• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar

Allergy Clean Environments

  • Products
  • Articles
  • Blog
  • About
  • Contact
  • Cart

February 17, 2016 By Jim Rosenthal

Understanding MERV or the Minimum Efficiency Reporting Value

How the ASHRAE 52.2 Helps You Select an Air Filter

For 30 years tests for air filters were done according to ASHRAE 52.1. This test provided an efficiency rating such as a 30%, 65%, 90%, etc. This system is relatively easy to understand. However, it has some major deficiencies in that it does not provide a very good guide for selecting filters for particular contaminants. In order to do that effectively you need to know two things – one, the particle size of the contaminant you are trying to remove and two, the level of efficiency of the filter on that particle size. Thus the ASHRAE 52.2 test was developed to provide this particle size efficiency data.

Obtaining the Data

An air filter’s performance is determined by measuring the particle counts upstream and downstream of the filter being tested. Particle counts are taken over the range of particles six times. One begins with a clean filter and then uses that same filter 5 additional times.

The particle generator creates particles of a known size in the air stream. 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

Range Size (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 efficiency of the filter is then measured on each of the 12 particle size ranges for each of the 6 cycles. Efficiency is measured as the percentage of particles captured by the filter. The lowest of the 6 readings is then taken to determine the Composite Minimum Efficiency Curve.

The twelve size ranges are then placed in three larger groups (E1, E2, E3) and the percentages in each group are averaged. This average is called the Particle Size Efficiency and these are used to determine the Minimum Efficiency Reporting Value (MERV).

MERV Parameters

MERV Value Group 1
Av. Eff. %
(0.30 to 1.00)
Group 2
Av. Eff. %
(1.00 to 3.00)
Group 3
Av. Eff. %
(3.00 to 10.00)
1 n/a n/a A-avg<65
2 n/a n/a 65<A-avg<70
3 n/a n/a 70<A-avg<75
4 n/a n/a 75<A-avg
5 n/a n/a 20<E3
6 n/a n/a 35<E3
7 n/a n/a 50<E3
8 n/a 20<E2 70<E3
9 n/a 35<E2 75<E3
10 n/a 50<E2 80<E3
11 20<E1 65<E2 85<E3
12 35<E1 80<E2 90<E3
13 50<E1 85<E2 90<E3
14 75<E1 90<E2 95<E3
15 85<E1 90<E2 95<E3
16 95<E1 95<E2 95<E3

When using the above table you move up each Group until you arrive at the highest true statement. This will correspond with a MERV number. The lowest MERV of the three groups is the MERV  for the filter.

Another important consideration is the speed of the airflow during the test. There are seven approved speeds:

# Feet Per Minute meters/secon
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.

One of the challenges of having both an ASHRAE 52.1 test and an ASHRAE 52.2 test is trying to make some comparisons between the two. The table below may help in this regard:

MERV Value Efficiency Particle Size Applications Filter Type
1-4 <20% >10 microns Residential Permanent
      Light Commercial Metal
      Equipment Foam
      Fiberglass Foam
      Poly Panel Foam
5-8 <20 to 35% 3 to 10 um Commercial Pleated Filters
      Industrial Tackified
      Better Res. Tackified
      Paint Booth Tackified
9-12 40 to 75% 1 to 3 um Res. – Best Best Pleated
      Commercial Rigid Box
      Telecommunications Rigid cell
      Industrial Bag
13-16 80 to 95% 0.3 to 1 um Smoke Removal Rigid Cell
      General Surgery Bags
      Hospitals V-Cell
      Health Care Mini-pleat
      Superior Comm. Mini-pleat

These comparisons are not meant to be definitive. They are guidelines only. If you have any questions about this subject, please give me a call at 817-261-3791 or e-mail me at jimrosenthal@allergyclean.com.

As a practical matter, let’s see how we can use these MERV numbers. One example would be mold spores. What filter would we recommend if we were concerned with pulling out a high percentage of mold? Mold varies in size from about 4 microns to 40 microns. The greatest number of mold spores are less than 10 microns in diameter. If we look at our chart a MERV 8 filter will pull out at least an average of 70% of the particles down to 3 microns. Not a bad recommendation. However, for just a little more money pleated filters are available in a MERV 11. The MERV 11 would give you an average of at least 85% mold removal. For all those but the most mold sensitive this should be adequate.

Note: This article is based on information provided by the National Air Filtration Association (NAFA). For more information on NAFA and the programs offered by this excellent organization please visit their website at www.nafahq.org.

Filed Under: Articles

Previous Post
Next Post

Primary Sidebar

Allergy Proofing Articles

  • A Face Shield is not Enough in the Battle against Covid-19. Listen to “Hairy Manne.” Always wear a Face Mask!
    July 25, 2020
  • How to Select the Right HVAC Filter
    February 17, 2016
  • Formaldehyde Increases through Exposure to Ozone from Ionizers and Ozone Generators
    February 17, 2016
  • Importance of Air Filtration
    February 17, 2016
  • There’s Something in the Air
    February 17, 2016
  • Problems with Ozone Generators and Ionizers that Produce Ozone
    February 17, 2016
  • You’ve Got Mold!
    February 17, 2016
  • What You Should Know About Ozone
    February 17, 2016
  • The Truth About Zeolites Used as Desiccants
    February 17, 2016
  • Allergy Proofing Your Home
    February 17, 2016
Pages: 1 2

Useful Information

  • Useful Links
    February 17, 2016
  • About Common Allergens
    February 17, 2016
  • Allergen Avoidance Checklist
    February 17, 2016
  • Frequently Asked Questions
    February 17, 2016
  • Controlling Irritants
    February 17, 2016

Copyright © 2023 Allergy, AIr & More · All Rights Reserved · Privacy Policy · Refunds/Exchanges