[nj_bartel]

What is it about powdered polystyrene that makes it react similarly to powdered metals, i.e. when blown into air, it combusts?

[Arkcon]

Hmmm...dunno.  Some things are pyrophoric, some things aren't.  Lead and iron are pyrophoric, so is flour.  Polystyrene is too, you say, and some other common plastics aren't?  Guess it's about reactivity to oxidation, and the ability to get a finely divided powder. 

[macman104]

I imagine it's similar to the flour bomb effect and grain silo explosions.  A finely divided organic powder in an air suspension, is highly flammable, I assume to the increased surface area.

http://en.wikipedia.org/wiki/Flour_bomb#Grain_elevator_explosions

[mnakhla]

I second that, its the increase in surface area 
at-least partially

[nj_bartel]

So what's the flammability a result of?  Increased contact with O₂?  And it applies to essentially any powdered organic material?

[macman104]

I believe so, just because there is so much larger area of reaction for combustion to occur at.