> Any form that doesn't have loose fibrils, microparticles, or dust.
Encasement breaks, materials erode, have to be manufactured and disposed. That's not a valid argument at all.
> I'd compare it to wood from walnut trees
Well, then you are very uninformed. The toxicity characteristics are nothing alike. If you want to point to a similar hazard, I'd suggest beryllium compounds.
I'd compare it to wood from walnut trees, which is perfectly safe to handle and use in dining tables, etc., but dust from its' woodworking is toxic.