Fitted carpets often develop a "Dirty Edge", which shows itself as a growing grey/black mark at the edges, near the walls. Most people think that the carpet is simply dirty but it doesn't necessarily happen in all rooms, or even to all the edges in the same room.

A clue to the reason is the fact that this effect generally happens "Only" at the edge.

Today carpets are normally secured using gripper rods very close to the edge, near the wall or skirting board. However, you can't position these gripper rods precisely against the edge and so there's a gap where there's only the floor boards under the carpet.

Air bricks are built into all buildings to create air circulation but where the floor boards aren't fitted closely to the skirting board it can cause an updraft, pushing air from below the floor up above them. Any small particles of dirt can be blown up through the floor boards and into a carpet above, causing a slow but surely increasing dirt mark (the same can happen for gaps in an underlay in the center of a carpet illustrated by dark lines of dirt appearing where underlay has gaps).

Lift the edge of the carpet from the gripper rods and cut thin strips of plastic that will cover the area between the rods and the very edge of the skirting. In fact, it's best to let the plastic bend slightly round the edge and slightly up the wall. If you select a thin enough plastic the gripper rods will retain their effectiveness and you can simply leave the plastic down to stop the problem recurring.

Now use a powerful vacuum cleaner to try and suck as much of the dirt out of the edge as you can. Hitting the edge with a brush and using a thin-nozzle attachment will help you get as much out as possible.

Once this is done clean the carpet using your chosen method and repeat if necessary. Even if you see some evidence of wicking back some dirt this now won't be from "NEW" dirt coming from under the floor boards so the problem should slowly get better.