Dust particles in the air.
The door closes on the carpet. The draught running underneath the door streams through the carpet and filters the small dust particles that become embedded in the fibres.
With synthetic threads and fibres, it helps to rinse with water using a spray extraction method. Contact James for professional cleaning.
Yarns mixed with wool or made from 100% wool cannot be cleaned. Cover the carpet area with a strip of (stainless) steel, or place a strip of a darker coloured carpet such that any discolouring is not visible anymore.