Choosing the Right MERV A or ISO ePM1 Rating for Your Facility
- MERV A and ISO ePM1 ratings measure the same thing — fine particle capture efficiency — but use fundamentally different test methods and reporting formats. MERV A (ASHRAE 52.2-2017 Appendix A) reports a single composite efficiency value at 0.3–1.0 µm. ISO ePM1 (ISO 16890) reports efficiency for particles ≤1.0 µm as a separate metric alongside ePM2.5 and ePM10.
- MERV A is mandatory for US federal and LEED v4.1 projects. ASHRAE 62.1-2019 and later versions reference MERV A rather than the earlier MERV (minimum composite) rating. If you specify filters for a US government building, school (LEED), or healthcare facility that follows ASHRAE 62.1, you need MERV A values from the filter manufacturer.
- ISO ePM1 is the global standard and is increasingly required in Europe, the Middle East, and Asia. ISO 16890 replaced EN 779 in 2018. It reports efficiency at three size fractions (ePM1, ePM2.5, ePM10) plus a coarse category. A filter rated ISO ePM1 ≥65% captures at least 65% of particles ≤1.0 µm, equivalent roughly to MERV 14–15.
- The two systems do not have simple 1:1 equivalents. A filter with MERV A = 13 (≥75% at 0.3–1.0 µm) typically corresponds to ISO ePM1 50–65%, but the exact mapping depends on the filter's efficiency curve characteristics. The only reliable method is to request both MERV A and ISO ePM1 test reports from the supplier — approximately 40% of KWS customers now do this.
- Most buyers should specify both ratings. For projects with mixed international requirements or uncertain future standards, requesting filters certified to both MERV A and ISO ePM1 protects your investment. The cost premium for dual certification is negligible (0–5% on most medium-grade filters).
Why This Decision Matters More Than You Think
I received a frantic email from a procurement manager in early 2025, right after his company had won a contract to supply air filters for a new hospital wing in Qatar. The mechanical engineer's specification required "ISO ePM1 ≥65% or equivalent MERV A 13 certified filters, with test reports traceable to ISO 16890-2:2018 and ASHRAE 52.2-2017 Appendix A." He had ordered F9 Bag Filters based on the old EN 779 standard — and none of his suppliers could provide both ISO ePM1 and MERV A certificates.
The problem was not the filter performance. The filter was perfectly capable. The problem was the test documentation — and in the world of Air Filtration compliance, if you cannot prove it with a test report, the filter does not meet the spec. He lost the order, worth $180,000, because his documentation gap was larger than the performance gap.
This is the reality of today's global air filter market. We are living through a transition period where two major rating systems — MERV A (ASHRAE 52.2) and ISO ePM1 (ISO 16890) — coexist, and neither is replacing the other quickly. Facilities that understand both systems can specify correctly, avoid compliance penalties, and actually improve indoor air quality. Those that rely on one system alone risk miscommunication, rejected shipments, or — worst-case — installing filters that do not deliver the performance their occupants need.
In this article I explain exactly how MERV A and ISO ePM1 differ, which one you should specify for different project types, and how to bridge the two systems without paying a premium.
In the world of air filtration compliance, if you cannot prove it with a test report, the filter does not meet the spec — regardless of how well it performs.
What MERV A and ISO ePM1 Actually Measure
Before comparing, let us be precise about what each system reports.
MERV A — The Updated ASHRAE Standard
MERV (Minimum Efficiency Reporting Value) is defined in ASHRAE Standard 52.2. The original MERV rating reports a single number (1–16) based on a filter's minimum efficiency across three particle size ranges: E1 (0.3–1.0 µm), E2 (1.0–3.0 µm), and E3 (3.0–10.0 µm). A MERV 13 filter, for example, must have a minimum composite efficiency of ≥75% in E1, ≥90% in E2, and ≥95% in E3.
In 2017, ASHRAE 52.2 added Appendix A — commonly called "MERV A." The problem with the original MERV system was that it allowed manufacturers to report the minimum efficiency from the entire E1 range (0.3–1.0 µm), which often meant a filter captured 80% of 0.8 µm particles but only 50% of 0.4 µm particles — and the final MERV rating hid this weakness. MERV A solves this by requiring efficiency reporting at every discrete particle size in the 0.3–1.0 µm range, then calculating a composite minimum across the full size spectrum. The result is a more honest and granular efficiency profile.
