Combustion Byproducts: Soot, Char, and Ash

Soot, Char & Ash Analysis: Frequently Asked Questions

Moody Labs analyzes surface and air samples for combustion particulate (soot, char, and ash) after structure fires. The questions below cover our methods, how to collect samples, what our reports contain, and how we can shape a report around what your project needs.

Our approach

What analytical methods do you use?

Our primary method is light microscopy, and tape lifting is our recommended surface sampling technique. Tape holds particles in place as they were deposited, which preserves fragile char and ash and lets us rate combustion particulate against the surface area examined. We examine the collected particulate using polarized light microscopy (transmitted light) together with reflected darkfield illumination, drawing on a range of optical techniques: reflected and transmitted light, plain and polarized light, and brightfield and darkfield illumination. Soot, char, and ash are identified by their structure and optical properties. Our approach follows the particle-identification methods taught by the McCrone Research Institute.

When tape lifting isn't an option, we can also accommodate wipe and micro-vacuum samples. These are analyzed by light microscopy for soot, char, and ash, and can also be analyzed for soot by transmission electron microscopy (TEM). Bulk samples of visible debris or ash are analyzed by light microscopy, with SEM/EDS available when elemental composition is needed.

For airborne soot, we analyze air samples collected on TEM cassettes and report results as soot structures per cubic centimeter of air.

Is there an industry-standard method for fire residue?

Not yet. No federal or Texas regulation specifies a testing method or limit for soot, char, or ash, and no single method has become the de facto standard.

Why don't you use ASTM D6602?

ASTM D6602 is sometimes referenced for fire residue, but it was written to identify manufactured carbon black (the N100–N900 rubber-grade materials), not combustion byproducts. It calls for wipe sampling and sonication, which break up fragile char and ash and can destroy the particle associations that support an identification. For that reason, we don't recommend it for fire residue work.

Can you tell combustion particles apart from ordinary household dust?

Yes. Every sample contains a mix of ordinary particles, such as cellulose and synthetic fibers, hair, and mineral grains. We identify and report these alongside any combustion particles so you can see what the surface looks like overall. We also watch for black particles that don't come from combustion, like rubber, paint, and mold spores, which are potential sources of false positives.

Can you tell what burned?

To a degree. Some burned materials leave recognizable residues:

Charred wood keeps its cellular structure.
Burned plastics and synthetics may form glassy, bubbly char with no cell structure.
Fiberglass, mineral wool, and gypsum from drywall can each be readily identified.

Soot is different. Its fine, chain-like (aciniform) clusters look much the same no matter what burned, so we don't claim to trace soot back to a specific material. Our reports describe findings as "consistent with a combustion byproduct" rather than naming a definitive source.

Reports built around your project

Can you tailor the report to our needs?

Yes, and we encourage it. Before sampling begins, we're glad to work with you on what the final report should show so it supports the decisions you need to make, whether that's documenting the extent of a loss, comparing affected areas with background, or checking results after cleanup. Tell us what the report needs to support and what information you want to see, and we'll build our reporting around it.

What information should we send with samples?

As much as you can about the sampling environment and site history. Helpful details include:

What burned, and when
Whether any cleaning or remediation has been done, and when
Where each sample was taken and what kind of surface it was (see "Which surfaces should we sample?")
Any other combustion sources nearby, such as generators, fireplaces, or regional burning

The more context we have, the better we can make appropriate analytical judgments and provide relevant information in the report.

What does a standard report include?

Each report includes:

A summary of the analytical method and sample preparation
The laboratory's credentials
Sample IDs and locations as you provide them
The particles identified in each sample, including non-combustion particles
A Low / Moderate / High rating for soot, char, and ash
Photomicrographs where helpful
A written conclusion

How are results expressed?

Surface results are semi-quantitative, going beyond a simple positive or negative. For each sample, we rate soot, char, and ash separately by the approximate percentage of the examined area they cover:

Rating	Approximate area covered	What it usually means
Low	Under 1% to 3%	Combustion particles are present but make up a small fraction of the sample. After a fire, this usually means the surface was cleaned well or reflects normal background.
Moderate	3% to 10%	A noticeable accumulation. Fire residue has settled or is settling on the surface. If the sample was collected after cleanup, this may indicate incomplete remediation.
High	Above 10%	Combustion particles are widespread, indicating heavy, unmitigated residue that can migrate to and cross-contaminate nearby areas.

Results should always be read alongside control samples and the site history.

Airborne soot analyzed by TEM is reported as soot structures per cubic centimeter of air.

Why not report particle counts per area of tape?

Counting structures per measured area of tape runs into the problem of defining what a single combustion structure is. Char, for example, readily breaks into smaller pieces, so a cluster of related fragments could be counted as one group or as dozens of particles lying close together. Rating particulate as a percentage of the examined area gives more consistent results, so that's the approach we use.

Collecting samples

Which sampling method do you recommend?

Tape lifts for surface residue. Tape collects particles efficiently from smooth to moderately textured surfaces and holds them in place as they were deposited. We advise against wipes and swabs for particle identification, because they smear and break apart delicate particle structures. For visible debris or ash deposits, a small bulk sample in a sealed container works well. Air sampling for TEM analysis can be added to characterize airborne soot.

What tape should we use?

Scotch Brand 3M Magic Tape (green box), sold at any office-supply store.

