What is Merv?






A practical look at ASHRAE 52.2 -1999 “Method of testing general ventilation air-cleaning devices for removal efficiency by particle size” 




  • Minimum – worst case scenario 
  • Efficiency – based upon specific particle size challenge
  • Reported –12 sets of particle sizes in three groups of four sets to make ranges E-1 (.3-1 micron), E-2 (1-3 microns), E-3 (3-10 microns)
  • Value – tested upstream vs. downstream and given as a percentage performance


  • ASHRAE 52.2-1999
  • Particle size performance
  • Minimum efficiency
  • Tested to maximum airflow advertised
  • Tested to realistic pressure drops
  • Consistent test aerosol
  • ASHRAE 52.1-1992
  • Dust spot efficiency
  • Average efficiencies
  • Moving pressure drop values
  • Atmospheric challenge changes
  • Dust Holding Capacity
ASHRAE Test Method 52.2-1999 
Test Duct Configuration


Possible Downside?


ASHRAE Test Method 52.2-1999 uses Neutralized dust meaning that the natural positive and negative charges in the atmosphere are diminished in the test

This could have a different effect on efficiency compared to natures effects

Average Efficiency?


Q: When is a 95% filter not a 95% filter?

A: When it is new, it is not 95% per ASHRAE test method 52.1-1992 (Dust Spot Efficiency)


The top five 95% V-cell filters today have an initial efficiency of 57 – 81%* on their best distributed test report

* - Per ASHRAE Standard 52.1-1992

Determine the NEED


The particle size which causes problems varies from industry to industry and methods or purpose of use

Once we determine the particle size we can choose a filtration efficiency – M.E.R.V.

WHO Sets the Particle Size?


Industries that know what the killer defect size particle is, require filtration systems to remove those particles from the air…


  • Healthcare and Hospitals
  • Semi-conductor Industry
  • School Districts and Universities
  • Commercial & Industrial Real Estate

Size Distribution of Atmospheric Dust Sample

dust samples

Ranges in ASHRAE Test Method 52.2-1999

  • E-3 particles between 3 and 10 microns
  • E-2 particles between 1 and 3 microns
  • E-1 particles between 0.3 and 1 micron
(efficiency is calculated on the composite range)

NOTE: there is no mathematical correlation between test methods 52.1-1992 and 52.2-1999 filter efficiencies

M.E.R.V. 1 - 4

  • Filters that fit within the 1 - 4 category are not tested by ASHRAE 52.2-1999 test procedure. Rather the Standard refers them back to ASHRAE 52.1-1992
  • Less than 20% Efficient on E-3 (3-10 microns) range
  • These are the historic arrestance type filters – autorolls, metal washables, etc

M.E.R.V. 5 - 8

  • Starting above 20% on the E-3 (3-10 microns) range
  • M.E.R.V. 5 must be 20-35%
  • M.E.R.V. 6 must be 35-50%
  • M.E.R.V. 7 must be 50-70%
  • M.E.R.V. 8 must be above 70%

   (Can be Filters from 20% to 60% per ASHRAE Standard 52.1-1992, Dust Spot Test)

M.E.R.V. 9 - 12

  • Measurable efficiency on the E-2 (1-3 microns) range and a minimum of 85% or more on E-3 (3-10 microns)
  • M.E.R.V. 9 must be up to 50%
  • M.E.R.V. 10 must be 50-65%
  • M.E.R.V. 11 must be 65-80%
  • M.E.R.V. 12 must be more than 80%

(Can be Filters from 40-90% per ASHRAE Standard 52.1-1992, Dust Spot Test)

M.E.R.V. 13 - 16

  • Measurable efficiency in submicron E-1 (.3-1 micron) and more than 90% in the E-2 (1-3 microns) and E-3 (3-10 microns)
  • M.E.R.V. 13 is up to 75%
  • M.E.R.V. 14 must be 75-85%
  • M.E.R.V. 15 must be 85-95%
  • M.E.R.V. 16 must be at least 95% in all categories E-1 (.3-1 micron), E-2 (1-3 microns) and E3 (3-10 microns)

    (Can be filters from 75  Dust spot – 95 DOP)

M.E.R.V. Reporting

A filter is given a M.E.R.V. at a given airflow
Example :
M.E.R.V. 11 at 492 fpm
(1968 cfm)
M.E.R.V. 11 at 295 fpm
(1180 cfm)

M.E.R.V. Reporting

Final pressure drops are per Range
E-3 (3-10 microns) is 0.6” wg
E-2 (1-3 microns) is 1.0” wg
E-1 (.3-1 micron) is 1.4” wg

Dust Holding Capacity is not a reported value

What does the 52.2-1999 Test Method Provide?

“When a specific particle size efficiency is identified as the target particle size it is possible to look at the test for that specific size rather than depend on the M.E.R.V.”

test data report


Coming Soon?

DHC – dust holding capacity

Research is looking for a more realistic loading dust to make the value relate to atmospheric dust loading

Has not been released for public review yet


ASHRAE 52.2-1999 Test Method Users Guide



Simple explanation of test procedure

Has all the ranges and particle size info

Rough equations to ASHRAE 52.1-1992 efficiency

And examples of the uses



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