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What is a MERV Rating?

Merv Rating

 

MERV is an acronym and a MERV Rating is an important concept to understand about air purification.

Minimum Efficiency Reporting Values (MERV): These values report a HEPA Filter's ability to capture larger particles between 0.3 and 10 microns (µm). If you need a refresher on microns, see our previous blog on microns. There are multiple MERV Rating charts out there but we've included the information from the EPA.

MERV Rating

Average Particle Size Efficiency in Microns

1-4

3.0 - 10.0 (less than 20%)

6

3.0 - 10.0 (49.9%)

8

3.0 - 10.0 (84.9%)

10

1.0 - 3.0 (50% - 64.9%)

3.0 - 10.0 (85% or greater)

12

1.0 - 3.0 (80% - 89.9%)

3.0 - 10.0 (90% or greater)

14

0.3 - 1.0 (75% - 84%)

1.0 - 3.0 (90% or greater)

16

0.3 - 1.0 (75% or greater)

 What does a MERV Rating Mean For Consumers?

When shopping for air purifiers you often read it’s best to go with something that is in the MERV 17-20 range. First, it is important to understand the use case for the other MERV rated filters. As you can see in the chart above, the lower MERV ratings target larger particles. Hence, many air purifiers are designed with multiple stages of filtration with pre-filters that capture larger particles and then higher rated post-filters as the stages progress toward taking aim at very small particles.

So, what about the other MERV ratings like 17-20?  It's good to appreciate that the MERV rating system was developed for particulate sizes down to .3 microns (which is why the enclosed EPA chart above stops there). Thus, technically speaking, there is no such thing as MERV 17-20.

In fact, HEPA filters aren't really tested using MERV testing methods. Rather, they are manufactured according to their own 'E', 'H', and 'U' numbering scales. MERV is a way to test the efficiency of different brands of filters in order to arrive at a standard for comparison.

Thus, the HEPA 'H' classification scale of H13-H14 changes to the 'U' classification scale of U15-U17 for particulates smaller than .3 microns. This range of filters are actually called ULPA. So, when manufacturers refer to MERV 17-20 they are really saying their "ULPA" filters are efficient at removing particles at .1 microns or even smaller at efficiencies from 99.9975% to 99.9999%.

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