Dark dirt marks around the edges of your carpet are caused by an issue called filtration soiling.
Dirt around edge of carpet.
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.
The first step to preventing filtration soiling is to be sure that your ducts are clean.
To clean up these stains you need water and vigorous scrubbing.
Along the lines of floorboards under carpet only in extreme cases.
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.
You may need to use detergent for more persistent stains.
Let the carpet dry then vacuum again to remove any surface dirt and loose fibers that remain.
Start with the hvac system.
While your carpet edges are still damp use your carpet edge cleaning brush toothbrush or grout cleaning brush to scrub the carpet.
Filtration soiling occurs when there is an imbalance in the air ventilation of the room.
Filtration soiling is pollution that comes from dirty ductwork and settles on your carpet.
You can find evidence of filtration soiling anywhere airflow is restricted near carpeting.
Along the edges of carpeted stairs.
The carpet acts as a filter gathering the dirt from the air.
According to procare the premium carpet upholstery care specialists.
Make 2 3 passes along the skirting boards with your scrub brush.
The black lines are a result of filtration soil.
Along the edge of walls where the carpet meets the wall.
This will help release any trapped dirt dust or debris.