I see several mentions of fumes here with regard to the dust
collection system. It has been my understanding that the hazard is
related to dust, not fumes. This is more similar to the grain dust
explosion that can occur in grain elevators - but on a smaller scale.

BTW - Woodsmith magazine ran an article, in this months issue,
regarding small shop dust collection and duct options. They discuss
metal, flex and PVC as options and PVC was presented as a viable
option. They state that "Studies suggest this (static explosion) is
highly unlikely to happen with dust collectors in a home shop."
However, to alleviate fears (and probably make their lawyer happy)
they briefly discuss grounding using copper wire,

RonB