Per ASHRAE 52.2-2017, a filter certified as MERV A 13 must achieve the same E1, E2, and E3 minimums as standard MERV 13, but the MERV A label explicitly confirms that the filter was tested using the Appendix A method with full particle-size-resolution reporting.
ISO ePM1 — The Global Standard (ISO 16890)
ISO 16890 was published in 2016 to replace EN 779 and create a single global classification for general ventilation filters. Instead of a single number, ISO 16890 reports efficiency at three particle size fractions:
| Classification | Particle Size Range | What It Captures |
|---|---|---|
| ISO ePM1 | ≤1.0 µm | Fine particles — bacteria, fine smoke, diesel soot, virus-carrying droplets |
| ISO ePM2.5 | ≤2.5 µm | Most inhalable particles — mould spores, fine dust, PM2.5 |
| ISO ePM10 | ≤10 µm | Coarse particles — pollen, dust, lint, large spores |
| ISO Coarse | >10 µm | Very coarse particles — visible dust, fibres, insect fragments |
The key number to focus on is ePM1. A filter rated ISO ePM1 ≥65% captures at least 65% of all particles with aerodynamic diameters ≤1.0 µm. Because PM1 particles penetrate deepest into the respiratory system and are the most difficult to capture, ePM1 is the most demanding metric in the ISO 16890 system — and the most relevant for human health protection.
A critical difference from MERV A: ISO 16890 requires discharge of electrostatic charge before testing, because many synthetic filter media lose efficiency when exposed to humidity and airborne contaminants over time. The standard measures the filter's minimum efficiency after this conditioning, representing "end-of-life worst case" performance. MERV A does not mandate electrostatic discharge — so a filter containing charged media may report a higher MERV A value than its stabilized ISO ePM1 value. This is a well-known discrepancy and the most common source of confusion when comparing the two systems.
MERV A vs ISO ePM1: Head-to-Head Comparison
The table below compares the two systems across nine dimensions that matter for specification and procurement. Use it as a quick reference when evaluating which rating to require from your filter suppliers.
| Dimension | MERV A (ASHRAE 52.2) | ISO ePM1 (ISO 16890) |
|---|---|---|
| Governing standard | ASHRAE 52.2-2017 + Appendix A | ISO 16890-1:2016, ISO 16890-2:2018 |
| Primary metric | Single composite MERV A value (1–16) | Three separate efficiency %s (ePM1, ePM2.5, ePM10) |
| Fine particle emphasis | E1 range: 0.3–1.0 µm | ePM1: ≤1.0 µm Broadly similar |
| Electrostatic discharge | Not required | Mandatory — conditions media before test |
| Reporting format | Single number (consumer-friendly) | Three % values + coarse group (engineer-friendly) |
| Adoption region | North America (dominant) | Europe, Middle East, Asia (dominant); growing globally |
| Regulatory linkage | ASHRAE 62.1, LEED v4.1, US Federal GSA | EU Energy Efficiency Directive, WHO IAQ guidelines, EN 16798-3 |
| Granularity | 16 discrete levels | Continuous % scale (e.g. ePM1 ≥50%, ≥65%, ≥80%, ≥85%) |
| Test aerosol | Potassium chloride (KCl), neutralized | Potassium chloride (KCl), neutralized + discharge conditioning |
| Approximate equivalence | MERV A 13 ≈ ISO ePM1 50–65% | ISO ePM1 ≥85% ≈ MERV A 15 |
Approximate Equivalence Table
The following table provides a best-estimate mapping between MERV A and ISO ePM1, based on published cross-reference data. Note: these are approximations — always verify with the manufacturer's test report for your specific filter.
| MERV A | ISO ePM1 Equivalent (Typical Range) | ISO ePM2.5 Equivalent (Typical) | Typical Application |
|---|---|---|---|
| MERV A 8 | ePM1 — not rated (coarse only) | ePM2.5 — not rated | Pre-filtration, light commercial |
| MERV A 11 | ePM1 50% | ePM2.5 65% | Office buildings, schools |
| MERV A 13 | ePM1 50–65% | ePM2.5 65–80% | Hospitals, labs, commercial IAQ |
| MERV A 14 | ePM1 65–80% | ePM2.5 80–90% | Healthcare, clean manufacturing |
| MERV A 15 | ePM1 ≥85% | ePM2.5 ≥90% | Pharma, semiconductor, cleanrooms |
| MERV A 16 | ePM1 ≥90% | ePM2.5 ≥95% | Pre-HEPA protection, critical zones |
⚠ Important caveat: The electrostatic discharge requirement in ISO 16890 means synthetic electret media filters can show ePM1 values 10–20% lower than their MERV A equivalency suggests. Glass microfiber or e-PTFE media (used in most HEPA filters) do not have this gap. Always ask for both test reports if your specification references both standards.
