Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16Identify, Diagnose, Remove Occupant Stains & Smudges in Buildings

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Dark indoor stain diagnosis:

Human or pet occupants: this article describes & diagnoses the cause of various how to recognize (and clean or prevent) stains and smudges on interior walls when these marks are caused by human occupants.

Examples of human-caused indoor stains include soiling from a person's head leaning against a light-colored wall or brown stains due to cigarette smolking. When investigating a building for a mold problem, you can save mold test costs by learning how to recognize stains that are basically harmless.

Don't waste money testing these stains. These are indoor marks and substances that you can easily learn to recognize.

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.

How to Recognize Wall Stains from Human Occupants

Photograph of dirt from a pet smudging a wallSave 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."

Black marks on interior walls such as painted drywall might be also be dark wall stains where people rested their heads in bed (as shown in this photo at left).

Look at the location of these stains and imagine a bed having been placed with its head against the wall shown in the photograph. Notice the two wall lights.

This is where a bed was almost certainly placed, and we can easily explain these black stains on the drywall. No further testing would be appropriate in normal circumstances.

We welcome more thermal tracking, soot tracking, air bypass leaks, and similar photos of indoor stains as well as text suggestions to expand this detail and would be glad to credit contributors.

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

Cigarette, Cigar or Pipe Smoker Stains in Buildings

Stains run down wall © D Friedman at

We have inspected a number of buildings inside which we observed brown nicotine/smoke stain patterns traced to smokers.

First, brown-colored stains that appear either uniformly or in thermal-tracking streaks on walls and ceilings.

Often these cigarette smoke stains are found principally in one or two rooms of a home occupied by smokers - rooms where, obviously, people spent more time smoking such as in a living room around a couch or recliner chair, or in bedrooms.

Watch out: smoking in bed is a well-known fatal fire hazard

In these homes, the observation that brown stains are found in just these occupied spaces is an argument against other causes that would produce more uniform staining, such as heating system problems or poor building wall or ceiling insulation.

Wall stains © D Friedman at

Our wall stain photos above illustrate a second instance, we find brown stains that appear to run in rivulets down building walls. This is essentially the same cause, and these stains will appear most severe in the same locations we described above.

The difference is that there have also been occasions of very high indoor moisture or even wet conditions in the "brown rivulet streak stain" homes. Moisture that condenses at high levels on a building wall will eventually produce droplets that run down the wall.

The dissolved nicotine and tobacco smoke stains are concentrated and redistributed by these moisture drip marks. Or conversely, we may see a tan-stained wall with clean rivulet areas where water droplets have carried stain material away.

Even when the interior wall has later been cleaned of smoke tar and nicotine marks, we may find remains of that material behind baseboard trim.

Watch out: any soiling on building walls can leave rivulet-pattern stains if the wall has been wet for any reason.

Mottled Pattern Indoor Smoker Stains

Mottled indoor wall stains © D Friedman at

Sometimes, depending on the water deposition pattern on building walls, these smoker stains may appear mottled in pattern rather than streaked.

Watch out: Mottled wall or ceiling stains may also appear not from smoking indoors but from building leaks that dissolve brown ingredients in wall, ceiling or floor cavity framing or insulating materials.

We've also found this condition in a room where someone attempted to wash a cigarette or cigar smoke stained ceiling or wall, using insufficient water or cleaner or other poor methods.

At HEAT LOSS INDICATORS we also discuss and describe brown stains found on building ceilings and walls that are traced to smokers.

That is, areas in buildings where people have spent time smoking cigarettes, pipes, or cigars.

Brown deposits blamed on smokers traced to unsafe gas furnace!

Question: a "sweet" strange smell in my house and brown dust

2017/01/04: anonymous (by private email) asked:

I am very concerned about a "sweet" strange smell in my house. I just bought the 11 year old house, it has a propane forced air furnace. I have washed every surface and I'm having the interior repainted.

What concerns me is the smell continues at irregular intervals and produces a tan granular dust. This discoloration is on light switches and outlets.

