Smoke testing standards: what the laboratory measures, what ANSI/IICRC S700 requires, and what "clear" means

What a laboratory can measure

Smoke leaves two different kinds of evidence. Particle analysis finds the solids; gas analysis finds the chemistry. An investigation that reports only one of them is describing half of the exposure.

Smoke is, in the EPA's words, a complex mixture of gases and fine particles produced when wood and other organic materials burn, and the agency identifies fine particles as the biggest health threat in it. Those particles are small: the EPA defines PM_2.5 as particles generally 2.5 micrometers and smaller, and notes that an average human hair, at about 70 micrometers, is thirty times wider than the largest fine particle. Nothing that small is identified by eye, which is why a smoke investigation is a laboratory exercise rather than a visual inspection.

Why every result needs a comparison sample

Neither soot nor VOCs are unique to fires. Candles, gas ranges, fireplaces and traffic all leave carbon particles, and the EPA's own exposure studies found about a dozen common organic pollutants at levels two to five times higher inside homes than outside, regardless of whether the homes were rural or industrial. A single number from inside a house therefore cannot show fire impact on its own.

The technical fix is a paired design: one or more samples from areas the fire could not have reached, taken with the same media, the same collection time and the same laboratory, reported side by side with the suspect samples. An elevated result is then a difference, not an absolute. When you read a report, the first thing to find is the comparison sample; without it the conclusion is an opinion.

Timing matters as much as location. VOC concentrations fall as a building ventilates, and particles settle and get cleaned away by ordinary housekeeping, so the same house sampled a month apart can produce two honest but different answers. A report should state the sampling date, the weather, whether windows and HVAC were open or running, and how long the building had been closed before air sampling began.

From results to a written protocol

Laboratory data becomes useful when an independent professional turns it into a scope of work. This is the sequence a standards-based project follows.

1. Define the boundary. S700's assessment language is about presence, intensity of impact and boundaries. Sampling is laid out to find where fire residue stops, not just to confirm it exists in the worst room, so rooms and systems can be sorted into affected and unaffected.
2. Sample by pathway. The EPA lists three routes by which outdoor smoke enters a building: natural ventilation through open windows and doors, mechanical ventilation through fans and HVAC fresh-air intakes, and infiltration through small openings, joints and cracks. A sampling plan that follows those pathways, from the intake to the filter to the supply registers and the attic above the ceiling penetrations, finds residue where it accumulates.
3. Keep custody. Every sample carries a unique identifier, the location, the medium, the collection time and the name of the person who took it, and the transfer to the laboratory is signed. A report without a chain-of-custody record cannot be relied on in a dispute, whatever its numbers say.
4. Write the Restoration Work Plan. S700 names the development of the Restoration Work Plan as part of its scope. The plan states, room by room and system by system, what will be removed, what will be cleaned and by which method, how the HVAC system will be handled, how contents will be sorted, and what clearance criteria will be applied at the end.
5. Document as you go. The standard's scope includes documentation of project-related events and compliance with local authorities. Photographs before and after, moisture and containment logs, and disposal records are what allow a carrier, a buyer or a court to reconstruct the project later.

What "clear" means, and what it does not

Clearance is a repeat of the initial sampling, after the work, against the same comparison samples and by the same laboratory. The IICRC's mold standard lists post-remediation verification as a named component of a professional remediation, and the same logic transfers to smoke: the project is finished when measured residue in the treated areas is no longer distinguishable from the unaffected reference, not when the odor is masked or the surfaces look clean.

Filtration during and after the work has physical limits worth knowing. A HEPA filter is defined by the U.S. Department of Energy as removing at least 99.97 percent of particles at 0.3 microns, the most penetrating size, with larger and smaller particles captured at higher efficiency. That covers soot. It does not cover gases: the EPA's air-cleaner guide notes that most filters handle either particles or gases, that activated carbon is the usual gas medium, and that there is no widely used performance rating for gas-phase filters. A clearance based on particle counts alone therefore says nothing about VOCs.

For central systems the EPA recommends a filter rated at least MERV 13, or as high as the system fan and filter slot can accommodate, and points out that HVAC filters only work while the system runs, which in most homes is less than a quarter of the time. Those constraints belong in the protocol, because they decide whether the air handler is a cleaning tool or a reservoir.

Technical questions, answered