Black stains like this one in a doorway are often caused by soot from burning candles.

Here are my questions:

1. I have an air filtration system, gas heating, a gas fireplace, and an air conditioner. Should I have anything else installed to help with the problem?
2. Would having the air ducts cleaned and sterilized help?
3. Does this mean I can't burn candles in my home? Would I still be able to burn a candle every once in a while and not have the problem reoccur?

I should point out that there is some irony in the fact that you--like so many others--are attempting, at a substantial cost, to apply Band-Aids so that you may continue with the practices that you suspect are causing the damage. Rather than deal with the source of the problem, you are seeking ways to mask it. Your letter indicates you suspect candle use as the cause of the soot deposition, and given your admission that you burn candles for most of the day, every day, it appears that this may be the likely culprit. Studies have shown that candle burning can, in some cases, result in soot deposition in the house. It would probably have been cheaper to stop burning the candles than it was to replace your carpet with a darker color.

However, you also indicate that you have a gas log fireplace, as well as a furnace. Are you certain that neither of those items may be the culprit? How about mold? Additionally, it could simply be dirt filtering through the carpet, as a result of mechanical pressures or stack effect, which gradually grows darker as the concentration builds. To rule out all the possibilities, you would need to hire a professional home performance contractor to do an investigation and take samples to determine the source of the black stains. A simpler way to determine if the main problem is your candles, however, is simply to stop burning the candles for a few weeks and see if the black stains stop forming. Once you've tried this process of elimination and feel pretty certain that your problem is candle related, then let's discuss the following: 

Regarding your air filtration system: You don't mention what type of filtration system you have. We have found electronic air cleaners to be fairly effective in reducing particulate count in the air. However, it is important to clean the air cleaner inserts frequently to maintain efficiency and to minimize any potential for ozone production. Six-inch pleated media filters also work reasonably well in reducing airborne particulates. Just keep in mind that no filter will guarantee a problem-free house. If the rate of pollutant production exceeds the rate of pollutant removal, you will have a problem, period. 

Soot particulate is cumulative and can remain airborne for extended periods of time (even days). To expect that all the air in your house will pass through the filtration system is optimistic, at best. Chances are excellent that if there is excessive particulate matter in the air, at least some of it will deposit somewhere in the house before the filter has a chance to remove it. If other forces are present, such as attraction (the static charge from plastic items such as blenders, toasters, and other appliances), the soot may deposit on these items before it has a chance to pass through the air filter.

Your second question, regarding duct cleaning, is a bit more difficult to answer. While the Air Duct Cleaners Association has established fairly rigorous guidelines for duct cleaning, studies by the U.S. Environmental Protection Agency have determined that these guidelines are not commonly practiced in the field and that duct systems are typically not much cleaner after having been serviced. (See "What Does Duct Cleaning Do?" p. 13.) I would not recommend it in this type of situation.

Finally, if you wish to continue burning candles, we suggest you choose only high-quality candles. Avoid cheap foreign imports, which often have unknown ingredients and are poorly made. However, not all domestic candles are of high quality. There are claims that beeswax candles burn more cleanly than petroleum-based candles, but it's too early to say for certain without further studies. Choose candles that have little or no fragrance, and follow the manufacturer's burning recommendations carefully. This includes keeping the wick trimmed to less than 3/8 inch at all times. Do not burn candles anywhere a draft may occur--in front of or near heating registers, fans, windows, or doors--or where children are playing. Follow burn time recommendations (some candles require extensive initial burn times to ensure proper melting, then reduced times for subsequent burns). Extinguish the candle by dipping the wick into the melted pool of wax rather than by blowing it out. Make sure that no debris (burnt match heads, wick trimmings, pet hair, and so on) is left in the melted wax. And last, avoid the use of jar candles, soft jelly candles, and liquid oil lamps, as these have been shown to be more prone to poor burning (which leads to increased soot release). 

Finally, remember the rule Amount + Time = Results. The amount of particulate that is in the air, plus the length of time it remains there, equals the result--that is, the potential for it to deposit on a convenient surface. Common sense and prudent use will minimize the potential for soot deposition.

Frank Vigil is a senior building science specialist for Advanced Energy in Raleigh, North Carolina. Vigil has been instrumental in research, testing, and diagnostics of black stain deposition in homes. 

Carpeting, as well as its backing materials (including pads) and adhesives, is a source of volatile organic compounds (VOCs) in indoor air. Up to 57 different VOCs have been identified as carpet-based emissions, but the most prevalent is 4-Phenylcyclohexene (4-PC), which originates from styrene-butadiene latex carpet backing. While there are no World Health Organization guidelines or OSHA permissible exposure limits to 4-PC, this VOC may cause headaches, dizziness, memory loss, and fatigue, and can be an eye and respiratory system irritant. Tolerance levels to VOCs vary greatly from person to person. In any case, total VOC emissions from new carpeting are low compared to those from many other building products, and these emissions decrease exponentially and can only be detected at trace levels after being exposed to air for one week. 

Far more important than VOCs in carpeting are other indoor air pollutants that accumulate from numerous sources. Carpeting can act as a sink for toxicants like lead dust, which can be tracked in from contaminated soil or can build up in carpets after it is released from sources in the house. Pollen can enter houses through windows and doors and also can accumulate in the carpets. Pesticides and herbicides can be tracked into a home on shoes and build up in carpets. Excess moisture levels in the house can contribute to the accumulation of biological contaminants in carpeting, including mold, mildew, and dust mites. 

Finally, carpeting should never be installed over an uninsulated concrete slab, because of the potential for moisture condensation on the slab and the resulting buildup of biological pollutants.

Joe Laquatra is an associate professor at the College of Human Ecology, Department of Design and Environmental Analysis, Cornell University. He teaches the course "Builders, Remodelers, and Indoor Air Quality" for the Homebuilders Institute, the nonprofit, educational arm of the National Association of Home Builders.
 

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