Start with the hvac system.
Dirt around edge of carpet.
Dark dirt marks around the edges of your carpet are caused by an issue called filtration soiling.
To clean up these stains you need water and vigorous scrubbing.
Let the carpet dry then vacuum again to remove any surface dirt and loose fibers that remain.
Make 2 3 passes along the skirting boards with your scrub brush.
As the air is forced through the ventilation system it picks up whatever dirt is in the ducts and carries it into the living areas of the home and thus potentially into the carpet.
Filtration soiling is pollution that comes from dirty ductwork and settles on your carpet.
The first step to preventing filtration soiling is to be sure that your ducts are clean.
While your carpet edges are still damp use your carpet edge cleaning brush toothbrush or grout cleaning brush to scrub the carpet.
You can also use a dampened white rag to scrub your carpet edges.
The black lines are a result of filtration soil.
This soiling is called filtration soiling because it comes from air passing through the carpet as it is attempting to get through the crack between the.
Preventing filtration lines filtration soil around the edges of the room is a fact of life in most homes with central air systems but you can minimize it by cleaning your furnace air filters regularly or replacing them when they are dirty.
Along the edge of walls where the carpet meets the wall.
The carpet acts as a filter gathering the dirt from the air.
You may need to use detergent for more persistent stains.
Along the edges of carpeted stairs.
Along the lines of floorboards under carpet only in extreme cases.
Along the corners of a room.
You can find evidence of filtration soiling anywhere airflow is restricted near carpeting.