This article was originally published in the September/October 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online September/October 1998

Sampling Stains for Fun and Profit

by Terry Brennan


In January, Home Energy presented a checklist to aid home performance specialists investigating mysterious dark stains. Since then, stains have turned into a major home performance issue. Here are more techniques to help find the causes of stainsÑlab sampling, examination under a microscope, and a close look at particle colors.


Is it mold? Dust? Dirt? Or soot? Stains around a supply register can be difficult to identify without sampling.
This stain is clearly mold, with its pattern of dark spots surrounded by areas of decreasing darkness. 
Door undercuts are a common location for every type of stain. This dark deposit resulted from cigar smoke and candle soot driven under the door by pressure imbalances.
One way to identify a stain as mold is to rub it with bleach. This stain turned white when bleached, revealing that it was mold.

Dealing with Mold

Here are my suggestions dealing with mold problems:
  • IDENTIFY the extent of contamination, the dynamics of the moisture sources, and the personal protection you are going to need.
  • DRY the areas. Use emergency measures, such as a carpet dryer, for the short term, and implement housekeeping and maintenance practices to prevent future problems.
  • DISCARD contaminated material that is unsalvageable or not worth saving.
  • DECONTAMINATE the materials that are worth saving. Some will need only simple cleaning and sanitizing. Others will need to be decontaminated using containment and worker protection.
Black stains are not restricted to the inside of a house. In this house, soot was produced in the basement and vented to the outside. Then it entered the house again through a leaky window frame. 

Do-It-Yourself Microscopy

Microscopes are not the most common tool in the home performance expert's tool kit. But if you find yourself dealing with particles over and over again, it might be wise to get a microscope and some training. That way you can get a good idea of what type of particle you're dealing with right there on-site, without waiting for the results to come back from the lab. I have learned to identify particles by broad class, and I often examine the slides in the field with an old Bausch and Lomb light microscope.

In one recent case, the company I work for was able to determine that a mystery deposit in a two-story school was not a fiber, mold, or soot, and that it looked like fine sand. It was being deposited on horizontal surfaces, which meant that the particles were fairly big. Because it wasn't present in the supply ductwork, we knew that it was settling downward. 

We found the solution by looking up. The ceiling was made of precast concrete planks, with a joint every 3 ft. While we were crawling around the air handlers, we noticed that something was coming through the cracks in the ceiling and landing on the ducts. My partner took a sample and put it under his microscope. It was degenerated carpet cushion sifting through the floor from above.

With a microscope that goes from 200x to 800x and a bit of experience and training, you can learn pretty quickly to solve mysteries like this one.

Researchers from the Environmental Protection Agency prepare to collect airbone samples. Particles are embedded in a sticky substance inside the metal cylinders. 

Sampling Resources

Sampling protocols are available from:
  • Harvard School of Public Health: 677 Huntington Avenue, Boston, MA 02115. Tel:(617)495-1000; Web site:
  • American Society for Testing and Materials (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959. Tel:(610)832-9585; Fax:(610)832-9555; Web site:
  • American Conference of Governmental Industrial Hygienists (ACGIH): 1330 Kemper Meadow Dr., Suite 600, Cincinnati, OH 45240. Tel:(513)742-2020; Fax:(513)742-3355; Web site:
  • Information on and ads for testing labs can be found in Indoor Environment Review, 7920 Norfolk Ave., Suite 900, Bethesda, MD, 20814. Tel:(800)394-0155; Fax:(301)913-0119; Web site:

It's happening more and more-home performance specialists are getting calls from homeowners complaining of a mysterious stain. Maybe the occupants think it's mold, and they're worried-could it be Stachybotrys atra, which has been linked to sudden infant death syndrome? Or maybe they're concerned about the potential destruction of thousands of dollars worth of art, carpets, and wall finishes. These stains can be found anywhere in the house: on the wall or ceiling, in the insulation or framing, spreading across supply diffusers, or even atop plastic cups in the kitchen cupboard.

Sometimes the identification is relatively easy. When black stains appear in an unventilated, humid bathroom, the cause is probably mold. If the occupants are cigar smokers who never open their windows and burn candles every night, the stains are most likely soot. But home performance specialists are sometimes confronted by stains with no obvious cause and no simple cure.

Much attention has been focused recently on candles as the source of soot and black stains, but there are many additional sources (see Black Stains in Houses: Soot, Dust, or Ghosts? HE Jan/Feb '98, p.15). As a building scientist, I have examined soot cases that were the result of exhaust from cars and buses, combustion gas spillage, and unvented kerosene heaters. I've seen stains that looked like soot but turned out to be degenerating plastics or mold, and I once observed a stain investigation in which a microscopic examination of the stain pointed to soot, but further tests showed that it was caused by tire particles, which look very similar.