Which Rating Should Your Facility Use?
The short answer is: it depends on your facility's location, regulatory environment, and internal IAQ targets. Here is a Decision Framework I use when advising KWS customers.
✓ Choose MERV A When:
- Your facility is in the United States or Canada
- The project specification references ASHRAE 62.1-2019 or later
- You are pursuing LEED v4.1 EQ credits (MERV A 13 minimum)
- The contract is a US federal, state, or municipal project
- Your facility engineering team is familiar with MERV nomenclature
- You need a single-number comparison for procurement simplicity
✓ Choose ISO ePM1 When:
- Your facility is in Europe, Middle East, Africa, or Asia
- The design specification references EN 16798-3 or ISO 16890
- You need multi-dimensional reporting (ePM1 + ePM2.5 + ePM10)
- Supplier base is international and needs a common standard
- You are evaluating total cost of ownership with energy efficiency
- You want "worst-case" stabilized efficiency (with discharge)
When to Specify Both
An increasing number of KWS customers now request dual certification. If your facility fits any of these profiles, specifying both MERV A and ISO ePM1 is the safest approach:
- Multinational corporate campuses — a single headquarters specification for facilities in different regions
- Global engineering firms — firms like AECOM, Jacobs, or WSP that design facilities worldwide and need a universal filter specification
- Pharmaceutical and semiconductor supply chains — where the product must be validated to the customer's standard, which may be either system
- Future-proofing — facilities being built now that may need to comply with a different standard in 5–10 years
5-Step Framework for Choosing Your Rating
Here is a practical process I have used with facility managers across 20+ countries. It takes about 30 minutes to complete and eliminates the guesswork from filter specification.
- Audit your regulatory and certification requirements.Review your local building code, any applicable green building certifications (LEED, BREEAM, WELL, Estidama), and your customer's specification requirements. List which standards are explicitly referenced. If ASHRAE 62.1 is named, you need MERV A. If EN 16798-3 or ISO 16890 is named, you need ISO ePM1.
- Map your filter positions to IAQ targets.For each filter bank in your facility, decide the target capture efficiency for particles ≤1.0 µm. Use either MERV A 13+ or ISO ePM1 ≥50% as the minimum for occupied spaces, and higher for critical zones (operating theatres, cleanrooms, pharmaceutical filling lines).
- Request dual test reports from your filter supplier.Ask every shortlisted supplier to provide test reports for both MERV A (per ASHRAE 52.2 Appendix A) and ISO ePM1 (per ISO 16890-2), on the same filter model and media. Compare the results side by side. A reputable manufacturer like KWS can provide both.
- Cross-check the electrostatic discharge gap.If the MERV A and ISO ePM1 values differ significantly (more than 10–15%), ask whether the media is synthetic electret. If yes, the ISO ePM1 value (with discharge) is the more conservative and likely more realistic performance indicator for long-term use.
- Write your specification in both systems.Include both the MERV A and ISO ePM1 minimum requirements in your specification, clearly stating which is primary and which is equivalent. Example: "Minimum MERV A 13 certified per ASHRAE 52.2 Appendix A, or ISO ePM1 ≥50% per ISO 16890, with test report showing both values."
A Semiconductor Fab in Penang, Malaysia
In 2024, a Tier-1 semiconductor assembly facility in Penang was upgrading its cleanroom HVAC and needed F9-grade bag filters for the make-up air units serving ISO Class 5 and Class 7 zones. The parent company (US-based) required MERV A 14 per its global standard. The local engineering team (Malaysian) preferred ISO ePM1 ≥65% per the regional standard. Neither side would compromise.
