These marks are caused by the movement of air passing through the carpet as it is trying to get through the gap between the carpet and the floorboard, or skirting board.
Air contains pollution and it will find its way into your home. The main reason is that hot air rises. Even where the carpet gripper pierces through the carpet backing the air will flow through and form black dots on the surface of the carpet.
Because most homes aren't airtight, the air moves between these gaps. The carpet then acts like a filter, trapping the solid particles.
A good way to prevent it is to lift up the carpet and lay thick sheets of card, or paper overlapping them and sticking them down with tape, then sealing around the edges with tape too. Once you have sealed the floor properly you will then be able to lay your carpet.
If the marks are already there, your options are limited. You can try to live with it. Or you can call an expert to tackle them. Trying to remove them yourself can make things much worse. I never take on draught marks if someone has tried DIY methods, for this very reason.
Happily, as in the photo above, many draught marks can be successfully removed. Even the worst examples can be greatly improved with the right products and know-how.
If you're annoyed by nasty draught marks, give me Perry as call on 01304 381165 and I can come by and test them and give you an idea of the likely success.