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This article describes & diagnoses the cause of interior wall, ceiling, flooring or carpeting stains and explains how to recognize their probable cause and source, including soot stains, house dust stains, pet or animal stains, and thermal tracking or thermal bridging stains associated with building air leaks, and building insulation defects. See Black stains from animals for details about pet stains on building floors (urine) and walls (various) and see Pet Stains on Walls for diagnosing stains such as the black marks left by pets on walls. Readers should also see STAINS on Indoor Surfaces: PHOTO GUIDE and for outdoor stains, see Stain Diagnosis on Building Exteriors.

Often these stains are mistaken for toxic indoor mold. When investigating a building for a mold problem, you can save mold test costs by learning how to recognize Stuff that is Not Mold or is only Harmless Mold but may be mistaken for more serious contamination - save your money. Because some clients have on occasion sent samples to our mold test lab that really should not have been collected, much less looked-at, we provide this library of photographs of things that are "not mold" and don't need to be tested.

These are substances that you can easily learn to recognize in buildings. Save your mold test money, and increase the accuracy of your mold contamination inspection or test for toxic or allergenic mold in buildings: review these items to learn recognize non-fungal materials or even possibly harmless cosmetic "black mold" often mistaken for "toxic fungal growth." © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Diagnostic Guide to Soot & Other Wall, Ceiling, or Floor or Carpet Stains in Buildings

If you are able to rule out specific air leaks causing normal deposition of house dust, and if you are quite sure that you have an abnormal level of stains/soot like material showing up on various surfaces then these courses of investigation are suggested as an aid to diagnosing stains on indoor ceilings, walls, floors, carpets, cabinet interiors, closet interiors, or even house contents:

How to Separate normal "thermal tracking" dust deposition patterns from other types of indoor staining

If stains or debris are appearing on surfaces which are not cooled by their location (such as exterior walls, hollow interior walls which are entertaining internal air movement due to convection from below to above, areas near cooling air registers), then it may be possible to state with confidence that the stains appearing are due more to a high level of particulate debris in the building than to the more common thermal tracking phenomenon.

How to Perform Dust or Soot Source Identification

look for the source or potential sources of abnormal levels of indoor airborne debris, soot, particles, dust, such as a malfunctioning oil or gas fired appliance, any other combustion sources, even a mal-adjusted pilot light on gas stoves or heaters can be a soot source as well as the oft cited candles, fireplaces, and even pets (for example lots of dog traffic between indoors and out brings in high levels of dust).

If/when we can identify an unusual source or a source producing an unusual level of particulate debris we have perhaps answered a key part of this question of the probable source of indoor soot, dust, or debris stains.

Measure Indoor Moisture Levels when Diagnosing Indoor Stains and Soot or Debris Deposits

OPINION: even in a relatively clean home, unusually high moisture levels may result in noticeable levels of dust deposition on indoor surfaces, regardless of the dust source, and even including normal types and sources of house dust.

If moisture levels are a factor in the home, say moisture regularly above 55% RH, we would expect to see more-stained surfaces on those building surfaces that are more likely to be a bit higher in moisture, such as cooler surfaces on walls, ceilings, or in closets or cabinets where temperatures are lower and moisture may condense at a slightly higher level.

Indoor Stain Diagnosis Using Air Movement Patterns

Sometimes we can identify particular sources of air movement, directions of air movement, which we can correlate with the areas where we see staining. A simple example is the higher amount of dust deposition that occurs around heating or air conditioning supply registers on ceilings and walls. Relating air movement patterns to dust or soot or other debris stains may be diagnostic.

Also take a look at a short paper we wrote on an analysis of suspect indoor dust, at An Investigation of Indoor House Dust Debris where we determined that indoor dust levels which had been suspected of originating in an HVAC system were actually carpet dust and fibers.

How to Use Particle Identification to Diagnose the Source of Dust, Dirt, Soot, or Debris Stains

It is often possible to collect samples of suspect dust or debris for microscopic analysis in order to suggest a source or type of source of indoor stains.

It is essential that you select a forensic laboratory whose staff includes people experienced and trained in the identification of a wide range of indoor particles. A lab specializing in mold or allergen identification, for example, may not consider much less apply methods used to identify oil burner soot, common components of ordinary house dust, mite fecals, pet dander, human skin cells, fabric fibers, or other indoor particles which, if properly identified along with a statement of relative frequency in the sample, may be diagnostic.

How to Collect Indoor Dust, Debris, or Stain Particles for Microscopic Lab Analysis

How to Collect Samples of Stains, Dust, or Debris on Hard Surfaces

For particles or stains found on hard surfaces indoors such as walls, ceilings, or furniture, often a simple adhesive tape sample will perform best. Furthermore, this sampling procedure is itself diagnostic, since if the adhesive tape is unable to lift and collect any particles from the surface, that also tells us something about the type of staining present. But remember that even an apparently "clear" tape sample (when viewed by the naked eye) may contain important diagnostic particles which will be quite evident when viewed as a properly prepared microscopic sample and at proper magnification and lighting in a forensic microscope.

See Six Easy Steps to Get and Mail a Mold Test Kit to Our Lab for 24-Hour Analysis and Report for a surface sampling procedure using adhesive tape. You can use this sampling method to collect surface particles for submission to any qualified forensic laboratory not just ours.

How to Collect Samples of Stains, Dust, or Debris from Soft Surfaces like Carpets or Furniture

Qualitative analysis of dust and debris: We use an air-sampling cassette connected to a vacuum pump to collect debris from carpets, upholstered furniture, or carpets. In the hands of an experienced investigator a useful non-quantitative analysis can be performed to collect particles which, examined in the lab, can tell us the dominant particles present in the debris. The lab should also be asked to cite other particles, even occurring at low levels, if the particle type is particularly diagnostic of a potential indoor air quality problem. An example is the presence of chains of Penicillium/Aspergillus mold spores since when found occurring in chains, these spores are suggestive of a nearby (potentially toxic or allergenic) mold contamination problem.

Quantitative analysis of house dust: other vacuum methods which use a calibrated flow rate and a sampling filter can collect nearly 100% of the particles from a surface, permitting a quantitative analysis of the number of particles per square inch. In our OPINION this is often a rather questionable procedure. Even studies which claim to report that the results of this sampling method are repeatable (a measure of sampling method reliability) we find that there is an enormous variation, probably several orders of magnitude, in the number and possibly even the type of particles collected in such samples depending on the investigator's choice of sampling location, sample duration, vacuum strength, and other factors.

The result may be an analysis which is impressive in its precision, say giving 1,243.275 particles of particle type X per square inch of surface, but completely inaccurate (because there maybe so much variability due to sampling procedure that sampling an area one foot away gives a particle level of 124,327.5 particles per square inch.).

See PHOTO GUIDE TO STAINS on Indoor Surfaces for examples of some common types of indoor stains on HVAC registers, doors, carpets.

Usually soot marks, thermal bridging, or thermal tracking stains appear, if at all, in the building interior locations listed just below discussed in the remaining sections of this article.

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• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.

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