There are 5 things that have to happen for a combustible dust explosion to occur:

1. There must be fuel—combustible dust is the fuel;
2. There must be an ignition source, i.e., heat;
3. There must be oxygen in the air—an oxidizer;
4. The dust particles must be dispersed in sufficient quantity and concentration so that they form a critical mass that reacts to heat; and
5. The dust cloud must be confined.

These 5 conditions can occur in any number of ways. The new OSHA combustible dust guidance describes two actual examples:

Plastic Particles from Micropulverizer

1991; Littleton, NH: A worker is feeding 400 pounds of granular polyalphamethyl styrene through a micropulverizer into a dust collector without venting. A piece of hot metal gets past the magnet in the micropulverizer and ignites the dust in the system. The worker is injured in the explosion and subsequent fire.

Plastic pellets don’t pose combustible dust hazards when they’re first produced. But when they’re processed or ground up downstream, they generate combustible dust.

Dust from Dried Aqueous Solution

2003; Kinston, SC: During the drying process, aqueous polyethylene solution used to produce rubber stoppers forms a dust that builds up above suspended ceilings. An explosion occurs killing six workers and injuring 30.

Source: OSHA, Hazard Communication Guidance for Combustible Dusts, July 2009, http://www.osha.gov/Publications/3371combustible-dust.pdf

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