A reader wants to know how to clean the edges of carpet that appear discolored. (Reader photo)

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?

Fredericksburg

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 tiny carbon particles that 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, Mass., 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 (540-841-3907; pristinepete.com ), 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 include Matrix Soil Out Filtration Soil Remover, $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 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. “People think it’s just dust, and it isn’t,” he said.

You don’t say what type of carpet fiber you have, other than calling it synthetic. If it’s nylon, you’re in luck. “It’s my favorite,” Diehl said. “It cleans best for us.”

Once the stains are removed, they will probably come back unless you address the underlying causes. To stretch the time between cleanings, eliminate sources of fine carbon particles as much as you can. If you burn candles, give that up. If you smoke, maybe this will prompt you to quit.

Beyond that, there isn’t much you can do except vacuum regularly. Use the crevice tool to clean along baseboards.

When it comes time to replace your carpets, seal gaps between the floor and baseboards with rope caulk or expanding foam, or call in a company that specializes in sealing drafts. And for your new carpet, choose a color other than white.