Yes, metal clay and carbon are non-toxic. That doesn't mean it is healthy to breath in dust. It is a good idea to wear a dust mask when sifting carbon or sanding your pieces. Save yourself a trip to the hardware store / nightmare home improvement warehouse by adding one of these to your clay order. 3M brand, 8110S model.
Don't worry, I'll send you a brand-new, never been worn mask. I just thought you might appreciate seeing what it looks like on my mug.