Why have I got dark grey or brown lines under doors, curtains and along the edges of stairs?

Filtration soiling is the accumulation of airborne soil – dust, smog, tobacco smoke and other pollutants - where airflow is concentrated and directed over or through carpet pile. The carpet "filters" out these pollutants and gradually becomes soiled and dark. It is not an indication of a defect in the carpet or low quality. It can appear on any carpet regardless of price, style, colour or fibre. In most cases the airflow is created by heating and air conditioning systems, thermal expansion and contraction of air or natural convection currents in the structure of the building. Filtration soil can be very difficult to remove as it consists of very fine particles, often oily in nature and typically the soil remains on the fibres for months or years. It can take several applications to remove, is labour intensive and therefore can be a costly process.  more