Is there a dark stain along the edge of your carpets or under doors that are often shut? The odds are you have what's called carpet filtration, which is not only ugly to look at but also a symptom of poor air quality and potential problems with your heating and air-conditioning system.

While carpet filtration has presumably been around as long as carpets, it's become a much more high profile problem in recent years, in part because of lighter-colored carpets and an increase in certain kinds of particulates in the air, particularly candle soot.

Here's what happens in carpet filtration. Air moves under baseboards due to air-pressure differences. As the air moves through the carpet, the carpet acts like a filter, removing particles from the air that are deposited on the carpet. This also can occur under a door that's often left closed. Air coming into the room from sources like the heating and air conditioning system pressurizes the room, and air is forced under the closed door, again causing the carpet to act as a filter. Carpet filtration can occasionally happen when air is forced up through joints in the plywood subfloor, leaving a series of dark stripes.

A Right Choice High Tech Audit can discover what is causing the airflow that's depositing the stain and can suggest ways to prevent the problem. It takes advanced testing to determine the cause, because the reasons for carpet filtration vary from house to house. A house operates as a complex system and various things can cause pressure differences.

One cause of pressure differences is leaks in ducts. If there are leaks on either the return or the supply side, more air will either be taken from the building or supplied back to it, causing either negative or positive pressure. Exhaust fans, such as those in bathrooms and kitchens, can also create pressure imbalances, because they send inside air outside. The reduced air pressure inside the house can draw in outside air. In addition, certain kinds of temperature differences can cause air to move because of convection — hot air rises from the first floor and in doing so, goes through the first available opening, the wall-floor joint.

The fixes may include repairing leaking ducts; caulking around windows and doors; caulking the top plate in the attic; sealing around penetrations into the attic for wires or recessed lighting; and making adjustments to the duct system.

In addition to airflow problems, the other factor required for carpet filtration to show up is a source of small particulates. Most experts see candles as a major culprit and see the current popularity of candles as being one reason why carpet filtration has become more common. To reduce candle soot, use high quality candles and keep the wick trimmed to about a quarter inch above the wax pool. If it's the romantic lighting level you're after, try adding dimmer switches to your overhead lights and buying decorative accent electric lamps.

Other sources of these tiny particles can be wood and gas fireplaces; water heaters; furnaces; pilot lights; incense; cigarette smoke; and automobile exhaust in houses with attached garages.

Remember, if you see these particles staining your carpet, they're also in the air and can affect your health, particularly if family members have asthma or other respiratory problems. The carpet filtration stains can also be a symptom of a problem that's reducing the efficiency of your heating and air conditioning system and raising your utility bills. Schedule one of our Right Choice Home Checkup services to find the source of the problem to make sure it's fixed right.