As to what causes this sort of explosion, dust (which is what flour becomes when it's flung into the air) has a large surface-to-mass area, and it is the surface area of materials that burns when items come into contact with an ignition source (such as a G.I. on fire). Said Bill Stevenson of CV Technology, a dust explosion consulting company, the "explosive pentagon" that works to create the phenomenon has five key components: 1) dust, 2) suspended in a cloud, 3) within a confined space, 4) enough air, 5) some type of ignition.
If there's a bottom line to be had in all this, if you absolutely can't resist trying out medical advice gained from an unsourced anonymous e-mail and must play about with flour in the wake of burning your hand, do so only after you've first fully brought the temperature of the injured area back down to normal with cool water, then dried the injured area. And never, ever toss flour onto a burning person.
Barbara "unbreaded" Mikkelson
Read more at http://www.snopes.com/medical/homecure/flourburns.asp#KgfyR1bl1QElMPKQ.99