Mold air testing and ERMI tests

How does a mold air test work?

A pump draws a measured volume of air through a small cassette called a spore trap. Spores stick to a sticky slide inside, and a lab counts and groups them under a microscope. The result is a count per cubic meter of air for each spore type the lab recognized.

The tester usually takes at least one sample outside at the same time. Outdoor air always contains mold spores, so the outdoor sample acts as the baseline the indoor samples are read against.

How do you read indoor vs outdoor results?

The general idea is a comparison, not a pass or fail:

Pattern	What it might suggest
Indoor counts lower than outdoor, similar spore types	Indoor air reflects outdoor air
Indoor counts higher than outdoor	Possible indoor source
Spore types indoors that are scarce outdoors	Possible indoor growth, worth looking for

These are hints, not diagnoses. A spore trap captures a few minutes of air. Counts move with wind, weather, time of day, and whether someone just vacuumed or walked through. One sample set is a snapshot.

There is also no number that counts as safe. The EPA says no federal limits have been set for mold or mold spores and there are no EPA standards for airborne mold.¹ The California Department of Public Health says no federal or state policies set safe exposure limits for dampness and mold in buildings.³

What is an ERMI test?

ERMI stands for Environmental Relative Moldiness Index. Instead of air, it uses a dust sample. A lab analyzes the DNA of 36 indicator molds and the result is placed on a scale built from more than 1,000 U.S. homes, running from about -10 to about 30.²

EPA scientists developed it for research on mold exposure and health. The EPA’s own fact sheet says ERMI has been peer reviewed for research purposes but has not been validated for non-research purposes, and that for this reason the EPA does not recommend the routine public use of ERMI in homes, schools or other buildings. The same fact sheet says inspection for water damage and mold remains the key to current EPA mold-assessment guidance.²

What about HERTSMI-2?

HERTSMI-2 is a shorter score calculated from a handful of the molds in the same dust DNA analysis. It was not developed by the EPA, and it shares ERMI’s basic limitation: no agency has tied a score to a health threshold.

California’s public health department states that the evidence consistently linked to health effects is current or past water damage, damp materials, visible mold and mold odor, not the number or type of mold spores or other markers of mold in indoor air or dust.³

When does air sampling actually help?

When a professional designs it to answer a specific question. The EPA says sampling should be done by people with experience designing sampling protocols and interpreting results.¹ Situations where that can make sense:

• A persistent musty smell with no visible source, where a comparison between rooms might narrow the search
• Documentation for a dispute, a claim or a sale, when someone else has asked for it
• Clearance after remediation, where the protocol has been agreed in advance

It rarely helps when mold is already visible. The EPA says sampling is usually unnecessary in that case.¹ The CDC goes further and does not recommend mold testing, noting that sampling can’t tell you whether someone will get sick.⁴ If a company recommends air or ERMI tests, ask how the result will change what you do next.

Does Clarity do air testing?

Yes, when it answers a real question. Our post-remediation clearance check is $745 for one work area and includes two air samples with lab analysis. During an inspection we can add air samples when a result would change a decision, and we price them before the visit. Surface samples are $125 each, lab analysis included, for growth that can’t be identified by eye. We don’t offer ERMI. Most calls are answered by an inspection and a moisture assessment, which look for the water that feeds mold.

If you see a small patch with an obvious cause, you probably don’t need us or a lab. Our guide to whether you need mold testing walks through each situation, and the mold check gives a quick answer for yours. If a home kit is what you were weighing, read do home mold test kits work first.