Carpet edges always look dirty? Cleaning is difficult, but not impossible.

Q: I live in an old house. I finished the attic stairs and living space with white synthetic carpeting. After several years, the edges of the stairs, especially where the risers meet the wall, have turned almost black. The carpet edges in the room also are turning dark. Vacuuming doesn’t budge this discoloration, and neither does carpet cleaner. It just looks dirty. Is there a solution?

A: You have what folks in the carpet-cleaning business call “soil filtration” and what energy-efficiency experts often call “ghosting.”

Where the risers meet the wall and under the baseboards at the edges of your living room, air is moving through gaps because of air pressure differences caused by temperature variations, wind or fans running your heating and cooling system. The carpet is filtering that air quite effectively, trapping enough tiny carbon particles to build up into stains.

The particles could come from tobacco smoke, a fireplace, incense, gas water heaters or furnaces, even pilot lights. When Building Science Corporation, a consulting company in Westford, Massachusetts, studied the problem, it identified candles as a key source and blamed the candle industry for not warning consumers that the smoke could stain carpets.

Edges of a room are especially prone to the stains. Thick carpet padding covers most of the floor, but there usually isn’t padding at the edges. Instead, there are tack strips to hold the carpet in place. Air can seep around the tack strips better than it can push through thick carpet padding. Builders today often install caulk, foam or another material between the sill plate (the bottom of the wall framing) and the subfloor to minimize drafts and air leaks, but an older home such as yours would not have that.

Unfortunately, the sooty particles are not easy to remove. Pete Diehl, owner of Pristine Tile & Carpet Cleaning in Fredericksburg, Virginia, said he’s found that only a cleaner designed specifically for removing soil filtration works, and even that is not always effective.

He uses Prochem Filter Out, available from cleaning-supply companies such as Jon-Don, which sells it for $15.26 a quart. Other brands also supply these cleaners. Matrix Soil Out Filtration Soil Remover is $11.56 a quart at Jon-Don.

Edge cleaning is expensive because the cleaning solution needs to be scrubbed in by hand with a soft brush or a carpet grooming tool and then extracted, ideally with a unit mounted on a truck outside. Cleaning edges of carpets is harder than cleaning the main expanse of a room because of maneuvering the carpet-cleaning tools.

Diehl’s usual carpet-cleaning fee is $100 for the first room and $25 for additional rooms. But when he needs to remove filtration stains from edges, he adds $50 more per room.

Edge stains are so common that when potential customers call and inquire about rates, one of the first questions he asks is whether they have a situation like yours.