A supplier quotes a pre-air filter with an ISO 16890 ePM classification. Another quotes the same duty point in MERV. Before either number can inform a selection decision, the buyer has to know what each rating actually measured, under what conditions, and whether the two results can sit on the same comparison sheet at all.

What each rating framework actually reports

ISO 16890 and ANSI/ASHRAE 52.2 are both laboratory test methods applied to general-ventilation filters, but they are not two measurements of the same underlying quantity expressed in different units. Each framework defines its own test procedure, its own particle-size treatment, and its own classification logic. A result reported under one framework describes what that framework’s test rig recorded for that filter under its specified conditions. It does not automatically describe what the other framework would have recorded for the same filter, because the two procedures are not designed to produce convertible outputs.

This matters at the point where a buyer is comparing two supplier quotations side by side. If one offer states an ePM classification and the other states a MERV classification, the temptation is to treat them as two expressions of one filtration grade and rank the offers against each other directly. That step is not supported by either test method. The classification a supplier reports is tied to the standard edition used, the rated airflow at which the test was run, and the particle-size convention that framework applies. Change any of those conditions and the reported number changes meaning, even where the physical filter media is identical.

The practical consequence is that a rating framework should be read as a self-contained report, not as a translatable score. Where a project specification calls out an ePM grade, the compliant evidence is an ISO 16890 report stating that grade under its own test conditions. Where a specification calls out a MERV value, the compliant evidence is an ASHRAE 52.2 report under its own conditions. Mixing the two, or asking a supplier to “convert” one into the other, introduces a step that neither standard supports and that the buyer cannot verify against the underlying test.

This distinction also affects how a buyer reads partial information. A data sheet that states only a classification, with no reference to the standard edition or the airflow at which the test was run, is not comparable to another data sheet even if it uses the same framework. The classification is the output of a specific test condition, not an intrinsic property of the filter that holds regardless of how it was tested.

ISO 16890 data to capture from the supplier report

Where an ISO 16890 report supports a quotation, the information worth extracting goes beyond the headline ePM figure. The first item is the standard edition the supplier tested against. Standards are periodically revised, and a classification generated under an earlier or later edition of ISO 16890-1 is not guaranteed to carry the same meaning if the particulate-matter classification system has changed between editions. Recording the edition alongside the result keeps the classification anchored to a known test definition.

The second item is the rated airflow at which the test was performed. ePM efficiency is airflow-dependent; a filter’s reported classification corresponds to a specific tested flow rate, not to whatever flow rate the filter will see once installed. A buyer comparing two ePM-rated filters needs both figures reported at the same or comparable airflow, otherwise the comparison conflates filter performance with test-condition differences.

The third item is initial resistance, meaning the pressure drop recorded at the start of the test, before any loading. This figure matters because two filters with the same ePM classification can carry different initial resistance values, which affects how the filter behaves in the ventilation system from the moment it is installed. A lower initial resistance at the same classification is not automatically preferable without knowing how the rest of the system is configured, but the figure needs to be visible for the comparison to be complete.

The fourth item is the exact reported classification string as stated in the report, not a simplified summary of it. ISO 16890-1 classifications are structured designations; abbreviating or paraphrasing them when logging supplier data risks losing the distinction the standard draws between classification bands.

Taken together, these four data points let a buyer evaluate an ISO 16890-based offer on its own terms, under its own stated conditions, before any comparison with a differently tested filter is attempted.

MERV test information needed for a fair comparison

The same discipline applies on the ASHRAE 52.2 side, and for the same underlying reason: a MERV value is not a standalone grade but the output of a laboratory procedure run under specific conditions. The standard edition matters first. ASHRAE 52.2 has been revised over time, and a MERV classification generated under one edition reflects that edition’s particle-size test method and reporting conventions. Treating MERV as a fixed, edition-independent scale overlooks that the underlying test procedure is what produces the number.

Rated airflow matters for the same reason it matters on the ISO 16890 side: the test is run at a defined flow rate, and the reported MERV value corresponds to that flow condition. Where a project’s installed airflow differs materially from the condition under which the supplier’s filter was tested, the reported classification describes the laboratory condition, not the installed one.

Initial resistance should be captured from the MERV report in the same way it is captured from an ePM report, because it is independent information from the classification itself. A buyer who only logs the MERV number and discards the resistance figure loses the ability to compare how two MERV-equivalent filters will differ in pressure drop at startup.

Finally, the exact reported MERV designation, not an approximate or rounded version of it, should be recorded. ASHRAE 52.2’s particle-size test information underlies the minimum efficiency reporting value, and the designation as stated in the report is the only form that ties directly back to that underlying test data.

With standard edition, rated airflow, initial resistance, and the exact classification recorded for a MERV-based offer, the buyer has the same four-field record used for an ePM-based offer. That symmetry is what allows a structured comparison in the next step, without yet claiming that the two frameworks are measuring the same thing.

