Expectation versus reality - examining air filter performance

Air distribution Filters

It is well established that efficient and effective filtration is one of the foundational pillars of any well-designed building heating, ventilating, and air conditioning (HVAC) system. David Schurk of Global Plasma Solutions and ASHRAE looks at maintaining both efficiency and effectiveness. 

Efficiency versus effectiveness

It is well established that efficient and effective filtration is one of the foundational pillars of any well-designed building heating, ventilating, and air conditioning (HVAC) system. With regard to air filters and cleaners, efficiency is a term that refers to the ability of a collecting medium to capture contaminants from the airstream that passes through it. The effectiveness of an air cleaner refers to its ability to reduce particulate concentrations in a space. The US Environmental Protection Agency (EPA) tells us “It should be noted that expected removal efficiencies for air cleaning devices are not the same as the effectiveness of the unit in actual use. Efficiency only measures the percent removal of contaminants in the air that flows through the air cleaning device. Effectiveness in use depends on how much of the interior air actually goes through the unit in a given time period”1. If space contaminants in the air do not pass through the air filter that is tasked with their removal, the effectiveness (actual contaminants removed) may be less than expected regardless of the filter’s rated efficiency. Simply put, a filter described as very efficient may not be all that effective in actual use.

Rating filter efficiency

The 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 has established a test procedure for evaluating the performance of air-cleaning devices (filters) as a function of particle size. This standard describes a method of laboratory testing to measure the performance of general ventilation air-cleaning devices using a test apparatus like the one shown in figure 1 (Below).

Summarizing the test procedure, a stable, dry, and uniform challenge aerosol (particulate) ranging in size from 0.30 to 10.0 micrometers (μm) is introduced downstream of an injection fan which is positioned immediately upstream of the air filter being evaluated. This propels test particles into the inlet of the filter at air velocities up to a number of meters per second (several hundred feet per minute (fpm). A particle counter then measures and counts the particles both upstream and downstream of the filter for the efficiency determinations. ASHRAE cautions that the user must be aware that the tests are made under laboratory conditions using synthetic dust for loading. They also specifically state: “Since the contaminant used for loading is not typical of natural atmospheric dust, its effect on the filter may not be the same as an equal amount of atmospheric duct. The value of the loading test is for rating and to some extent ranking filters. The performance values obtained in accordance with this standard cannot be used by themselves to predict air cleanliness of a specific ventilated space or the service life of an installed filter.”

While the ASHRAE rating methodology may be a useful measure of an air cleaner’s tested efficiency, it may not be indicative of its effectiveness in true application. Most real-world contaminants are not synthetic, and the vast majority of particles in the indoor air we breathe are smaller (much smaller) than the minimum 0.30 μm size used for these rating procedures. Typical room air velocities may range more on the order of 0.1 to 0.25 meters per second (20-50 fpm), and many HVAC systems do not have return air injection fans. As well, the contaminants of concern in the air may be located considerable distances from the filter itself and are not force-fed at close range and high velocity into the inlet of the device tasked with removing them.

The National Air Filtration Association (NAFA) Guide to Air Filtration, Sixth Edition 2021, reminds us that while we are inclined to think of air filtration as an irreversible action where a dust particle is captured once and then stays captured, this is not true of dry media filters, especially those in the high-efficiency range. A particle may be captured by a fiber only to become restrained in the airstream and then either be captured by another fiber downstream or pass through the filter altogether. When considering commercial HVAC applications, multiple filters may be installed for side access, and slid into the filter housing on formed metal tracks. Formed metal tracks may offer no sealing mechanism at the filter-to-track interface, or between filters installed side-by-side. Often standard-size filters do not completely fill these tracks. This lack of sealing may result in significant leakage via air bypass, which can degrade overall filtration efficiency 2. All of the real-world dynamics mentioned above will certainly have an impact on any air-cleaners true effectiveness.

Applying current guidance

Figure 1: Schematic of test duct - ASHRAE Std 52.1-2017
Figure 1: Schematic of test duct - ASHRAE Std 52.1-2017

Scientific study has concluded that small particles and pathogens (including SARS-CoV-2 and other viruses) may be of insufficient mass to be controlled by bulk airflow and can remain suspended in the indoor air for days or even weeks. It reported that significant fractions of the fine and ultra-fine particles in the air may not be effectively transported to or removed by conventional air filters, and that even ultralow penetration and HEPA filters are only effective on the particles and pathogens that can reach the filter, not those that remain suspended in the space 3. ASHRAE stated in a November 2020 interview that research has not provided evidence of the transmission (of SARS-CoV-2) virus through HVAC systems, although within a space, air motion caused by HVAC system components as well as fans can be a factor 4. On March 23, 2021, the Centers for Disease Control Preventions (CDC) updated their COVID-19 Ventilation FAQs titled: Can COVID-19 be transmitted through HVAC ventilation systems? They stated that the risk of spreading SARS-CoV-2 through ventilation systems is not currently clear, and that while airflows within a particular space may help spread disease among people in that space, there is no definitive evidence to date that viable virus has been transmitted through an HVAC system to result in disease transmission to people in other spaces served by the same system.

This latest information could be telling us that traditional HVAC strategies might have a reduced impact on cleaning the indoor environment because the small pollutants of concern are not being effectively moved out of the space and into the HVAC system to be exhausted, filtered, or additionally treated. HVAC system air cleaning strategies should certainly be capable of more than just waiting for virus and contaminant-laden air to “get to it” for treatment. Easily understood is a simple concept that if the pollutants in the space are removed, they are no longer a concern to its inhabitants. This would be a major accomplishment for all building air cleaning systems, therefore something different than what we are now doing may warrant consideration. Understanding these dynamics is key in helping us to properly design building HVAC systems that can contribute to the well-being and productivity of occupants breathing the indoor air.

 Improving IAQ

Because ventilation and filtration are the foundation of any well-designed building HVAC system, implementing additional measures to ensure they perform better makes sense if we are to achieve maximum results. Designs that enhance ventilation rate effectiveness, help maintain proper indoor relative humidity, and actively treat to clean the air within the indoor space itself (where the contaminates of concern originate and may remain) could prove prudent. Any technology that improves the efficacy of traditional ventilation and filtration warrants thoughtful consideration. Understanding the subtle but important difference between efficiency and effectiveness and then taking steps to improve both, maybe a major step toward making the air we breathe indoors as clean, safe, and healthy as it can possibly be.


  1. Introduction to Indoor Air Quality, A Self-Paced Learning Module-United States Environmental Protection Agency, Office of Air and Radiation, EPA/400/3-91/002, July 1999.
  2. American Conference of Governmental Industrial Hygienists (ACGIH), Bioaerosols Committee, Engineering Controls for Bioaerosols in Non-Industrial/Non-Healthcare Settings, 2021.
  3. Mark H. Ereth MD, Donald H. Hess MSME, Abigail Driscoll BS, Mark Hernandez PhD, Frank Frank Stamatatos BEE, Particle Control Reduces Fine and Ultrafine Particles Greater than HEPA Filtration in Live Operating Rooms and Kills Biologic Warfare Surrogate, American Journal of Infection Control, 2019.
  4. ETF Chair: Limited Virus Knowledge is Key Challenge, ASHRAE Journal, November 2020.

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