• Filter/Air Cleaner Efficiency Tests(top)

  While reviewing this section please refer to Characteristics of Particles and Particle Dispersoids and  Airborne Pathogens Database for information on size of airborne particulate.

  • The DOP Test (MILITARY-STANDARD 282)(top)
    High Efficiency Particulate Arresting Filters ( HEPA ) are a popular filter type used in indoor air cleaners.  They represent state of the art technology and are one of the most efficient types of air cleaner filters used for general air cleaning.  By definition, a true HEPA filter in contrast with a near HEPA filter,  must remove 99.97% of all particles of 0.3 microns mean diameter.  Near HEPA filters generally are considered to have efficiencies at 0.3 microns of less than 99.97% but greater than 90%.  In the theory of air cleaning a 0.3 micron size particle is near the most difficult size particle to filter from the air.  It is intuitive to see that 0.3 micron particles are more difficult to filter than larger particles.  But, intuition is contradicted by the fact that 0.3 micron particles are actually more difficult to filter than smaller particles¹.  As particles become very small, the effects of Brownian motion allow the particles to impact on filter fibers not in the most direct path to the exhaust side of the filter.
    
    When particles become extremely small compared to 0.3 microns this rule breaks down and the particles may no longer be filtered from the air.  In this case, another process called adsorption is used to remove particles that approach the size of small gas molecules. 
    
    The DOP test obtains it's name from an abbreviation of the organic material Di-Octyl Phthalate evaporated to create 0.3 micron particles used for this filter efficiency test.  Because DOP is a recognized carcinogen, safer synthetic oil Emery 3004 (Poly-Alpha-Olefin; PAO) is sometimes substituted for DOP.
    
    Since a filter’s efficiency increases as it accumulates particulate matter, the initial efficiency when the filter is clean, is the lowest efficiency during the life of a filter.

    Figure from FILTRATION OF AIRBORNE MICROORGANISMS: MODELING AND PREDICTION, W. J. Kowalski, M.S., P.E., ASHRAE Student Member, W. P. Bahnfleth, Ph.D., P.E., Member ASHRAE, T. S. Whittam, Ph.D., Copyright © 1999 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

    _________
    ¹ "Filtration of aerosol particles by fibrous filters has been subject to numerous theoretical and experimental studies....... As a result, the dependence of filtration efficiency on particle size is now well established.  An increase in particle size will cause increased filtration by the interception and inertial impaction mechanisms whereas a decrease in particle size will enhance collection by Brownian diffusion.  As a consequence, there is an intermediate particle size region where two or more mechanisms are simultaneously operating yet none is dominating.   This is the region where the particle penetration through the filter is a maximum and the efficiency of the filter a minimum.  Although the minimum filter efficiency and the particle size at which the minimum efficiency occurs will vary depending on the type of filter and the flow velocity, the existence of such minimum filter efficiencies is well established, not only for fibrous filters but also for membrane and Nuclepore filters.  For most fibrous filters operating at relatively low filtration velocities, the minimum filter efficiency is generally known to occur in the vicinity of 0.3 mm.  This is the basis of the widely used dioctyl phthalate (DOP) test for high efficiency particulate air filters (HEPA) which makes use of the monodisperse 0.3 mm diameter DOP particles for testing the filter.  However, at high filtration velocities, the most penetrating particle size may become substantially smaller than 0.3 mm, as Liu and Lee have shown." 
    Above excerpt from: K.W. Lee and B.Y.H. Liu, University of Minnesota,   "On the Minimum Efficiency and the Most Penetrating Particle Size for Fibrous Filters", Air Pollution Control Association (APCA) Journal, Vol. 30, No. 4, April 1980

    Also see: MODELING FILTRATION OF AIRBORNE PATHOGENS
     

  • Atmospheric Dust Spot Test (ASHRAE Standard 52.76 Atmospheric)(top)
    Usually used to rate medium efficiency air cleaners (both filters and electronic air cleaners). The removal rate is based on the cleaner's ability to reduce soiling of a clean paper target, an ability dependent on the cleaner removing very fine particles from the air. Exhibit 1 shows typical applications and limitations of filters rated using the ASHRAE Standard 52-76 atmospheric dust spot test.
    (above excerpt from document EPA 400/1-90-002, February 1990)
    

    Exhibit 1. Filter Applications for In-duct Systems Based on ASHRAE Standard 52.76 Atmospheric Dust Spot Test

    Efficiency	Applications
    10%	n Used in window air conditioners and heating systems n Useful on lint. n Somewhat useful on ragweed pollen. n Not very useful on smoke and staining particles.
    20%	n Used in air conditioners, domestic heating, and central air systems. n Fairly useful on ragweed pollen. n Not very useful on smoke and staining particles.
    40%	n Used in heating and air conditioning systems, and as pre-filters to high efficiency cleaners. n Useful on finer airborne dust and pollen. n Reduce smudge and stain materially. n Slightly useful on non-tobacco smoke particles. n Not very useful on tobacco smoke particles.
    60%	n Use same as 40%, but better protection. n Useful on all pollens, the majority of particles causing smudge and stain, and coal and oil smoke particles. n Partially useful on tobacco smoke particles.
    80%	n Generally used in hospitals and controlled areas. n Very useful on particles causing smudge and stain, and coal and oil smoke particles. n Quite useful on tobacco smoke particles.
    90%	n Use same as 80%, but better protection. n Excellent protection against all smoke particles.

