[Adam Eastman]

Just wondering if anyone knows the most common reason for build up of dirt around the perimeter of the room is? I see it a lot when going to replace old carpets, there is a dark line around the perimeter of the room or in the doorway and never fully understood what causes it. 

[Deep Cleaning Solutions]

Search the forum for draught marks or filtration marks as they are the main culprits. Also some carpets are fitted at an angle on the edges by some fitters that gets missed by the hover unless the home owner can be bothered to use the cresive tool which most can't including me 

[Hilton]

It's a draught mark caused by carpet filtering out dirt in the air that's flowing around the home.
Its most common around the edges because that where the skirting meets the flooring ,

[Deep Cleaning Solutions]

Adam. I have found Prochem Ultra Pac Renovate to be far the best product to use on draught marks. Sometimes I will use Prochem citrus gel watered down if I have no ultra pac in stock which works okay but is less economical.

[Steve Chapman]

Could also be mildew from condensation on the walls........

[Dave_Lee]

Filtration or Draught marks, can be caused from fine dirt particles being blown or sucked through the carpet fibres and can originate from underneath or above, at skirting board gaps, floor board gaps, beneath doors and through the holes in carpet backing from carpet fitters knee kicker teeth penetrating right through.
I was always of the opinion that most of it was caused by particles from underneath, but most evidence seems to indicate that it comes from above.
Whatever the main cause is it is down to air movement within the home.
Dave.

[John Kelly]

Near busy roads it is usually Diesel particulates. You even get it in new builds if there is bus and lorry traffic passing. Worst I seen was 12 inches all round room in every room.

[Hilton]

Worst I have seen it the outline of every floor board in a very plush and very large central London apartment,It looked quite impressive.

I think I have said before on here that I knew a guy that used to do draught mark 'mitigation' for London stores including John Lewis, doubt they do it now but he had a very good business.