On Wednesday, 15 November 2017 10:27:18 UTC, Recliner wrote:

Not asbestos. Iron and brake pad dust, plus human skin detritus as you say.

The skin thing is a myth:
"Sometimes a specific percentage of dust is said to be skin, usually about 70 or 80 percent, but unless you’re a moulting bird or reptile very little of your environment is composed of dead body parts.

There are far more common sources of dust pollutants, including animal dander, sand, insect waste, flour (in the kitchen), and of course lots of dirt."
Source: the internet.