Dust Masks

Dust masks are NIOSH approved and are much better than the single usage paper model, but still very affordable.
These masks have a thin metal strip across the bridge of the nose that you can bend to fit your face for a good seal. They also have a foam pad inside for comfort across your nose.
Powdered dyes are micro fine and prone to suspend in the air. Sometimes they are so small they are barely visible. The instructions for using these powdered dyes (and most of the books on the subject) recommend the use of a dust mask.
Soda Ash Fixative is a powder that is somewhat caustic and should also be mixed when wearing a dust mask. Common sense really dictates wearing a dust mask when mixing any powdered substances.