Over the past 15 years, my eyes have seen a lot of mysterious matter, from large scaly particles along or under baseboards that were deposited by insect colonies, to carpet matter that has deteriorated into a brittle, powdery substance. Common stain materials include the following: mold, skin flakes, fabric fibers, fiberglass, wood fibers, paper dust, degenerated plastics, tire particles, fine sand, clays or silts, and soot from a variety of sources.

While there is no universally accepted method of identifying stains, this article describes my informal protocol for determining their sources. After using this for an initial identification, if you wish to go further, you can go to a specialized lab.

That Important First Impression Stain sources are identified by combining a number of clues, which you can discover simply by using your senses. When you first arrive to do a stain investigation, take a good look at the building and its surroundings, because stains don't always originate from within the residence. Is there an attached garage? A bus depot across the street? Heavy vegetation surrounding the building? Exposed soil that could be the source of dust?

As you enter the house, note any odors. Generally, each type of stain has its own unique bouquet. Molds smell earthy and musty. Bacteria smell like old socks or vomit and are usually found in water. Combustion fumes tend to smell like exhaust, like something hot or metallic. If the heating system has been turned on after being off for a long period, particles that have collected on the surfaces of heat exchangers, fin tube radiators, or other heating system components will vibrate off when the surfaces heat up, producing a metallic smell.

But remember-your nose quickly tunes out most smells, so the first entry is your best opportunity for recognizing them and tracking them down. You may need to go outside and let your nose rest for a while if it gets saturated. You also might need to rest your lungs (if you sniff continuously for several minutes, you may hyperventilate).

After you record your visual observations and the results of your careful smell assessment, interview the residents. They may offer a glimpse of what's going on. Sometimes their ideas of the physics or biology of the situation pushes into Lewis Carroll territory, but they have often discovered worthwhile clues. Get a history of the stains. When did they first appear? Did they come gradually, or all at once? Have the residents tried cleaning them? With what? What happened?

Getting to Know Them Next, take a close look at the stains, without disturbing or inhaling them. If you need to disturb the particles, wear a respirator. I use a little aluminum respirator that fits my face snugly and also fits nicely into my tool bag.


Look at the color of the stains. Generally, if the deposited material is white or light gray, it is likely to be paper, fabric, or mineral fibers; mold; or skin flakes. Green or red means mold. Yellow or pink stains may indicate mold or mineral fibers. Brown, tan, gray, or brick red stains tend to be soil dust or fabric fibers. Black stains can be from soot, mold, soil dust, tire particles, or mineral fibers. Because a house is usually filled with cotton dust from clothing, blue stuff is almost always denim fibers.

Shapes and Patterns

Note the shapes of the deposits, and in particular, check for patterns. These can indicate whether a mold is present, how the stains were deposited, or whether a stain is from a combustion source.

If you see concentric circles or clusters in a splotchy pattern, the stains are probably mold (see Dealing with Mold). Mold forms competing colonies on a surface, so all other things being equal, these colonies tend to spread themselves out.

If you see a uniform layer of deposition (on vertical as well as horizontal surfaces) the stains are probably made up of small particles that have been deposited by diffusion in slow-moving or still air.

If you see streaks and eddies, the air was moving across a surface. Streaks and eddies are typical for stains from combustion sources because combustion produces tiny particles that behave like gas molecules. Little sand dune-shaped deposits indicate that the air has made a hard turn. I commonly find these on door and window jambs.

By examining the shapes and locations of the stains, you can tell which direction the air was moving when the deposits were made. They also indicate air pressure relationships. Noting these patterns will pay off later when you are looking for the source upstream.

Make a List

At this point, make a list of potential particle sources, and ask the occupants what activities go on inside the building. Burning or heating anything produces tiny particles, so ask if they light candles, make leaded glass windows, or fire clay in a kiln. Also ask if they have a gas stove or a kerosene heater. If they own a car, ask them to describe the starting procedure. Is it a diesel? Where is it parked? Do they let it warm up before driving?

Keep an eye out for potential sources of moisture. For most kinds of mold to grow, a nearby moisture source is essential. Do the occupants know of any damp spots in the house? Roof leaks? Basement leaks? Is the dryer vented to the indoors? Moisture sources sometimes are obvious, as with leaky plumbing or rotting roofs. Other times, they are more subtle, as in a house with an oversized air conditioner, or in a room where residents use a humidifier.

