Why is wearing a mask recommended when carving antler?
I think @matt's response is exactly correct, but I wanted to add a bit more to it. There are two concerns about inhaling dust--the size of the dust particle and the chemical composition of the particle.  There are lots of articles on-line that talk about how dust inhalation causes problems, but I will summarize this one from the Canadian Centre of Occupational Health and Safety.

In general, your body has lots of systems designed to move dust particles back out of your lungs when you breathe them in. The first line of defense is a layer of mucus--the dust gets caught in this mucus and then these teeny hairs called cilia all wave together (imagine a crowd doing "The Wave," or someone crowd surfing at a punk music venue) to move that dusty mucus out of your lungs and into your throat where you can cough it out (or swallow it--blech).  

Some very small dust particles can get past the mucus and cilia and end up in the very tiny air sacs in your lungs called alveoli. When this happens, your body produces cells called macrophages that surround the dust particles, and physically move it to the cilia so that they can do their job and move the particle out of your lungs. The lungs also produce a protein that neutralizes the dust particles.

But, if you are inhaling a large amount of dust at one time, or over a long period of time, it can overwhelm your body's ability to produce microphages.  The dust particles accumulate in your lungs and reduce your ability to breath deeply--I imagine it something like a roof drain pipe clogged with leaves. 

And some dust particles are composed of chemicals that interact with your body in nasty ways--apparently cigarette smoke actually paralyzes the cilia--so even if your microphages were working perfectly, it wouldn't matter. And there could be pathogens, bacteria or mold spores in the dust that could make you ill.

So, in general, if you are working on something that is likely to create small dust particles like woodworking, carving antlers, using dye powder to dye fabrics, working with ground glass for fusing, etc., you want to limit the amount of dust that you inhale into your lungs.  Each exposure may not be dangerous on its own, but combined exposures over the course of your life can really build up and cause problems.