I sit directly under a air vent in my office building which is an older 8 story building. Every year when the air conditioner get going my desk gets sprayed with small black flakes. They are very small and hard but can be easily crushed into a power. I would guess its some type of carbon build up. The real question is this harmful to my health? The particals are gross and a pain to clean but it not normally anything I concern myself with. What is this stuff? Do I need to worry? Thanks for your info.

It is tough to say what it is. It could be mildew that forms on the insulation of the ductwork if it is lined on the interior of the ducts as in many commercial applications. The higher blower speed is knocking it loose. Yes it can be dangerous, and all of this is just a guess, since I can't see the system.