The realtor said the previous owners were smokers and that was the cause for this smell and the dust material being everywhere. Help. I don't know what to do. My attempts so far seem to be failing. - Anon in Michigan

Reply: suggestions for sealing stains and odors left by smokers vs. diagnosing powdery dust deposits

Smoking leaves a sticky, tarry brown deposit on indoor surfaces but not a powdery dust. The smoking deposit can be wiped off but usually with difficulty unless using a liquid cleaner that acts as a solvent. Even then in my experience that won't eliminate smoking-related odors.

At FIRE & SMOKE ODOR REMOVAL we discuss the use of sealants to manage odors and stains left in buildings after cleaning has proved insufficient.

In a home where there were heavy smokers I've found it necessary to first clean all interior surfaces: ceilings, walls, cabinets, trim, etc., and then I might have to paint ceilings and walls, possibly trim, with an odor-sealing paint such as types used after fires in buildings. See MOLD SPRAYS, SEALANTS, PAINTS to see examples and products.

You might want to use such a sealer as your primer coat before re-painting. Particularly, I use a lacquer primer sealer over bad stains that might otherwise bleed through even multiple coats of latex and even some alkyd paints.

Plastic switch and receptacle covers should be removed for a decent paint job anyway. Soak them in a household cleaning solution and then use a green scrubby sponge to clean them and let them dry before reinstalling - or if labor gets excessive just replace them.

Look for the source of the tan dust

The tan dust, if that is really recurring, is a different problem and we need to track down its source. I'd look at particle and dust or debris sources, starting with any fuel burning appliances.

Watch out: gas fired appliances, in particular, that are putting out dust or debris are unsafe. A minimum safety step besides inspection for trouble signs would be the installation of smoke and CO detectors.


At the top of any InspectApedia page where you see EXPERTS DIRECTORY you might find an IAQ consultant or inspector who can do some useful consulting or onsite work. I'll offer a few comments, questions, suggestions pro-bono.

Reader follow-up: sand and dirt sucked into the furnace mounted on a dirt/sand crawl space floor spreads tan dust through building

I thought I'd get back to you about the "tan dust" question I posed previously. Some time in the past an air conditioning coil was added to the heating system. In order to add the ac, the furnace had to be lowered and was set on a couple of 2" paving blocks which put the furnace right at the level of the crawl space sand floor.

Over time, sand/dust was sucked into the furnace, circulated, some exited through un-sealed duct joints and due to static electric charge, clung to the Styrofoam used to insulate the block in the crawl space. I taped and sealed all of the duct joints and had a sealed vapor barrier placed over the entire crawl space floor.

I previously had the ducts cleaned (hind sight would have reversed the order) and hope that by running the furnace fan, constantly changing the filter and providing lots of outside air to the main floor of the house the system will eventually "air out" and the problem will be solved.


Thank you for this update - that's certainly a problem that I'd not have guessed.

There are further implications from what you've told me. Return air should not be drawn in at the furnace in any event.

Watch out: taking return air at the furnace risks (for gas systems at least) fatal carbon monoxide poisoning, and both gas and oil systems, improper burner operation. Sufficient negative pressure around the air handler / furnace to draw in local sand, debris, grit, dust, tells me that the system is probably starved for - has inadequate - return air from the occupied building space.

An effect of that inadequacy is less effective heating or cooling and higher heating and cooling costs.


and also


Reader follow-up: Carbon Monoxide Poisoning from return air inlet at furnace

You keep nailing my situation. Last Tuesday I took sick and found out I had elevated carbon monoxide (0 - 1.5 normal for non smoker, I was 2.6). I had paid a heating contractor to check my furnace in December (I specifically asked him to check the exchanger as Lennox had problems a few years back with cracked exchangers).

He said all was fine and put in some new burners.I found out last week that I did indeed have a cracked exchanger and opted to replace the furnace. The first contractor said he would "make it right with me financially." So, I will see.

Reply: importance of carbon monoxide and smoke detectors

Watch out: Be sure you have working CO detectors properly located, installed, tested as even with new equipment, something can still go wrong, such as a chimney blockage, inadequate combustion air, or return air interference at the heater.


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