I visited the site and proposed a simple solution: supply the filter with dual certification. We tested our standard F9 synthetic bag filter (KWS-MB-F9-592x592x600-6P) per both ASHRAE 52.2 Appendix A and ISO 16890-2. Results:
- MERV A: 14 (E1 minimum 78%, E2 93%, E3 97%)
- ISO ePM1: ≥60% (after electrostatic discharge, the charged media showed a 12% reduction)
The 12% gap was documented in the test report, and both parties accepted it. The order — 1,520 bag filters worth $62,000 — was placed. The key takeaway: dual certification cost approximately 3% more per filter compared to a single-certification alternative, but it eliminated the compliance risk that could have stalled the entire HVAC commissioning for the cleanroom.
How KWS Certifies Filters to Both Standards
At our factory in Xiamen, we maintain test rigs for both ASHRAE 52.2 and ISO 16890 testing. Since 2023, every medium and high-efficiency filter we produce can be ordered with dual certification.
- MERV A testing: Conducted on our automated flat-duct test rig per ASHRAE 52.2-2017, using KCl aerosol with a neutralizer. We report the full Appendix A efficiency curve and the calculated MERV A composite value. Our medium and high-efficiency product range includes certified MERV A values for every model.
- ISO 16890 testing: Conducted on the same rig with added electrostatic discharge conditioning (2 hours at 70°C, 80% RH, per ISO 16890-2:2018). We report ePM1, ePM2.5, and ePM10 separately.
The most important document we issue is the dual test report — a single PDF showing both MERV A and ISO ePM1 results for the same filter, on the same media batch, tested on the same rig. This eliminates the documentation gap that cost that Qatar procurement manager his $180,000 contract.
Frequently Asked Questions
No — there is no reliable 1:1 conversion formula. The relationship depends on the filter's efficiency curve shape, media type (especially whether it uses electrostatic charge), and pleat geometry. The only reliable method is to request actual test data for both standards from your supplier. The approximate equivalence table in this article is a starting point, not a specification tool.
Because ISO 16890 tests after electrostatic discharge conditioning. Most F9 bag filters use charged synthetic media that loses 10–25% of its initial efficiency after discharge. Under EN 779, the same filter was tested in its charged state. This is not a manufacturing defect — it is a different test condition. For long-term performance, the ISO ePM1 value is actually the more honest representation of how the filter will perform after 3–6 months of operation in humid conditions.
For the filter itself, the manufacturing cost is identical — we are testing the same product. The additional cost comes from the test report generation and third-party verification. At KWS, the premium for dual certification is 0–5% depending on volume, and we absorb this cost for orders exceeding 500 units. The cost of not having the right certification — lost contracts, rejected shipments, compliance penalties — is orders of magnitude higher.
ASHRAE 62.1-2019 and later versions reference MERV A in an informative appendix, not a mandatory requirement — but in practice, virtually all US federal projects (GSA, USACE, VA), LEED v4.1 projects, and healthcare facilities following FGI guidelines require MERV A certification. If you supply filters to the US market, having MERV A test reports is now a competitive necessity rather than an optional enhancement.
No — HEPA and ULPA filters are classified under EN 1822:2019 (European) or IEST-RP-CC001.6 (US), not MERV A or ISO 16890. HEPA H13 (99.95% at MPPS) and H14 (99.995%) are tested using different aerosol challenges (liquid DOP/PAO), different measurement methods (photometry or particle counting), and different pass/fail criteria. However, the staged filtration before HEPA — typically G4 + F9 or MERV A 8 + MERV A 14 — uses MERV A or ISO ePM1 ratings for the pre-filters.
Your Next 30-Day Action Plan
If you manage filter procurement for a facility, here are three actions to take within the next 30 days:
1 Audit Your Specs
- Review all current filter specifications for every building you manage
- Identify which standards each spec references (ASHRAE 52.2, ISO 16890, or both)
- Note any specs that reference EN 779 — these need updating to ISO 16890
2 Check Your Inventory
- Pull test reports for your five most-used filter SKUs
- If they do not include both MERV A and ISO ePM1 values, request them from your supplier
- Flag any filters that depend heavily on electrostatic charge for their MERV rating
3 Update Your RFQ Template
- Add a line requiring "MERV A per ASHRAE 52.2 Appendix A + ISO ePM1 per ISO 16890" for all medium and fine filters
- State that test reports must be supplied with each order
- Require that the test reports show both certified values on the same document