Why do people get so concerned over antler dust when knife carving. I wear a mask when I work with it. But what if you didn't wear a mask? What would the dangers be?
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.
I will make a comparison with wood. Ever have a splinter before? They can hurt and bleed and become infected in some cases. Now imagine small microns of bone dust in your lungs. They can cause minute tissue scars inside you that you cannot repair.
This is not because of the carving itself so much as the smoothing and finishing with other tools like sand paper.
Wood dust is a known carcinogen and I imagine that bone dust is in the same boat for the same reason. Most types of dust are in this category. Once that gets into your lungs it can cause irreparable damage. It takes very little to get into the air and stay there for a while before settling long after the work is done. This increases the exposure time. For more details of what happens you can read magerber's answer.
Not only should you be wearing a mask and eye protection but you need to be sure you are wearing a mask that is capable of stopping small microns. Something simple like a bandanna or a cheap mask used for simple paint jobs will not be enough. True, you might only be doing this for a short period of time in your life but it is very easy to mitigate the risk of this and getting new lungs is hard.
When looking at masks or respirators pay attention to the micron size they are rated for. Something in the 5 or lower range would be a good fit.