Labs Are Your Friends Many problems can be solved using the information you collect by direct observation and by talking to the occupants. However, sometimes a sample of the material must be sent to a lab for further analysis.

Whether or not you sample depends on a number of things. If you don't know what a stain is and can't find a likely source, you should take a sample. Sometimes you may want to test a hypothesis, such as This massive amount of paper fiber I'm finding is the same as the cellulose insulation in the attic. But you won't need to sample for airborne fungi, for example, when a patch of mold from a leaky roof is plainly evident.

If folks are trying to assess the damage to a building and determine who is responsible for having caused the stains, you should always sample. And you should identify and document in such a way as to convince an outside party of your results. There is sometimes a lot of money on riding on the correctness of an identification.

The Right Place, the Right Sample The secrets to a good lab analysis are good samples and the right lab. Different sampling methods are called for, depending on whether you are curious about the stuff on the surfaces, airborne particles, or biological particles. By carefully following protocols, you can collect samples that can be definitively identified under a microscope.

Before you collect any samples, you must find a lab that can deal with the type of contaminants that are present. This may seem like a Catch-22-you have to identify the pollutants to identify a lab that can tell you what the pollutants are. I solve this problem by using the identification protocol mentioned earlier to narrow the range of choices, and by using labs that deal specifically with indoor environments.

I haven't found any one-stop labs that can completely identify all particles. Use the closest environmental lab that says they can answer your questions. You might ask something like, Are all these mineral fiber samples from the same source? Are some of them fiberglass, and some mineral wool? or Are these samples diesel particles, soot from candles, or tire particles?

Some labs have the ability to find out which species of mold are present. Other labs have the ability to identify individual fibers-telling you, for example, whether the fibers found on a desk are the same as the ones taken from the inside of a return air plenum. Labs that are more specialized in asbestos identification, or ones that are essentially microbiology labs, can often identify particles by their generic source (paper fibers, fabric fibers, or mold spores, for example), but they do not typically feel competent to give a detailed analysis of something that isn't in their area of expertise. Many indoor environmental labs advertise in industry magazines such as Indoor Environment Review.

Another option is to contact a local college or university. They usually have many resources available for identifying particles. They have the technology, and they have knowledgeable faculty who are frequently very excited to help solve a mystery from outside the academic community.

Sampling Protocols Whichever lab you use, talk to its personnel before you begin sampling, because you need to use its protocols for taking and shipping the samples. The lab may have special procedures for you to follow. It is crucial to pay close attention to the lab's needs, or you may not get useful results. For example, you need to know how many samples to take; when, where, and how to collect them; how to store them; how to ship them; and how to interpret the results.

There are some types of sampling that you may not be able to do without proper certification. In most places, for example, only someone who is certified as having received the proper training can collect asbestos samples. Find out about, and comply with, regulations in the state and locality in which you are working.

If the sampling is not regulated, protocols from national consensus associations, such as the American Society for Testing and Materials (ASTM) or the American Conference of Governmental Industrial Hygienists (ACGIH), may be available (see Sampling Resources). It is best to use them in the absence of specific instructions from your testing lab.

Protocols for emerging issues are best found by searching the scientific literature. For example, I like the vacuum-cleaner and cellulose-thimble filter protocol used by the Harvard School of Public Health to collect samples of dust from surfaces and then have the dust assayed for biological components.

This Sample Is Just Right There are three types of useful samples to take: bulk samples, surface samples, and airborne samples. After the samples have been collected, you can either use your own microscope to identify the particles, or send them to a lab for analysis.

Get It in Bulk

Collect bulk samples by cutting out pieces of the contaminated material with a utility knife or sheetrock saw, and putting them into zip-lock plastic baggies. The samples do not have to be large. Bear in mind that a sample is representative only of the spot where you collected it. With bulk sampling, the most common error is sloppy record keeping. You must have a system that lets you track accurately which sample was taken where, when, and under what circumstances.

On the Surface

Surface samples are collected by vacuuming the particles into a filter (see cover photo), swabbing them with cotton swabs, or trapping them on a sticky material such as tape or plastic wrap.

When vacuuming surface samples, gather a sample of material from one square meter of carpet or floor, and collect the sample in a cellulose thimble filter. This does not require a special vacuum. Drop the filter in a plastic bag (labeling everything), and ship it to a lab.

