Cascade air filter system (C) Daniel Friedman Air Filter Effectiveness Table & Guide
Air purifiers & air filters: what do they remove from indoor air

  • AIR FILTER CLEANER EFFECTIVENESS - CONTENTS: How do we measure the "real world" or practical effectiveness of different types of air filters in improving indoor air quality? Table of air filter effectiveness by application and filter type. What is the real-world effectiveness of air filters?. Definition of MERV, arrestance efficiency, dust spot efficiency, & DOP filters
  • POST a QUESTION or READ FAQs about the effectiveness & efficiency of different types of air filters used in HVAC systems or air cleaners, furnaces or air conditioners
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Effectiveness of air filters: 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.

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Measuring Air Filter Effectiveness

Table of filter efficiency and particle removal (C) J Wiley, Steven BlissAs discussed in Best Practices Guide to Residential Construction (available at REFERENCES):

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.

[Click to enlarge any image or table]

Our page top photo shows the first level of filtration in the cascade or in-series filter system discussed

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:

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.

Readers who are concerned about fiberglass release from air filters should

Real-World Effectiveness of Air Cleaners & Filters

Table of real world performance of upgraded furnace filters (C) J Wiley, Steven Bliss

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.

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

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

Air purifiers or home air filtration systems including portable and central systems vary in their effectiveness in removing particles and other pollutants from air and their effectiveness in improving indoor air quality for different types of occupants, for example people suffering from allergies versus people suffering from asthma also varies.

Watch out: In our opinion, while the best air filtering systems may reduce the overall level of airborne particles or pollutants, no air filtration or air cleaning system can remove a significant indoor air quality hazard whose source is inside the building. For example, if a building suffers from significant mold contamination (more than 30 sq.ft. of contiguous allergenic, pathogenic, or toxic mold whose spores are capable of being airborne or of releasing mycotoxins in MVOCs), it is essential to find and remove the source of contamination as well as to correct its original cause so that it does not recur.

In listening to a report on tests of an air cleaner effectiveness reported by Burge at a PAAA conference we [DF] saw a graph demonstrating that the air cleaner produced a significant drop in the airborne particle level during its test. But the air cleaner was tested in a sealed chamber into which a known quantity of particles of a known size and type was introduced. At best this test represents the use of an air cleaner in an enclosed space within which the actual particle source is not present. The test provided useful data on the air filter's abiltiy to remove particles from air but it did not accurately describe a "real world" case of a home with a persistent contaminant source.

Approaches that rely on filtering to remove airborne mold spores or similar contaminants cannot remove the problem source any more than you can vacuum dust bunnies from under the couch by standing two rooms away in the kitchen, waving your shop vacuum wand in the air. - opinion, Daniel Friedman, based on more than 30 years of forensic building & indoor environment field & lab investigation & testing

Kurutz, who performed his own test on six air purifiers (using HEPA filters, UV_C light, ionic filtration, hydroxyl radical molecule generation) and reporting for the New York Times, found his own initial experience inconclusive. He further noted that

"Purifiers are somehow more than electronic gadgets, yet they aren't considered medical devices, so are not regulated by the Food and Drug Administration. ... Over the years, the Federal Trade Commission has taken action against several makers of air purifiers, including brands like [Alpine], Honeywell and Oreck, for unsubstantiated allergy-relief claims or for advertising that their devices removed virtually all impurities from the indoor air people breathed. ... several articles in Consumer Reports suggesteing that some air purifiers were not very good at performing the latter function, [and that ] that most people don't need one." - Kurutz, Steven, "Eat My Dust: A veteran sneezer challenges several air purifiers to live up to their promimses of eliminating pollen, mold and even pet odors", The New York Times, 16 May 2013, p. D1, D6.

Kurutz also noted an important difference in the type of building occupant who may or may not be helped by the use of air filtration systems: asthma sufferers are more likely to benefit from indoor air filtration than people suffering from allergies. - op. cit.

Watch out: Similarly, relying on "kill it" approaches to try to "remove" hazardous biological materials such as mold spores risk leaving still-harmful particles in the air. Some mold spores that contain mycotoxins remain toxic even if the spore is no longer viable. For this reason, do not rely on ozone generators to correct indoor air quality issues, and beware of ion generators that may be adjusted to produce high levels of indoor ozone.


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.

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.


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