We all have seen those dark lines under a closet door or around the edges by the walls. Those are called air filtration lines. Let’s take the line at the closet door as an example.
When your air conditioner or heater turns on, the air pressure shifts around in your house to reach equilibrium. So the air moves back and forth under your closet door through those carpet fibers. All the dirt and allergens that are in the air of your house are deposited on those fibers creating a dark line. Those dark lines are an indication of your indoor air quality. If your lines are particularly dark then there are some steps that you need to take to clean up your air. Look for indoor air quality tips on our website.
Back to those dark lines. The carpet in the open areas are acting like filters too, just not in such a concentrated way so it takes longer for them to show dirt.
Carpet is designed to hide up to a pound of dirt every square foot. So if you are developing soiled traffic lanes then you have waited too long to have you carpets cleaned.