Learn something new every day, I guess. Maybe others here knew it. Local news item:
http://wjbdradio.com/LocalNews/Centralia-Home-Receives-Minor-Damage-From-Sawdust-Fueled-FireI had been collecting clean sawdust from my power saw pickup bags as raw material for scenery. So it gets spread on the lawn as soon as I go there today. Risk in my case was low, thankfully. In quick research, the hazard from spontaneous combustion seems to be driven by sawdust containing finishing products such as stains and varnishes, but that it doesn't take much.
Anyway, something to be aware of if you're using old-school scenery methods, or even new-school techniques such as sawdust+glue over wire armatures for trees.