Dark grey or black lines along the edges of a carpet, at the skirting boards, under doors and around the bottom of stairs, are filtration soiling. They form where air is drawn through the edge of the carpet and the pile acts as a filter, trapping fine airborne soot and dust in a sharp line. It is not ordinary dirt, it is very fine, oily soot bonded to the fibre, which is why a vacuum does nothing and the lines are genuinely hard to remove.
Why the lines appear exactly where they do
Air moves through the small gaps under skirting boards, around doorways and at the edges of stairs, drawn by draughts and the pressure differences in a house. As that air passes through the carpet pile at those edges, the carpet does what a filter does: it catches the fine particles the air is carrying and lets the air through. Over months, those trapped particles build into the dark line you see. The soiling is concentrated at the edges precisely because that is where the air is being filtered.
Where the soot comes from
- Burning candles, a major and often surprising source of fine soot indoors.
- Fireplaces and any indoor combustion.
- Cooking, especially frying, which puts fine oily particles into the air.
- Outdoor pollution and traffic drawn in through the home.
The particles are extremely fine and slightly oily, which is what makes them cling to the fibre rather than vacuum away like ordinary grit.
Can filtration lines be removed?
Often they can be greatly reduced, but honesty matters here: filtration soiling is one of the hardest marks to remove and it cannot always be eliminated completely. It needs a specialist approach, working the edge with the right chemistry to break the oily bond and lifting the soil out, rather than the general clean the rest of the carpet gets. How much comes out depends on how long it has built up and the carpet fibre. We will tell you what is realistically achievable on your carpet before we start rather than promise a perfect result.
How to slow it down
- Burn fewer candles, or switch to cleaner-burning ones, this alone makes a real difference.
- Ventilate well when cooking and using a fireplace.
- Seal gaps under skirting boards and around doors where you can, to reduce the air being filtered through the carpet edge.
- Clean the edges periodically before the soiling has years to build and bond.
Common questions
What are the black lines around the edge of my carpet?
They are filtration soiling: fine airborne soot and dust trapped where air is drawn through the carpet edge at skirting boards, doorways and stairs. The carpet acts as a filter and catches the particles in a line. It is not ordinary dirt, but very fine, oily soot bonded to the fibre.
Why won't the black lines vacuum out?
Because the particles are extremely fine and slightly oily, so they cling to the fibre rather than lifting like ordinary grit. Removing them needs specialist chemistry to break the oily bond and proper extraction, not vacuuming or general cleaning.
Can filtration soiling be removed completely?
It can often be greatly reduced, but not always removed completely, it is one of the most stubborn marks there is. How much lifts depends on how long it has built up and the carpet fibre. An honest assessment will tell you what is realistically achievable on your carpet.
For filtration lines along your carpet edges, contact us or request a quote for an honest assessment of what can be improved. See our carpet cleaning page.