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Here we summarize the key measures of air filter effectiveness and we discuss the real world effectiveness of filters for improving indoor air quality in homes. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Readers should also see  AIR FILTER EFFECTIVENESS where we provide added detail. Our page top photo shows the first level of filtration in the cascade or in-series filter system discussed at How to Cascade Air Filters. See ENVIRONMENTAL HAZARDS - INSPECT, TEST, REMEDY for our full list of environmental hazard identification and remedy related to buildings.

© Copyright 2012 InspectAPedia.com, Daniel Friedman, Steve Bliss, Wiley & Sons, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

Measuring Air Filter Effectiveness

As discussed in Best Practices Guide to Residential Construction:

Different types of air cleaners use different rating systems. It is important to know what is being measured, since a “90% efficient filter” may actually capture 0% of respirable particles.

In-Duct Filters. Under ASHRAE Standard 52-89, low-efficiency HVAC filters are evaluated for “arrestance” efficiency and medium- and high-efficiency filters are rated for “dust spot” efficiency. HEPA filters have their own measure as follows:

• Arrestance efficiency: This measures how well a filter captures large, heavy particles and is generally used only for the low-efficiency filters typically found in residential HVAC systems.
• Dust-spot efficiency: This measures how well a filter captures a mixture of fine particles ranging from 0.3 to 6.0 microns, and it is a good guide to how well a filter handles many respirable particles. However, it does not accurately predict the filtering of particles less than 1.0 micron. Also, two filters with the same dust-spot efficiency may perform very differently depending on the size of the particles being captured (see Table 7-3 at left)
• DOP: Based on Military Standard 2823, this uses a fine aerosol of the chemical dioctylphthalate (DOP) to rate high-efficiency filters and requires removal of 99.97% of 0.3-micron particles. Filters meeting this standard are often called “true HEPA” filters. High- efficiency filters that do not meet the standard may still have very good performance and may be called “near HEPA” or similar terms.
• MERV: The Minimum Efficiency Reporting Value is based on ASHRAE Standard 52.2 1999. This new rating system evaluates a filter at a particular air velocity over 12 particle sizes ranging from 0.3 to 10 microns. MERV values range from 1 to 16. A higher MERV indicates that a filter is more efficient and more effective against smaller particles. Filter manufacturers are beginning to report MERV values. see AIR FILTER EFFECTIVENESS for details.

Portable air cleaner units units. While tabletop units are generally ineffective, larger console-style filters can be effective when operating in a limited space provided that the rooms are kept closed and that there is not a large problem particle reservoir such as mold contamination. These units may contain one or more of the filter types described above. No portable air cleaner can remove a problem particle reservoir in the building. Many include HEPA filters and some also use adsorbents to capture odors and gases.

The Association of Home Appliance Manufacturers (AHAM) has developed an ANSI-approved standard for portable air cleaners called “Clean Air Delivery Rate” (CADR), which measures how quickly the filter removes dust, smoke, and pollen particles. The CADR is a function of both filter efficiency and airflow rate and assumes the filters are new and clean. To calculate how large a space a unit can effectively handle, AHAM recommends using the formula:

Square footage of room = Smoke CADR x 1.55

So, for example, a unit with a CADR of 100 can service a 155-square-foot room. The formula is based on the requirement that the filter remove 80% of the smoke particles on a continuous basis.

For details about air filter effectiveness, see AIR FILTER EFFECTIVENESS. Readers who are concerned about fiberglass release from air filters should see FIBERGLASS & AIR FILTERS.

Real-World Effectiveness of Air Cleaners & Filters

While air cleaners work efficiently in laboratory tests, their effectiveness in the typical household is less clear. Research conducted by the Canada Mortgage and Housing Corporation (CMHC) found that upgrading the HVAC systems to use medium and high-efficiency filters had only a modest effect on personal exposures to particulates in homes.

During peak activity periods, the best performing filter in the study, an electronic air cleaner, reduced particulate levels in the duct- work by 95%, but cut household levels by only 31%. In nonactive times, like the middle of the night, the filter reduced particulate levels by over 70%, but levels were already so low that the reduction had little impact on the occupants’ personal exposure or health (see Table 7-4 at left).

Researchers attributed the modest effect of the filters to two key facts: First, the filters only worked while the furnace fans were operating, about 20% of the time on average. Second, personal exposures to high levels of particulates were almost always caused by the occupants themselves, who in their daily tasks of cooking, vacuuming, or just walking on the carpet stirred up small clouds of surrounding dust.

Once the activity ended, the particles tended to rapidly settle out on their own before the filter, far away down a duct, could have much of an impact. If furnace fans were run continuously or the filters were part of a continuously operating ventilation system, their impact might have been greater.

Also, many of the larger, heavier particles linked to allergies, such as pollen, house dust, animal dander, and some molds, are more likely to be found settled on surfaces than in the air. A high-efficiency vacuum is needed for these, not an air cleaner.

Conclusions About Air Filtering Cost, Effectiveness, and Indoor Air Quality

The conclusion drawn by most air-quality experts is that filtering the air is costly and the effects are modest unless a high-efficiency filter is used, maintained well, and run frequently. A properly sized console unit can be effective at limiting personal exposure to pollutants, at least for the time spent in the room with the device. First costs for whole-house systems, typically in the hundreds and sometimes thousands of dollars, plus maintenance costs, may be a justifiable expense for individuals with special health problems—but probably not for the average homeowner.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Anyone intent on keeping their household air clean should focus first on keeping pollutants out of the house in the first place (see “Source Control,” page 268 in Best Practices Guide to Residential Construction), along with regular vacuuming with a HEPA vacuum or central vacuum exhausted to the exterior. That, along with filtering any outdoor air brought into the home’s ventilation system, will go a long way toward providing wholesome indoor air.

For optimum indoor air filter placement, air filtration design, and filter alternatives on central heating and air conditioning systems, see AIR FILTERS for HVAC SYSTEMS and AIR FILTERS, OPTIMUM INDOOR.

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Technical Reviewers & References

• InspectAPedia.com^® - Daniel Friedman - Publisher & Editor.
• Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education. Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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  AIR CLEANER PURIFIER TYPES
  Air Filter Effectiveness
  Air Filtering Strategies
  Air Pollutants, Health Effects
  Air Pollutants, Common Indoor
  Air Pollutants, Finding & Reducing

• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
  

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
• Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com