Assemblage Analysis of Wildfire Combustion Particles vs. Soot/Char/Ash Analysis

Why is Assemblage Analysis of Wildfire Combustion Particles superior to the typical soot/char/ash analysis?

For years, the most common smoke damage analysis has been collection of tape lift samples and analysis for soot, char, and ash, typically reported as percentages in reports. Consultants, however, often tell us that the results of these soot/char/ash/carbon black/etc. reports are virtually worthless, as there is no regulatory standard for what an analysis result of 2% ash or 4% char mean in terms of definitively identifying smoke damage. There are a host of issues with these traditional that are discussed below:

• What's normal?  How much soot and char and ash should a building have?
• What is "elevated"?  Why can't I find percentage guidelines anywhere?
• What does it mean if dust is 2% ash?  Is that a lot?  How do we know if it's even from wildfire?  Lots of typical household activities makes ash.

The biggest issue with Soot, Char, and Ash Analyses is that they are not necessarily indicative of wildfire deposition.

Ash is made by any combustion activity, and ash from one source looks like ash from any other source.  So, while ash is a reliable indicator that combustion has occurred, there is no way to distinguish cigarette ash from furnace ash from industrial ash from wildfire ash.  Ash, therefore, is a poor indicator that combustion particulate is from a wildfire.

Soot "presence" actually contraindicates wildfire.  While all combustion activities will produce soot, individual particles of soot are near or below the ability of a light microscope to resolve.  Therefore, soot is only identified during analysis when it agglomerates (adheres together) into large structures.  Trees, shrubs, grasses, etc. produce a very dry soot when burned that does not agglomerate and therefore will not be identified during analysis.  Large identifiable soot agglomerates are created by "liquid" fuels — materials which melt prior to combustion.  There are a host of these found around the house — PVC, candles, plastics, synthetics, some caulks, etc.  These are the materials that create identifiable soot when combusted.  Therefore, soot presence in the analysis actually indicates structure fire rather than wildfire.

Char, as a category, is not necessarily an indicator of wildfire deposition. There are literal tons of wood in most houses (framing).  Therefore, a great deal of charred wood is created in a structure fire. When determining if a house has particle deposition from a wildfire, as opposed to structure fire or other combustion activity, ash is meaningless, soot is actually a counter indicator, and char is inconclusive.

Assemblage Analysis of Wildfire Combustion Particles — A Superior Method

PEL Labs utilizes what is known as "assemblage analysis" and we believe this method is far superior to traditional soot, char, and ash analyses. Assemblage analysis essentially utilizes multiple clues to "prove" a conclusion. For instance, a child's shoe in a random front yard may demonstrate a child lives there. If that shoe is accompanied by a tricycle, the potential a child lives there increases. If there is also a clothesline in the backyard with children's clothes drying on it, in addition to the shoe and the tricycle, the likelihood of a child living there increases even further. The analysis to confirm wildfire deposition is similar— specially trained analysts are only selectively analyzing samples for materials that are generally only going to be created by wildfires.

What actually fuels a wildfire?  While some tree heartwood burns, the main fuels are grasses, leaves, bark, and twigs. When partially combusted into char, these materials retain enough of their cellular structure that they can be identified.  An experienced expert can even speciate grasses by looking at the char, although for purposes of this analysis, identification as grass char is sufficient. PEL analyzes samples for the "assemblage" of grass char, leaf char, bark char, and small crystals called phytoliths (naturally occurring mineral crystals in exterior portions of plants) in a single sample.  None of these will be found in an ordinary home in any great quantity, and, individually, all are strong indicators of wildfire deposition.  Found together, they establish wildfire deposition beyond a reasonable doubt.

If all members of this assemblage are found, and in significantly greater quantities than soot, ash, or wood char, it can be said conclusively that the sample appears to be wildfire deposition. The only indoor burning of leaves and grasses would be from tobacco, marijuana, or incense, and those chars are easily identifiable due to the huge amount of ash associated.  Small amounts of bark are burned in wood fireplaces, but the amount of bark char will be hundreds of times less than wood char in that instance.  If analysts identify grass char, leaf char, bark char, and phytoliths (which evidence the combustion of grasses, leaves, and bark) at much higher concentrations than wood char, ash, and soot, then there is no reasonable explanation other than wildfire.

How Is the Assemblage Concentration/Percentage Reported?

It's not.  Once we have confirmed that the particles present indicate wildfire deposition beyond reasonable doubt, the absolute concentration isn't significant.  There are no federal, state, or even industry guidelines identifying what is or isn't smoke damage nor what mitigation activities should be performed for a given concentration. What is of value to the end user are relative concentrations. These can help the onsite consultant establish the path of deposition through the house and focus on what areas need to be cleaned and which might not. Typically, the highest concentrations are identified where the particulate entered, dropping off as one moves away from the entry site. PEL's reports classify levels as "high", "medium", or "low" based on what surface area of the tape sample needs to be analyzed in order for us to find the entire assemblage.  These are approximately logarithmic; "high" is approximately 10x the concentration of "medium" and 100x the concentration of "low".

Why do many labs continue to perform analyses the other/old-fashioned way?

Cheap, easy, and unregulated are the most likely answers.  It is very easy to request an asbestos analyst to utilize their current microscope to estimate a percentage for tiny black (soot), large black (char) and tiny white (ash) particulate.  In addition to the problems referenced above, this analysis will include a significant amount of material that is opaque/black on a standard asbestos microscope that has nothing to do with combustion whatsoever — rubber wear, metallic and vinyl slivers from windows, fungal material, etc. Due to these concerns, industry consensus is rapidly moving to assemblage style analysis, especially in the (current draft) IICRC S760 Standard for Professional Wildfire Investigations and Restoration of Impacts to Structures, Systems, and Contents. PEL is simply ahead of the curve for the vast majority of labs performing the analysis the way they've always performed it.