I just checked my artists' handbook of hazards. By the way, according to this book, just about everything is hazardous. Some things that I have been working with my entire life (lead and turpentine) are supposedly so hazardous that you should never use them. Anyway, the book say the following about horn.
"... bone, antler, and horn dusts; and shell dusts may cause respiratory irritation and allergies and are usually slightly toxic by inhalation."
Ivory and mother of pearl dust may cause a pneumonia type illness and worse in the young. It would seem that most plant fibers are highly toxic if inhaled. Also, oak and beech dust are known carcinogens.
So, in the grand scheme of things, horn dust is about as harmless as anything gets. That is, as long as it is sterilized and cleaned. Unsterilized and uncleaned(with a degreaser), it can harbor all kinds of nasty things.
Mark E.