MERV Ratings and ASHRAE 52.2: What the Numbers Actually Certify
A lot of published MERV charts overstate what the ratings promise, sometimes by a factor of two in the particle range that matters most. This page carries the actual thresholds from ANSI/ASHRAE Standard 52.2, Table 12-1: what the test measures, what each rating guarantees as a minimum, and what the rating system deliberately does not tell you.
Last updated: August 15, 2026
The short version
- MERV comes from ASHRAE 52.2, which counts particles from 0.30 to 10 microns in three size ranges: E1 (0.30 to 1.0), E2 (1.0 to 3.0), and E3 (3.0 to 10.0).
- The rating is a floor. MERV 13 means at least 50 percent capture in E1, 85 percent in E2, and 90 percent in E3, built from the lowest efficiencies observed during the loading test.
- 52.2 is a test standard, not an application standard. Where a rating is required comes from other documents: ASHRAE 62.1, California Title 24, LEED, CDC and EPA guidance.
- MERV-A (Appendix J) repeats the test after conditioning that strips electrostatic charge from the media. ASHRAE 241 specs filtration by MERV-A, not MERV.
- UL 900 on a filter label is a fire test, not an efficiency claim.
What ASHRAE 52.2 is and how the test works
ANSI/ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size, is the standard behind every legitimate MERV rating sold in the United States. It is a lab test method. A filter is mounted in a duct, a potassium chloride (KCl) test aerosol is fed upstream, and particle counters measure concentrations upstream and downstream across twelve size channels spanning 0.30 to 10 microns.
The twelve channels are grouped into three ranges: E1 covers 0.30 to 1.0 microns, E2 covers 1.0 to 3.0 microns, and E3 covers 3.0 to 10.0 microns. The test runs the filter from clean through several dust-loading steps, and the reported value in each range is the composite of the minimum efficiencies observed. That word minimum is the whole point of the system: the rating reflects the filter at its weakest, not its best day.
Two more rules from the standard are worth knowing when you read a spec sheet. The minimum final resistance used in the test must be at least twice the initial resistance, and the full reporting designator pairs the rating with the airflow it was tested at, for example MERV 13 at 0.93 m³/s. A manufacturer may only use the term MERV if the entire procedure was followed.
Not an application standard: 52.2 says this about itself in its own foreword. It measures performance. It does not tell you what rating a given building needs. That comes from ASHRAE 62.1, state energy codes such as California Title 24, certification programs such as LEED, and public health guidance.
The MERV rating chart (ASHRAE 52.2 Table 12-1)
These are the actual minimum composite efficiency thresholds from Table 12-1 of the standard, in percent, for each size range. MERV 1 through 4 all land below 20 percent in E3 and are separated by average arrestance, a coarse dust holding test.
| MERV | E1: 0.30 to 1.0 µm | E2: 1.0 to 3.0 µm | E3: 3.0 to 10.0 µm |
|---|---|---|---|
| 1 | Not required | Not required | Below 20%, arrestance below 65% |
| 2 | Not required | Not required | Below 20%, arrestance 65 to below 70% |
| 3 | Not required | Not required | Below 20%, arrestance 70 to below 75% |
| 4 | Not required | Not required | Below 20%, arrestance 75 or higher% |
| 5 | Not required | Not required | 20% or higher |
| 6 | Not required | Not required | 35% or higher |
| 7 | Not required | Not required | 50% or higher |
| 8 | Not required | 20% or higher | 70% or higher |
| 9 | Not required | 35% or higher | 75% or higher |
| 10 | Not required | 50% or higher | 80% or higher |
| 11 | 20% or higher | 65% or higher | 85% or higher |
| 12 | 35% or higher | 80% or higher | 90% or higher |
| 13 | 50% or higher | 85% or higher | 90% or higher |
| 14 | 75% or higher | 90% or higher | 95% or higher |
| 15 | 85% or higher | 90% or higher | 95% or higher |
| 16 | 95% or higher | 95% or higher | 95% or higher |
Reading the chart: a jump from MERV 8 to MERV 11 is the point where a filter must demonstrate capture in the E1 sub-micron range at all. The jump from 11 to 13 is where E1 capture moves from 20 percent to 50 percent. That sub-micron range is where wildfire smoke, exhaust particles, and respiratory aerosols live, which is why so much guidance since 2020 lands on MERV 13.
