Have you noticed in your house or apartment a darkening line forming around the edges of your baseboards? Something that looks like a dark lint collecting along the edges of your carpet. These are called filtration lines and are a pretty common issue in homes with carpets.
Most people believe that walls are solid, impermeable barriers that would not pass air (that's what the insulation is for, right?) the reality is that its not. Frequently air will go under the baseboards and into the wall, where it goes through cracks, holes and structural gaps to create an air current. When there is carpet is present, dust, fibres and other particulates will act like a filter for the air passing through. Otherwise it usually collects as dust balls on hard floors, which are easily swept up.
Removing filtration lines is not an easy task. Regular Vacuums can get some of the larger dust particles or lint, but it is unable to remove fine particulates that get trapped into the fibres. They often have stronger attraction to the carpet than normal dust, either due to electrostatic attraction or due to oils.
The other problem with filtration lines is its placement. Since they only occurs close to a wall, a regular cleaning head on the machine (with a wide mouth like the one on a stand up vacuum)can't create enough suction on the edge to remove the line. Instead we use a special attachment on the extractor.
Really it is just a more advanced version of the needle nosed attachment you see on your home vacuum, but with a narrow spray nozzle for the hot water. This is critical, as the dust and debris often has a stronger attachment to the carpet fibres than normal dirt.
So here is an corner of an apartment that hasn't had its carpets cleaned for several years if ever. The filtration lines are pretty apparent, despite the dark carpet colour. Alex starts on these first, allowing him to come back later if his first pass is insufficient. He also takes care not to damage the baseboards by applying too much pressure.
And that's that.... looks pretty good, I'd say