Use about 3 inches of tape for a single lift from a flat surface.
Stick the lifted tape flat onto the inside of a clean Ziploc or sample bag.
Seal and label the bag.

Don't fold the tape onto itself, lift the same spot more than once, or sample more than once with the same piece of tape.

We specify this tape for a technical reason. Its cellulose ester backing dissolves readily in acetone, and its adhesive has a known refractive index that we match with our mounting media. The result is a clear, high-quality mount. Clear packing tape and other office tapes haven't been evaluated for their optical properties, can't reliably be prepared the same way, and may give noticeably poorer results.

How should we collect air samples for soot?

Collect air on 0.45 µm MCE TEM cassettes, drawing 1,200 liters of air, to reach an analytical sensitivity of 0.005 soot structures/cm³. We sell TEM cassettes by the box at the lab.

Do we need control samples?

Generally, yes. Low levels of combustion particles are common in homes that have never had a fire, from cooking, candles, and fireplaces. Even light bulbs can pyrolyze airborne organic material. Outside sources, such as generator exhaust or regional burning, are also worth considering. A comparison sample from an unaffected area of the same property, and ideally one from outdoors, is the most useful tool for interpreting results. Without one, a low result is hard to judge either way.

How many samples do we need?

There isn't a standard number. The right count depends on the size of the loss, how far residue traveled, and what the results need to support. A sound plan is built around three ideas:

Replicates within a room. Take a few samples per room, from different surfaces, so the results corroborate one another. A single sample can be skewed by one surface that was recently wiped or unusually dusty.
A gradient across rooms. Sample several rooms moving away from the fire, including along the path residue would travel (hallways, stairwells, HVAC returns). Results that decline with distance from the fire tell a much clearer story than scattered single samples.
Unaffected areas for background. Include rooms that appear unaffected, and ideally an outdoor sample, to establish what "normal" looks like for that property.

We're happy to talk through a sampling plan for a specific project.

Which surfaces should we sample?

Target undisturbed, flat surfaces, and keep in mind how often a surface was cleaned before the fire:

Seldom-cleaned surfaces, such as the tops of door and window frames, may carry heavy dust loading that can act as a matrix interference.
Regularly cleaned surfaces, such as tables, countertops, and furniture tops, usually give lighter loading, with combustion particles more prominent.

Please record which type of surface you sampled, since it bears on how results are interpreted.

Additional analysis

Do you offer SEM/EDS or other advanced analysis?

Yes. Optical microscopy is our main tool for this work, particularly for characterizing char, while also identifying soot structures and ash and estimating their share of the examined area.

TEM is used mainly for ultrafine airborne soot. We examine soot structures at 20,000–60,000× to observe their characteristic morphology and calculate a concentration in structures per cubic centimeter of air. Our TEM is equipped with an energy-dispersive X-ray (EDS) detector for chemical characterization.
SEM/EDS is available for bulk samples where elemental composition is needed, such as confirming ash chemistry or characterizing inorganic building-material residues. Because SEM/EDS is billed by instrument time, we generally recommend it for selected bulk samples after the light-microscopy results are in, rather than across a whole project.

Accreditation and quality

Is your soot, char, and ash analysis accredited?

No. No federal or Texas regulation governs this testing, and there is not yet an AIHA-approved proficiency test for combustion particles. AIHA LAP can accredit laboratory-developed methods like this through its Unique Scopes program, but Moody Labs has not yet sought that accreditation for soot, char, and ash. AIHA's proficiency testing program is developing a soot, char, and ash pilot study, and Moody Labs has signed up to participate. The first round is expected in early 2027. (EMPAT, AIHA's environmental microbiology program, covers mold, not combustion particles.)

What accreditations does Moody Labs hold?

Moody Labs holds ISO/IEC 17025 accreditation through two bodies:

AIHA LAP for environmental microbiology (EMLAP, Lab ID 102577)
NVLAP for asbestos analysis (Lab Code 102056)

We are also licensed by the Texas Department of State Health Services (No. 30-0084). Our soot, char, and ash service runs under the same quality system, but it falls outside those accredited scopes, and our reports state that plainly. Current certificates are on our Accreditations and Licenses page.

Pricing, turnaround, and ordering

What are your turnaround times?

Standard turnaround is 3 business days for both surface and air samples. 24-hour turnaround is available for all analyses at the standard rate plus 50%.

What does it cost?

Service	Standard (3-day)	24-hour
Combustion particulate on surfaces (tape lift)	$200 per sample	$300 per sample
Soot in air by TEM	$150 per sample	$225 per sample
SEM/EDS	$450 per hour, one-hour minimum	$675 per hour, one-hour minimum
TEM air cassettes	$70 per box plus tax	—

How do we submit samples?

Download our chain-of-custody form:

Note the sample location and date for each sample, and request "Combustion Particulate (Soot, Char, Ash)" or "Soot in Air by TEM" as the analysis.

Do you provide sampling procedures or training?

We don't offer hands-on field training. We can support your program with a written sampling procedure for tape lifts and TEM air sampling that your technicians can follow, and with phone or email guidance when a project raises a sampling question.

Contact us

Call (972) 241-8460, Monday through Friday, 9:30 a.m. to 5:00 p.m. Central, to talk through a project, a sampling plan, or what you'd like your report to show. For billing and payment-portal questions, email customerservice@moodylabs.com.