The standard also includes an informative appendix pairing rating ranges with typical settings: MERV 5 to 8 for commercial buildings and industrial workplaces, MERV 9 to 12 for better commercial buildings and hospital laboratories, MERV 13 to 16 for superior commercial buildings, hospital inpatient care, and general surgery. Informative means guidance, not requirement.
What MERV 13 actually means, and what it does not
Retail listings routinely describe MERV 13 filters as capturing 98 percent of particles down to 0.3 microns. The rating supports no such claim. MERV 13 means the filter demonstrated at least 50 percent capture in the 0.30 to 1.0 micron range, 85 percent from 1.0 to 3.0, and 90 percent from 3.0 to 10.0. A specific filter can certainly test above its floors, but no tier certifies a number like 98 percent at 0.3 microns. Even MERV 16 requires 95 percent averaged across the whole 0.30 to 1.0 micron range. Claiming it for a MERV 13 product without a matching 52.2 test report is marketing, not measurement.
The floor structure is also why two MERV 13 filters can perform differently in service. One may clear the E1 threshold at 52 percent, another at 70. The rating tells you both crossed the same bar. If the application is critical, ask the manufacturer for the 52.2 test report with the efficiency curves rather than relying on the tier alone.
Watch for: capture percentages quoted without a size range or a test airflow. An efficiency number means nothing until you know which particle range it covers and that it came from the 52.2 procedure. The EPA publishes the same Table 12-1 thresholds if you want a second public source to check a claim against.
MERV vs MERV-A: the conditioning step
Most commercial pleated filters reach their rating with electrostatically charged synthetic media. The charge boosts capture while it lasts, and it can fade in service as the media loads. Appendix J of 52.2 addresses this with an optional conditioning step: the filter is exposed to fine KCl aerosol until its efficiency stops dropping, then tested. The result is reported as MERV-A. The standard is direct about why the step exists, noting that charged filters may show high efficiency when clean and a drop during their actual use cycle.
This is not a reason to avoid charged media. It is a reason to know both numbers when the duty is critical. A filter whose MERV-A equals its MERV holds its rating mechanically, without depending on the charge. A filter with a large gap between the two is leaning on the charge.
The A rating is moving from a lab detail to a spec item. ASHRAE Standard 241, Control of Infectious Aerosols, requires mechanical filters to be at least MERV-A 11 or equivalent to earn filtration credit in its infection risk management mode, with the MERV-A requirement in effect since January 1, 2025. Standard 241 is voluntary unless a jurisdiction adopts it, but specs that reference it will ask for the A rating.
FPR, MPR, and ISO 16890
Two other rating systems show up on retail shelves. FPR (Filter Performance Rating) is a retail scale used by The Home Depot, and MPR (Microparticle Performance Rating) is 3M's scale for its own filters. Both are proprietary marketing scales, not consensus standards, and neither is defined by ASHRAE. Published crosswalks between them and MERV are approximations. For anything written into a spec, purchase order, or maintenance plan, state the requirement as a MERV rating per ASHRAE 52.2 and let the retail scales fall away.
ISO 16890 is a different case: it is the international consensus test standard, published in 2016, and it replaced EN 779 as the European classification framework when the transition ended in mid-2018. Instead of a 1 to 16 scale it reports efficiency against particulate matter fractions, the same PM1, PM2.5, and PM10 categories used in air quality reporting, with classes such as ISO ePM1 50%. Its procedure differs from 52.2 in an important way: for the ePM1 and ePM2.5 classes the media is conditioned to remove electrostatic charge, so the classes already account for the effect MERV-A was created to expose.
No direct conversion exists between ISO 16890 classes and MERV ratings. The tests differ in aerosol, loading, and conditioning, so any crosswalk table is approximate. Documents that accept both, such as ASHRAE 62.1, list the acceptable value in each system separately, which is the honest way to do it.
MERV vs HEPA
HEPA is not the top of the MERV scale. It is a separate specification tested by a different method: at least 99.97 percent capture of 0.3 micron particles. MERV 16, the highest MERV rating, requires 95 percent in the same neighborhood. The two systems meet at the top but they are not the same test, and the difference in resistance is substantial. HEPA filters need fan power that ordinary commercial air handlers are not built to deliver.
