Before we look at the MERV rating chart (1-20 rating for 0.3 – 10 micron particles), let’s answer this question:
Do you know why HEPA filters are called ‘HEPA’?
It’s actually based on the MERV rating. MERV stands for Minimum Efficiency Reporting Value. The rating was devised by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to answer one question:
Which air purifier is the most effective?
In the most basic sense, the MERV rating gives us an idea of how well filters (air purifiers, furnaces) can filter out 0.3 to 10 micron (10-6m) particles. That’s the size interval that includes most air pollutants smoke, dust, cooking oil smell, mold spores, and bacteria, to name just a few.
Simple use of MERV rating chart for air purifier filters: The higher the MERV rating, the better the filter.
Here is Ron Wilkinson’s (P.E.) commentary on the MERV scale when it first came to light in 1987:
(MERV rating) represents a quantum leap in the precision and accuracy of air purifiers.
We’ll look at MERV 1-20 ratings in the MERV rating chart (you’ll find the chart further on). We’ll also look into how high a MERV rating an air purifier should have to best filter out different air contaminants (pollen, mold spored, smoke, dust, etc.).
In the end, we’ll finally see why HEPA filters are called HEPA (and reveal which filter is even better than the HEPA filter). But let’s start with what kind of particles are between 0.3 and 10 microns:
Air Contaminants Between 0.3 And 10 Microns
Let’s look at examples of two filters with 10 MERV and 14 MERV ratings.
A 10 MERV filter removes:
- 50%-64.9% of air pollutants with an average particle size between 1 and 3 microns.
- More than 85% of air pollutants with an average particle size between 3 and 10 microns or greater.
A 14 MERV filter removes:
- 75%-84% of air pollutants with an average particle size between 0.3 and 1 microns.
- More than 90% of air pollutants with an average particle size between 1 and 3 microns or greater.
When you’re buying an air purifier for a specific purpose, you need to know how big the air contaminants you’re looking to remove are. These are the 4 average contaminant size intervals MERV rating applies to:
| Pollutant Size | Type Of Pollutant |
|---|---|
| 0.3-1.0 μm | Smoke, dust, oil smell, bacteria, sneeze droplets |
| 1.0-3.0 μm | Car emissions, bigger dust particles (lead), Legionella |
| 3.0-10.0 μm | Hair spray, mold spored, mite debris, fabric protector, cat, and dog dander |
| 10.0+ μm | Pollen, sanding dust, cockroach debris, textile fibers |
These are just a few contaminants that we want to get rid of with a good MERV air purifier. You can check the whole EPA’s list of air pollutants here.
MERV Rating Chart
Here is the full MERV rating chart:
The most prevalent MERV ratings are MERV 4, 6, 8, 10, 12, 14, and 16:
| MERV Rating: | Filtering Efficiency For Specific Pollutant Size: |
|---|---|
| MERV 1-4 | Less than 20% for 3.0-10 μm particles |
| MERV 6 | 49.9% for 3.0-10.0 μm particles |
| MERV 8 | 84.9% for 3.0-10.0 μm particles |
| MERV 10 | 85% for 3.0-10.0 μm particles, or greater |
| 50% – 64.9% for 1.0-3.0 μm particles | |
| MERV 12 | 90% for 3.0-10.0 μm particles, or greater |
| 80% – 89.9% for 1.0-3.0 μm particles | |
| MERV 14 | 90% for 1.0-3.0 μm particles, or greater |
| 75% – 84% for 0.3-1.0 μm particles | |
| MERV 16 | 75% for 0.3-1.0 μm particles, or greater |
MERV Rating In Air Purifier Specification Sheets (Or Lack There Of)
A Higher MERV rating automatically means a better filter.
You can see from the MERV scale above that MERV 10 filter will filter out 85% of 3.0-10.0 μm particles. MERV 12 filter, for comparison, will filter out 90% of the same particles.
Here is a list of the best air purifiers; the best ones have to have filters with as high MERV rating as possible.
What is interesting is that you’ll rarely see MERV rating in an air purifier specification sheet. It is quite an important filter-specific factor.
Here’s the reason why:
It isn’t easy to navigate through 20 classes of MERV ratings. That’s why air filter producers don’t usually specify the MERV rating. Rather, they use simplified words that everybody knows.
Why Is HEPA Filter Called HEPA?
HEPA stands for High-Efficiency Particular Air. Here’s the definition of what that actually means:
Definition: HEPA filter is a mechanical filter that must remove 99.97% (according to U.S. DOE) of particles whose average size is equal to 0.3 microns.
The ‘M’ in MERV rating stands for ‘Minimum.’ As you can see, the HEPA filters surpass the most restrictive 0.3-1.0 micron particle size interval MERV rating is based upon.
In other words: HEPA filters are better than anything MERV rating can measure. They are not ‘Minimum’ efficient.
In fact, they are high-efficiency filters. Their rating is based upon filtering out a particular size of air particles, 0.3-micron ones. You can check the best medical-grade air purifiers here; these use H13 HEPA filters.
Because they go ‘off the MERV chart’, they are referred to as HEPA filters or high-efficiency filters.
Can I purchase a 2x14x20 air filter with rating of 20, or 30 if possible, online?
Hi Diane, MERV 20 are highly specialized filters and are not sold online. MERV 30 filters don’t really exist, the MERV rating gets only up to 20.
Hi, great article! Can you provide some insight on how ASHRAE 52.2 Appendix J plays a role in the data tables mentioned above? I normally see a product spec sheet with ASHRAE 52.2 testing, but I don’t normally see Appendix J associated with it. Thanks for any clarity.
Hi Nash, ASHRAE 52.2 is the source of all MERV effectiveness data. You will find exactly the same MERV rating chart there in a bit more technically language.
Is the merv rating simply to help purify my air or does my furnace require a high merv rating to protect itself? The installer put in a merv 16 filter but I’m wondering if it will be bad for my furnace to get a merv 12 filter (less than half the cost of merv 16 filters)
Thanks!
Hi Steve, furnaces don’t need high MERV rated filters to protect themselves; high MERV filters are used for us humans to breathe cleaner air. Rather, the high MERV rating (MERV 13, 16) can damage the furnace. These high MERV filters are very fine and present a big obstacle for the furnace; this can lead to overloading the furnace, restriction of airflow, and all kinds of problems. In your case, it is highly recommended to use MERV 12 filters instead of MERV 16 to protect the furnace. Hope this helps.
I use merv 16 in conjunction with my wood stove. A simple bathroom exit fan pulls air though it. Been in use for years with no problems.
Hi there, usually a MERV 16 is a bit overkill for a wood stove. MERV 11 or 13 would be more suitable. However, if you don’t have problems, you can keep it the way it is now.