to be frank,i wouldnt worry about any of it personally.

if there was anything out there that was that bad we would already know of it.can there be something that technically isnt good for you....sure but in reality how much harm can it do?

honestly when i cook i have never given a thought as to what type of wood i burn or use few skewers and never really will,except like i said i wouldnt want to use poison ivy as a skewer even though i'm not allergic to it