Note: Kevyn's uniform is was a sophisticated weave of powered monomolecular carbonan. One of the features of the weave is that it conducts heat fairly evenly and quite quickly across its surface. The fabric's designers recognized that in the event of thermal overload (engineer-speak for "it is waay too hot") it would be better for the fabric to dump heat into the surrounding environment rather than into the suit's occupant.

The most effective way to radiate heat in this way is through increased surface area. Dust and ash may not appear to be good for much, but they both have a lot more surface area than fabric. In short, while it may look like Kevyn's low-profile armored uniform has failed in the face of the Sergeant's careless wash of plasma, it has actually performed its function admirably, keeping Kevyn from roasting.

It is a good thing the room is full of air -- otherwise the dust and ash would have nothing into which to quickly dissipate their heat other than Kevyn, and Kevyn would be surrounded by a fog of skin-scorching particulates. Oh, and he wouldn't have any air.