    (above table based on table in document EPA 400/1-90-002, February 1990)

  • Arrestance Test (ASHRAE Standard 52.76 Arrestance)(top)
    Generally used to evaluate low efficiency filters designed to remove the largest and heaviest particles; these filters are commonly used in residential furnaces and/or air-conditioning systems or as upstream filters for other air-cleaning devices. For the test, a standard synthetic dust is fed into the air cleaner and the proportion (by weight) of the dust trapped on the filter is determined. Because the particles in the standard dust are relatively large, the weight arrestance test is of limited value in assessing the removal of smaller, respirable-size particles from indoor air.
    (above excerpt from document EPA 400/1-90-002, February 1990)

  • How the tests compare(top)
    It should be noted that ASHRAE Standard 52-76 addresses the overall efficiency of removal of a complex mixture of dust. However, removal efficiencies for different size particles may vary widely. Recent studies by EPA, comparing ASHRAE ratings to filter efficiencies for particles by size, have shown that efficiencies for particles in the size range of 0.1 to 1 µm are much lower than the ASHRAE rating5. A filter with an ASHRAE dust spot rating of 95 percent only removed 50-60 percent of particles in the 0.1 to 1 µm size range. Many of the respirable-size particles in indoor air (e.g., cigarette smoke) appear to be in this size range.

    In contrast to the ASHRAE Standard 52-76 ratings, efficiencies derived by the DOP method in Military Standard 282 are expected to be more representative of capture efficiencies for respirable-size particles.
    (above excerpt from document EPA 400/1-90-002, February 1990)

    Exhibit 2. Efficiency Comparison Scales for Major Filter Tests

    Above comparative scale prepared by David Murphy, P.E., Air Filter Testing Laboratories, Louisville, Kentucky, March 1987 for A.S.H.R.A.E Review Committee; reproduced from a printing by B.C. Air Filter, Vancouver, Canada October 1987

    Exhibit 3. Relative Particle Size & Efficiency Ratings

    Range of Efficiency	Typical Particle Size Retained by Filter
    Arrestance	10 micron - smallest particle visible to human eye
    Atmospheric	1.5 micron
    DOP - HEPA	0.3 micron

    One micron(um) = 1/25,000 inch = one micrometer = one millionth of a meter
    
     

    ASHRAE Standard 52-76 (1976) ratings have been replaced by ASHRAE Standard 52-1 (1992).  These two standards are very  similar.  In addition, a new standard, ASHRAE Standard 52-2 (1992), provides for filter efficiency ratings by evaluating the fractional efficiencies in three particle size ranges.  The filter efficiency ratings are designated by Minimum Efficiency Reporting Value (MERV) between 1 and 20 as shown in Exhibit 4 below.

    Exhibit 4. Comparison of ASHRAE Standards 52.1 and 52.2

    U.S. Standards						European Standards	Particle Size Range, um	Applications
    ASHRAE 52.2				ASHRAE 52.1		EN779 & EN1822
    MERV	Particle Size Range			Test		European Efficiency Class
    	3 to 10 um	1 to 3 um	0.3 to 1 um	Arrestance	Dust Spot
    1	<20%	-	-	<65%	<20%	G1,G2	>10	Residential, light, pollen, dust mites
    2	<20%	-	-	65 - 70%	<20%
    3	<20%	-	-	70 - 75%	<20%
    4	<20%	-	-	>75%	<20%
    5	20 -35%	-	-	80 - 85%	<20%	G3	3.0 - 10	Industrial, dust, molds, spores
    6	35 - 50%	-	-	>90%	<20%	G4,F5
    7	50 - 70%	-	-	>90%	20 -25%
    8	>70%	-	-	>95%	25 - 30%
    9	>85%	<50%	-	>95%	40 - 45%	F5	1.0 - 3.0	Industrial, Legionella, dust
    10	>85%	50 - 65%	-	>95%	50 - 55%	F5,F6
    11	>85%	65 - 80%	-	>98%	60 - 65%	F6
    12	>90%	>80%	-	>98%	70 - 75%	F6
    13	>90%	>90%	<75%	>98%	80 - 90%	F6,F7	0.3 - 1.0	Hospitals, Smoke removal, bacteria
    14	>90%	>90%	75 - 85%	>98%	90 - 95%	F7,F8
    15	>90%	>90%	85 - 95%	>98%	-95%	F8
    16	>95%	>95%	>95%	>98%	>95%	H11
    17	-	-	³99.97%	-	-	U13-14	<0.3	Clean rooms, Surgery, chem-bio, viruses
    18	-	-	³99.99%	-	-	U15
    19	-	-	³99.999%	-	-
    20	-	-	³99.9999%	-	-

Note: Exhibit 4 is adapted from Table 5-1, U.S. Federal Emergency Management Agency (FEMA) document 426, Risk Management Series, Reference Manual to Mitigate Potential Terrorist Attacks Against Buildings, December 2003,  [adapted from American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) Standard 52.2: Method of Testing General Ventilation Air-cleaning Devices for Removal Efficiency by Particle Size, Atlanta, GA, 1999 and Spengler, J.D., Samet, J.M., and McCarthy, J.F., Indoor air quality Handbook, New York, NY: McGraw-Hill, 2000] and Airguard filter efficiency selection table.

Modeling Filter Bypass: Impact on Filter Efficiency (PDF)

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