In general, you should ALWAYS wear a dust mask when mixing media, no matter what it is, and also keep a spray bottle handy to mist it and keep the dust down. Asbestos is dangerous when it becomes airborne; keep your medium damp and your face covered and you are pretty safe.
I got the worst double sinus infection of my life from mixing bat guano and mycostim without a mask once... 6 weeks later, I had learned my lesson.