MERV stands for Minimum Efficiency Reporting Value and is a measure of how effective the filter is in stopping dust and contaminants, including viruses, from passing through its media. The numerical value in the MERV rating (e.g., MERV10) is obtained by following the indoor air quality (IAQ)- ASHRAE Standard 52.2-1999 Test Procedure. We explain how the MERV testing works and what the results mean.
ASHRAE stands for: American Society of Heating, Refrigeration, and Air Conditioning Engineers
We went to their website for the details on how they test for particulate matter such as: spores, molds, pet dander, pollen, asbestos, fiberglass, smoke, insecticide, dust, smog, tobacco, etc.
What is fascinating about the IAQ-ASHRAE testing is that they create their own materials to exacting specifications in order to have a controlled environment. Their proprietary particle generator creates individual particles of a defined size and specific material. They also carefully control the air flow rate.
The objective is to create particles of sufficient numbers to obtain meaningful counts in all of the measured particle ranges which are the following:
ASHRAE 52.2 Particle Size Ranges (in microns) Group
- 1 – 0.30 to 0.40 E1
- 2 – 0.40 to 0.55 E1
- 3 – 0.55 to 0.70 E1
- 4 – 0.70 to 1.00 E1
- 5 – 1.00 to 1.30 E2
- 6 – 1.30 to 1.60 E2
- 7 – 1.60 to 2.20 E2
- 8 – 2.20 to 3.00 E2
- 9 – 3.00 to 4.00 E3
- 10 – 4.00 to 5.50 E3
- 11 – 5.50 to 7.00 E3
- 12 – 7.00 to 10.00 E3
The test is repeated for 6 cycles, and the efficiency of the filter is measured on each of the 12 particle size ranges for each of the 6 cycles.
Another important consideration is the speed of the airflow during the test. Airflow, which directly correlates to the speed at which the test particulates impact the filter materials, will affect how much particulates can pass through the filter materials.
There are seven approved speeds, measured in Feet Per Minute (FPM) and Meters Per Second (m/s)
- 1 – 118 FPM 0.60 m/s
- 2 – 246 FPM 1.25 m/s
- 3 – 295 FPM 1.50 m/s
- 4 – 374 FPM 1.90 m/s
- 5 – 492 FPM 2.50 m/s
- 6 – 630 FPM 3.20 m/s
- 7 – 748 FPM 3.80 m/s
The Minimum Efficiency Reporting Value (MERV) must be stated with the speed of the airflow at which the filter was tested. Here are the most common MERV Ratings, along with their Efficiency (as a percentage), the Particle Size, common Applications and the Filter Type materials:
MERV 1 to 4, <20% Efficiency , >10 microns, Residential, Permanent
- Light Commercial Metal
- Equipment Foam
- Fiberglass Foam
- Poly Panel Foam
MERV 5 to 8, <20% to 35% Efficiency , 3 to 10 microns, Commercial, Pleated Filters
- Industrial Tackified
- Better Residential Tackified
- Paint Booth Tackified
MERV 9 to 12, 40% to 75% Efficiency, 1 to 3 microns, Residential, Pleated Filters
- Commercial Rigid Box
- Telecommunications Rigid cell
- Industrial Bag
MERV 13 to 16, 80% to 95% Efficiency , 0.3 to 1 microns, Smoke Removal, Rigid Cell
- General Surgery Bags
- Hospitals V-Cell
- Health Care Mini-pleat
- Superior Commercial Mini-pleat
As an illustration of what most home owners would use:
The best filters for home use will have a MERV rating of around 11, 12 or even 13. These are typically made from deep pleated electrostatic-ally charged synthetic gradient media.
One example, and has been recommended by many contractors, is the Aprilaire Space Gard 201 Alternative Pleated AC Furnace Air Filters.
This is a rated MERV 11 and costs about $60 for 2 filters. Each filter will last about 6 months for regular usage; Although depending on your conditions, replacements can easily be made annually instead.
