The most recent and increasingly common form of staining is caused not by dirt or dust but by soot (see "Dirt and Dust Also Cause Ghosting Stains"). Ghosting from soot is seen primarily in more recent construction, but diagnosticians have detected soot stains in older residences as well. Typically, newer homes--often still under warranty--are the focus of attention. There have even been reports of the problem in newly built, still unoccupied, model homes. Unfortunately, there are as many opinions about the causes of ghosting as there are occurrences of the mystery.

The black soot will outline items, such as ornaments and pictures hanging on walls. Some of the substances seem to have a particular affinity for plastics, such as coffeemakers, blenders, or garbage cans. Deposits have also frequently been observed along the traceline between carpeted flooring and the edge of draperies, vertical blinds and bed ruffles--even on the inside of refrigerators! The marks may be random smears or they may form clear geometric patterns, following the lines of the framing behind the surface. The marks range in size from small and isolated spots to soot running along the entire height of a wall.

Sources of the Soot

Investigations in buildings across the nation reveal multiple sources of the stains. Lab analyses indicate ingredients ranging from carbon soot (that might come from fireplaces, water heaters, furnaces, standing pilot lights, candles, cigarette smoke, cooking byproducts, and even automobile exhaust) to other ingredients such as paraffin, benzene, toluene, silicates, iron oxide, cellulose and cotton, dirt or clay, pollen and carbonates (typically found in airborne dust), common grease, and nicotine. One lab even reported that these black deposits "could be the result of carbon from automobile tires which becomes airborne as tires become road-worn" (although the authors were unable to substantiate this information). Occasionally, tests do indicate mold spores and/or mildew, but these are easily identified and should not be confused with the increasingly more common forms of black stains that are cropping up.

Carbon molecules act in ways that can make precise identification difficult. Although the black color often leads to the assumption that the sole ingredient of the material is from a combustion-related source, we have learned that this is not always the case.

Hydrocarbon compounds will seek equilibrium with the surrounding environment (Frick's law). That is, they will absorb whatever is in the surrounding air. As the concentration of pollutants increases or decreases, the concentration in the carbon molecules will change. The length of time required for this to occur depends on many different factors, ranging from ambient temperature and molecular weight of the particles to the polarity of the compounds involved. Although laboratory analysis can tell us if the sample contains carbon soot commonly found from incomplete combustion, it cannot positively identify the actual source of the material.

Cleaning Up the Mess

Unfortunately, we don't have all the answers yet. We still have much to learn about the interactions among the various forces and pollutants involved in ghosting. What combinations of relative humidity, air velocity, and source pollutants are necessary to create a problem? How long must these factors be present before a problem is apparent? What factors might exist that we don't yet know about?

We do know enough, at least, to be able to begin taking positive action. We know that houses work as systems. We know how to deal with driving forces by ensuring that the pressure and thermal boundaries of our houses are aligned. We know that houses should be tight and well sealed at the top, including wall-to-attic connections. We know that the air handler and ductwork function as an integrated system within the house system, and that they should be designed to complement one another and installed to meet recommended industry standards. We know that houses should be pressure balanced when the air handler and exhaust fan are in use and interior doors are closed. We know that insulation should be installed without compression and voids to avoid cold interior surfaces. On top of all of this, we know that houses should be performance tested to ensure that all recommendations and specifications are met. Finally, we know that homeowners--as well as builders and trade allies--must be educated on house-as-a-system issues and the consequences of various actions.