Boundaries that rule out a simple ePM-to-MERV conversion

Once both sets of report data are in hand, the question becomes what can honestly be concluded from placing them side by side. The answer depends on which reports exist and what decision the comparison is meant to support.

Where only an ISO 16890 report exists for a given offer, the defensible use of that data is to read the ePM classification within its own stated standard edition and rated airflow. It is not defensible to infer what MERV value the same filter would have received under ASHRAE 52.2 testing, because no conversion procedure between the two frameworks is established in either standard. The reverse holds where only an ASHRAE 52.2 report exists: the MERV classification is usable within its own test conditions, but it does not support an inferred ePM grade.

Where both types of report are available, for the same filter or for competing filters, the comparison that is supported is a side-by-side reading of framework, edition, rated airflow, initial resistance, and reported classification for each. This lets a buyer see where two offers agree or diverge on test conditions even though their classifications come from different systems. What remains unsupported is a claim that the two classifications are directly equivalent to each other; the frameworks were not constructed to produce interchangeable outputs, and no amount of side-by-side tabulation changes that.

A separate boundary applies once the decision shifts from filter selection to room-level acceptance. A pre-air filter’s laboratory classification, under either framework, describes component-level performance measured on a test rig. It does not by itself describe how the installed room or clean zone performs once the filter is in service as part of a larger system. Where room-level cleanroom acceptance is the actual decision in front of the project team, the applicable evidence is a cleanroom test method such as ISO 14644-3 applied with project-specific acceptance values for that installed room or zone, not an extrapolation from the pre-air filter’s component rating.

Service life sits in a similar position. Neither an ePM classification nor a MERV classification predicts how long a given filter will perform acceptably once installed, because service life depends on the operating conditions and the system the filter is installed within, not on the laboratory rating alone. Where a loading condition changes, such as a shift in the upstream air quality the filter is exposed to, the laboratory classification stays fixed while the field performance can diverge from it. This is a condition the buyer confirms against the actual installation, not something either standard’s classification resolves in advance.

Specification wording that preserves the chosen test basis

The data-capture discipline above only protects a buyer if it is written into the specification itself, rather than left to be reconciled after quotations arrive. A specification that asks for “a MERV 13 or equivalent ePM filter” invites exactly the unsupported conversion the comparison boundaries rule out, because “equivalent” implies a translation that neither test method performs.

Naming the test basis explicitly, meaning the selected standard and its edition, keeps every submitted classification tied to one framework rather than leaving suppliers to choose whichever standard produces a more favorable-looking number for their product. Stating the rated airflow at which supplier data must be reported closes the gap where two offers might otherwise report classifications generated at different flow conditions, making their numbers look comparable when they are not. Requiring the initial resistance to be stated in the submission keeps that figure visible at the comparison stage rather than buried in a separate data sheet. Requiring the exact reported classification, in the designation format the standard itself uses, removes the risk of a paraphrased or rounded figure substituting for the tested result.

A further element belongs in the specification precisely because it sits outside what either laboratory rating can establish: a separate statement of the applicable cleanroom test method and the project-specific acceptance values that govern the installed room or zone. Keeping this as its own specification element, distinct from the filter’s component classification, prevents a reviewer from later treating the pre-air filter’s laboratory rating as if it were evidence of room-level acceptance.

When project information such as the intended test basis, rated airflow, and installed configuration is defined this clearly before a quotation is requested, it gives a supplier’s configuration or quotation review something concrete to work from, rather than a specification that can be satisfied by either framework interchangeably. A specification written this way also gives the buyer a basis for discussing the physical filter family, whether a panel configuration or a bag-and-pocket configuration, against the stated test basis, since the construction type and the rating framework are independent choices that both need to be fixed before offers are compared.

Frequently Asked Questions

Q: Can I use an ePM-to-MERV conversion chart to choose between two pre-air filter offers?
A: No direct equivalence should be assumed. Record the standard and edition, rated airflow, initial resistance, and exact reported classification for each offer, then compare those values side by side while keeping each rating within its own test framework.

Q: What should I do if two supplier reports list ratings at different airflows?
A: Treat the ratings and resistance data as condition-specific until the offers are aligned to the airflow required by your project. Ask each supplier to state the rated airflow and initial resistance with the reported ePM or MERV classification so the comparison uses a clear test basis.

Q: Does a pre-air filter’s ePM or MERV rating prove that the cleanroom will meet its acceptance target?
A: No. The rating is laboratory component data, while room-level performance must be checked using the applicable cleanroom test method and the project’s own acceptance values for the installed room or zone.

Q: Can the reported ePM or MERV classification be used to predict filter service life?
A: Not by itself. Treat service life as dependent on the selected system, installed airflow, and operating conditions, and define the operating evidence and replacement criterion that will be used after installation rather than assigning a life from the laboratory rating alone.