It’s important, not only for your health, but also for the performance of all your electronics. This is especially true for any devices such as PCs, Laptops, Routers, etc. which have cooling fans. Dust and particulates will eventually build up on the components inside your electronics. This can cause overheating, short-circuits and equipment failure.
When dust gets inside your hard disk drive (HDD), CD / DVD players, and other gear it can cause errors in data writing and reading.
When looking for a filter, be sure to note the exact measurements of the one you are replacing, as well as the manufacturers type. Most filters will have the dimensions listed on the box / frame for easy reference when it’s time to order replacements.
Recent updates and improvements to the ASHRAE Standard for testing include:
- substitution of chemically generated potassium chloride (KCl) dust for the test dust
- de-coupling of filter loading from filter arrestance
Let’s look at each of these in more detail.
Using KCl enables the testers to more easily and consistently created particulate down to 0.3 microns. Since repeatability is key to the cycle testing, there is a more uniform application of the particles to the materials resulting in a more true evaluation of the filter.
With “arrestance” – gravimetric (e.g., by weight) measure of the ability of a tested filter to remove ASHRAE synthetic dust from the test air – as there is more dust in the air, there will be more dust accumulating on the filter. This method better models the expected increases in filter efficiency with higher particulate loads, and the associated pressure drop as there is material build-up on the filter.
As mentioned earlier, the filters go through 6 cycles of testing starting with a new, clean filter and progressing through to additional material loading until reaching a maximum pressure drop. After each stage of loading, the filter is tested for its ability to mask out KCl particles of 0.3-10 microns. This testing methods provides a more realistic and accurate measurement of arrestance at each stage, rather than the average produced by the
It may seem counter intuitive that dry-media filters exhibit an increase in efficiency as they collect material. This
increase in efficiency corresponds to a decrease in open area as collected fibers and particles cover the media. The contaminant particulates are acting as an extension of the filter since they are now blocking other particulates from passing through the material.
However, the trade-off is there is now a decrease in the volume of air which can pass through the filter, as well as an increase in the amount of work the fan system must to do to maintain a defined air flow.
Also, this increased efficiency may not be enough to accomplish the required objectives in a dust-critical environment, which can’t wait for the efficiency to increase to a suitable level.
Once all the testing has been completed, the MERV rating is based on the worst-
case performance of a filter through all six stages of dust loading and all particles sized
0.3-10 microns. Because the rating represents the worst-case performance, specifiers can use it to assure performance in applications where a maximum particle count must be maintained over the filter’s entire life.
Again from the ASHRAE website, we have the specifications for high-efficiency particulate air (HEPA) filters and
ultra-low penetration air (ULPA) filters:
- MERV values of 8 and higher correspond to efficiencies > 30%
- MERV values of 13 and higher correspond to efficiencies > 80%
- MERV values of 15 and higher correspond to efficiencies > 95%
- MERV values of 16 to 20 correspond to HEPA and ULPA
A typical MERV rating tells a specifier the arrestance rating of the filter at three particle sizes of 0.3-10 microns at a certain air velocity on the filter surface. For example, a rating of:
- MERV 10 at 0.93 indicates a filter with a MERV 10 arrestance when operated at an airflow of 0.93 cubic meters per second, which is a little less than 500 feet per minute, or 2,000 cubic feet per minute (cfm) for a standard 2-foot-square filter.
- MERV 10 corresponds to 50-65 percent efficiency for particles 1-3 microns and above 85 percent efficiency for particles 3-10 microns.
- MERV 13 corresponds to less than 75 percent arrestance efficiency for particles 0.3-1 microns, above 90 percent efficiency for particles 1-3 microns, and above 90 percent efficiency for particles 3-10 microns.
- MERV 15 corresponds to 85-95 percent arrestance efficiency for particles 0.3-1.0 microns, above 90 percent efficiency for particles 1-3 microns, and above 90 percent efficiency for particles 3-10 microns.
If you need more details, the entire list of MERV ratings based on particle arrestance efficiency is found in Table 12-1 of Standard 52-2.