Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16Carpet Stain Test Procedure
Carpet Contamination or Stain Test Guide

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Tests to identify carpet or furnishing stains:

This article explains how to recognize, identify and if necessary, actually test stains that appear on indoor carpets, rugs, or perhaps on upholstered furniture. We help sort out stains from air leaks & house dust, pet stains, mold contamination & stains, spills, water damage and other stain causes. We also explain that most carpet stains do not need to be tested - save your money.

This article series describes methods for diagnostic testing of interior stains on carpets, rugs and on other surfaces in buildings. While we can usually identify the key components of a building stain by appearnce, location, surroundings, & apparent cause, some stains & marks in or on building materials & surfaces can be ambiguous and some may represent harmful or even dangerous contaminants or unsafe building conditions.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Diagnostic Test Guide for Mold, Pet Stains, Soot & Other Carpet Stains in Buildings

Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16Before testing an indoor stain for identification or hazards

  1. Rule out natural color variations: Some apparent building stains may not be an actual stain at all but rather the original color or color variations in natural materials such as the birch plywood door shown at left. Sometimes what you think is a new building stain is actually a color variation that has been in place for some time, but was not previously attended.
  2. Identify thermal tracking stains from normal house dust: 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. We will explain this distinction in more detail next.

Black or Gray Stains on Carpets: Perform Dust or Soot Source Testing & Source Identification

Black stains on carpet at walls (C) Daniel Friedman JBSoot, dirt or abnormal thermal tracking from air leaks can leave black marks on walls and on carpeting, often at the wall-floor juncture. 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.

Usually it is not necessary to test stains such as the one in our photograph to understand the cause and cure of the trouble.

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.

For an explanation of air leak dust deposition and staining on carpeting see CARPET STAINS by AIR LEAKS.

The full explanation of soot and dust deposit formation in patterns in buildings is at (THERMAL TRACKING & THERMAL BRIDGING).

Pet Stains on Carpets

What about pet urine or poop stains on carpets or rugs? Those stains will of course be found in homes where there are or have been indoor pets, and they tend to be round or oval and tend to be located in the same area, as once an animal urinates or leaves feces in an area the remaining odor marks that spot as their toilet. Usually you do not need to test these stains.

Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16

See details at ANIMAL STAINS & MARKS in BUILDINGS where we discuss animal marks on walls, floors, carpeting or other surfaces. We offer suggestions for cleaning, deodorizing, or removal of pet stains.

Mice pee indoors too, but you're not likely to see their pee as carpet stains.

Spills on Carpeting

Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16

Spills of drinks, food, soup, or water, even if mopped up and cleaned promptly enough to leave no immediately visible mark may later produce discoloration that shows up as a stain or even as an area of mold growth. That's because of the difficulty of cleaning carpeting enough to remove all of the nutrients left in the carpet fibers by the spill.

In our photo above, the two round blotches of mold growth were ultimately explained by spills on the floor in that area.

Usually you do not need to test to identify mold stains on carpeting or rugs. Knowing the the genera and species of mold found in a building will not change the recommended mold cleanup and removal procedure. But if there is a large mold reservoir (more than 30 sq.ft. of contiguous mould) and if your doctor says that speciation of the dominant mold genera/species is useful for medical diagnosis and treatment might you need to identify the mold.

Watch out: Before testing for "toxic or allergenic mold" see these two articles


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

OK so you've decided you want to collect test samples of stuff: stains, dirt, debris, suspected mold, perhaps to try to identify the source of mysterious particles or soot.

Hard Surface Stains: 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 un-upholstered 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 DUST / MOLD TEST KIT INSTRUCTIONS 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.

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

Carpet stains in rental  house 3W © D Friedman at Details about particle collection using adhesive tape or vacuum cassette test methods for carpet debis or stain or suspected-mold contamination are detailed

The dark areas of stain on the carpeting at left were found in a rental home owned by the author [DF] after tenants had moved out. The stain pattern suggests filth and debris staining, perhaps combined with spills, but there may also be mold contamination if the carpeting was wet.

If you see stains in an organic growth pattern, typically round colonies that can vary up to a foot or more in size, or perhaps following an area on which food was spilled or water leaked, the stain may be any color: brown, red, black, yellow, and may be mold growth.

See MOLD APPEARANCE - WHAT MOLD LOOKS LIKE for help in recognizing mold growth in buildings.

If you are sure the growth is mold, testing is not necessary. Wall to wall carpeting that has become wet or moldy needs to be discarded.

But for uncertain stain identification testing might be useful in deciding the cause of staining as well as in answering the question of whether or not the carpeting must be discarded.

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.

Watch out: 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 THERMAL TRACKING BRIDGING GHOSTING article series.

Carpeting and IAQ Articles


Continue reading at CARPET CONTAMINATION TEST PROCEDURE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see ANIMAL STAINS & MARKS in BUILDINGS for pet stain analysis, treatment, and removal.


Or see CARPET STAIN DIAGNOSIS - various causes of stains on carpeting, mold, dust, pets, debris

Or seeMOLD APPEARANCE - WHAT MOLD LOOKS LIKE for help in recognizing mold growth in buildings.


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CARPET STAIN ID TESTS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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