Cotton swabs in sterile water are used to sample surfaces for fungi and bacteria. A lab can provide the swab in its own little bottle. After the swab is rubbed on a surface to collect material, it is returned to the bottle and shipped to the lab.

Another method of surface sampling is to stick a piece of tape to the sample and then transfer the stuck material to a microscope slide. With a piece of clear plastic tape (not the frosty stuff), make a loop between your index finger and thumb, with the sticky side facing out. Make the loop carefully, so there aren't any thumbprints on the center of it. Press the sticky part of the loop on the deposit, and then tape it onto a slide that has a drop of immersion oil on it. (Slides and oil are both available from scientific supply stores and catalogs.) The immersion oil should fill any air bubbles between the tape and the slide. Label the slide, place it in a slide shipping box, and send it to the lab.

The sticky method is also used when you ask for an analysis of the particles under an electron microscope. An electron microscope provides much higher magnification than a light microscope, and can even identify the elements contained in the particle. To collect surface samples for an electron microscope, you use a special small metal stud with a disk of sticky carbon that is touched to the surface and then sent to a lab.

Going Airborne

Airborne sampling is a complex process; I use it mainly for research projects or to verify containment of a pollutant. It should only be done if visual inspections and bulk or surface samples prove inconclusive.

Airborne samples are generally collected by slamming the particles against a sticky surface. This is done with a device that looks like an electric pencil sharpener. The device sucks air through an inlet that narrows to a slit or hole. This makes the air move very fast toward the sticky surface. When it hits the sticky surface, the air makes a sharp turn, but heavier particles don't make the turn. They instead slam into the sticky surface, which is made up of petroleum jelly or agar jelly on a microscope slide or petri dish.

Testing of this kind can be very complex, and there are a number of caveats to keep in mind.

  • First, the process often misses large particles because they are too heavy to remain suspended in the air for very long. This is important in the case of mineral fibers, which can cause eye and upper respiratory irritation and skin rash on the forearms. If you suspect your stain is from mineral fibers, take surface samples first.
  • Second, for an accurate analysis, airborne samples need to be collected in duplicate and sometimes in triplicate. You must also sample both indoors and out, because often the particles that are located indoors have been brought in from outdoors. When sampling the air for fungi, take samples from complaint areas (where the stains or problems occur), from non-complaint areas, and from outdoors. That way, you can identify which species of fungi are concentrated in the complaint areas.
  • Finally, remember that airborne sampling reflects what's in the air for a short time period. It's easy to miss a species of fungi because fungi only release their spores at certain times, and the one you are looking for may not happen to release its spores when you are sampling.
Prodding, Culturing, Burning After you have collected your samples, you can break out your microscope (see Do-It-Yourself Microscopy) or send them to a lab. How the lab analyzes the samples depends on whether they are biological or inanimate.

At the lab, biological samples (from a swab, for example) will be examined by a microbiologist under the microscope, or they will be cultured. To culture the sample, the analyst will put it into a petri dish in hospitable conditions and with a nutrient jelly for food, and watch what grows. Inanimate particles will be examined by a particle specialist under a microscope (sometimes an electron microscope), or they will be burned or weighed.

A general lab analysis of your sample will cost approximately $150. An analysis under an electron microscope, which can also include a breakdown by elements, is usually available for an additional $100.

If your samples cannot be identified by microscopic examination or by culturing, you can use a specialized, wet chemistry lab. There, your samples will be subjected to various solutions, dilutions, or liquid chromatography analysis (in which the stain's chemicals are identified by the colors they make on special staining paper). Such a lab will charge you up to $500.

Once the stain is identified, you have to figure out where it's coming from, how it's being transported, and how to stop it. But that's another story.

Make New Files, but Keep the Old Over the years, I've developed a large collection of case histories of stain identifications, which have proven invaluable. Each case history includes slides or photos of the stains I investigated, detailed accounts of what they looked and smelled like, and what the final verdict was, whether my own or from a laboratory. I frequently refer to these files when I come upon a similar case, and I recommend that you do the same-especially the slides and photos, which can be taken to a stain site and compared with the stains.

I recommend caution if you plan to keep any of your samples, and don't suggest that you hang on to any biological samples at all-or you may get more intimate with the problem than is good for you!

Finally, keep in mind that for certain substances, such as asbestos, there are regulations for the storage and documentation of samples and investigation records. This type of problem should be done by a pro.

Good luck!

Terry Brennan is a building scientist at Camroden Associates in Rome, New York. He has been peering at things in microscopes since he was 13.

Steven Bodzin contributed to writing this story.

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