Encountered pollutants in the house?

Hello everyone,

I was just working on a suspended ceiling in my mother's house with a sanding machine because a window no longer opened well and was scraping along the ceiling. After sanding off the paint, a brown base appeared, a bit like cardboard, which forms small beads or fibers when sanded. I attached a photo – does anyone know what this is and if it is harmless?

Thank you for the help!!
Sebastian

Looks like the box of a drywall panel. Poke in with the tip of a utility knife and see if it’s white behind the brown layer. And tap on the surface, does it sound hard like a stone or more like 1.25 cm of compressed gypsum ;-)