One of the most frequently asked carpet cleaning questions I’ve come across. What are those black lines? The answers is as follows -This type of soiling is called Soil filtration or draught marks . Its the airflow contaminants that are trying to pass through a door, the crack in the wall or gaps in floor boards. This contamination is similar to what you find in your air conditioning filter.As the air passes through the edge of the carpet pile, the microscopic soiling is deposited there, (hence the term filtration soiling). This build up occurs over a extended period of time and is not noticed until a build up has occurred.
This type of soiling is very difficult to remove because the soil particles are so fine that they penetrate deep into the carpet pile, sometimes into the backing of the carpet. So even when you remove them from the pile you may still see the stained carpet backing.
Removing this type of soiling requires special cleaning agents to assist in the process.
To help prevent soil filtration, vacuum(with a good vacuum) and wipe the edges of the carpet on a regular basis.