First, please enjoy this video of "Stupidity Captured at 2500 Frames per Second," preferably in full-screen view at high resolution. Take special note of the "Flour + Candle" segment that starts around the 2:50 mark:
If, like me, you were left wondering how and why ordinary cooking flour would ignite in a giant fireball, allow me to share with you the strange science of explosive dust.
We don't ordinarily consider all-purpose flour to be a flammable, and indeed, a cup of flour sitting on the kitchen counter poses no danger. Yet flour does contain calories, and calories are just another form of energy.
Fires require just three factors—fuel, oxygen and heat—and our cup of flour can't get enough oxygen to burn. Throw that flour into the air, however, and it disperses into a fine haze of dust particles. All these tiny particles have a huge surface areas compared to their volumes, which means there's a lot of oxygen surrounding each bit of grain. Add a spark—or, in this case, a candle—and our fibrous fuel turns into a fireball.
The problem becomes most acute in grain silos or other industrial settings, where fine dusts collect in confined spaces. Here an errant spark—or even static electricity—can set off a massive explosion. In the late 1970s a spate of explosions in grain elevators killed 59 people. Agricultural grains aren't the only danger. Dust is a constant worry in coal mines, and anything from sawdust to aluminum to powdered milk can ignite under the right conditions. In the first two months of 2003, a dust explosion in a Kentucky acoustic insulation manufacturing plant killed seven, while six died during a polyethylene dust explosion in a North Carolina pharmaceutical plant.
As the video says: please do not under any circumstances try any of this at home.