Not a physics background(see mundane question above as proof) but my guess is that for a chain reaction to spread uncontrollably to that scale, basically everything in the air would have to be fusable. And my non-educated guess it that things like N2 and O2, even at the temperatures this would generate, don't fuse that well.
Fission reactors chain react out of control because fission can happen at much much lower temperatures and there are safety measures(control rods) put in place to inhibit and control the rate of reaction. Those controls malfunction, you get a meltdown. Here, I think loss of control would be its own safety measure because of how much heat is needed just to sustain the reaction.
But, if that were possible...god, think of the weaponry you could create from that.