In a lot of fires, the smoke hurts people before the flames ever reach them.
Smoke moves faster than you'd think
Smoke is hot, so it rises. In a room, it collects at the ceiling and works its way down toward the floor, and it pours through open doors and up stairwells. That's why the cleanest air in a smoky room is down low, and why closing a door behind you on the way out slows it down.
Here's what that means if you're ever in it:
- If the smoke is thick and dark, or it's coming down from the ceiling, don't try to fight the fire. Get out.
- If you have to go through smoke, stay low and keep moving.
- Close doors behind you as you leave.
- Once you're out, stay out.
Smoke is also the early warning
The same smoke that can hurt you is what a smoke detector is looking for, and different fires make different smoke. A fast, flaming fire throws off a lot of very fine particles, and an ionization detector is quick on those. A slow, smoldering fire, like a cigarette down in a couch cushion, puts out bigger, heavier particles, and a photoelectric detector is quick on those. That's why a lot of buildings use both types, or a detector with both sensors in one unit. We explain how each one works in how do ionization smoke detectors work and how do photoelectric smoke detectors work.
In a business, those detectors are usually part of a fire alarm system wired to a panel. We install fire alarm systems, test the whole system every year, and offer monitoring. If you're not sure what's on your ceiling or whether anybody's testing it, call 1-800-348-9604.