In practice: central HVAC systems run MERV-rated filters, with MERV 13 to 16 in the demanding applications. HEPA belongs to portable air cleaners, cleanroom and hospital equipment, and purpose-built housings. Upgrading a central system to true HEPA is an engineering project, not a filter swap.
UL 900: the other marking on the frame
Most commercial filters also carry a UL 900 marking. UL 900 is the Standard for Air Filter Units, and it covers fire behavior: how much flame and smoke a clean filter contributes when exposed to fire. It is referenced by NFPA 90A and 90B and by the mechanical codes, which is why it appears on nearly every filter sold into commercial buildings. It says nothing about filtration efficiency.
One dating detail: UL removed the separate Class 1 and Class 2 designations effective May 31, 2012. Filters listed since then simply meet UL 900. Plenty of manufacturer spec sheets still print "Class 2 under UL 900" out of habit, and it reads as a legacy phrase rather than a current UL classification.
Frequently Asked Questions
What is a MERV rating?
MERV stands for Minimum Efficiency Reporting Value. It comes from ANSI/ASHRAE Standard 52.2, which measures how well an air filter removes particles between 0.30 and 10 microns from the air passing through it. The lab counts particles upstream and downstream across twelve size channels and groups the results into three ranges (E1, E2, E3). The rating is built from the lowest efficiency each channel shows during dust loading, averaged within each range, so it reflects the filter near its worst rather than on its best day. A MERV 13 filter held at least 50 percent capture in the hardest range on that basis.
Is a higher MERV rating always better?
Not automatically. Higher MERV means finer capture, and it usually comes with more airflow resistance for the same filter depth and media. A system with limited fan capacity can lose airflow if you jump tiers in a 1 inch filter. The practical fix is depth: a 2 inch or 4 inch pleat at the same MERV has more media surface and lower resistance. Match the rating to what the space needs, then pick the deepest filter the track accepts.
What MERV rating do I need for my building?
It depends on the space and what applies to it. ASHRAE 62.1 requires MERV 8 ahead of wetted cooling coils and MERV 11 on outdoor air in PM2.5 nonattainment areas. California Title 24 requires MERV 13 in covered systems. CDC and EPA recommend MERV 13 or better for infection control and wildfire smoke. Our MERV selection guide walks through offices, schools, healthcare, industrial, and data center applications one by one.
What is MERV-A and how is it different from MERV?
MERV-A comes from Appendix J of the same standard. The filter is conditioned with fine KCl aerosol before efficiency testing, which strips the electrostatic charge that many synthetic filter medias rely on when new. Charged media can test high when clean and lose efficiency in service, and MERV-A is designed to show that drop. A filter whose MERV-A matches its MERV holds its rating mechanically. ASHRAE 241 requires at least MERV-A 11 for filtration credit in its infection risk mode, so the A rating is starting to show up in specs.
Does ASHRAE 52.2 say where each MERV rating is required?
No. The standard says of itself that it is not an application standard. It measures and reports performance, and it includes an informative appendix that pairs rating ranges with typical settings, such as MERV 5-8 for commercial buildings and industrial workplaces and MERV 13-16 for superior commercial buildings and hospital inpatient care. Requirements come from other documents: ASHRAE 62.1, California Title 24, LEED, and whatever your local code or spec references.
Is MERV 13 the same as HEPA?
No, and the gap is large. HEPA means at least 99.97 percent capture of 0.3 micron particles, tested by a different method. MERV 13 requires at least 50 percent in the 0.3 to 1.0 micron range. Even MERV 16, the top of the scale, requires 95 percent. HEPA filters also run much higher resistance, so they need fan power that ordinary HVAC systems do not have. Central systems get MERV-rated filters; HEPA lives in portable air cleaners, cleanrooms, and purpose-built equipment.
What does UL 900 mean on an air filter?
UL 900 is a fire test, not an efficiency rating. It evaluates how much flame and smoke a clean filter produces when exposed to fire, and it is referenced by NFPA 90A and 90B and the mechanical codes. It says nothing about how well the filter captures particles. Spec sheets that say "UL 900 Class 2" are using legacy wording: UL retired the Class 1 and Class 2 split on May 31, 2012, and everything listed since simply meets UL 900.
Was this resource helpful?
Your feedback helps us improve our technical resources.