Standards and guidelines for air filter efficiency have been around for several decades, but the standards have continually evolved over time. This has caused some confusion within the industry, especially for contractors.

ANSI/ASHRAE Standard 52, issued originally in 1968, was one of the first formal standards to address the testing of air filter efficiency. This standard was updated to 52.1 in 1992. It also led to the creation of ANSI/ASHRAE Standard 52.2, “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.” The standard was meant to address new filtration concerns and improve some outdated practices of the previous standard, but Standard 52.1 is still valuable, at least for the time being.

Confused? Don’t be. The News spoke recently with H.E. Barney Burroughs, head of the ASHRAE Standing Standards Project Committee in charge of Standard 52.2. Burroughs explained how Standard 52.2 has been instrumental in creating new and more effective guidelines for air filters. He also outlined the evolution of both standards and explained how they will continue to evolve over time and why contractors need to stay knowledgeable about the changes.


According to Burroughs, the two test methods under Standard 52 include the “arrestance test” and the “atmospheric dust spot test” (ADST). He said that the arrestance test is a gravimetric test that determines the dirt arrestance capacity of low-efficiency panel filters. The ADST test looks at discoloration of the filter to determine the average smudge removal efficiency of higher-efficiency filters.

The dust-spot procedure was born over 50 years ago by the National Bureau of Standards (NBS) as a Federal Specification. The practice evolved over the years and created procedures for testing the efficiency of air filters. The arrestance test came out of a now defunct group called the Air Filter Institute. Since the practices were developed so long ago, it’s fair to say that the recommended procedures got a bit “dusty” and outdated.

These were the basic testing procedures for several decades until a standard project committee was formed to update the standard published in 1992. The goal, according to Burroughs, was to take these testing procedures and update them for new measurement and application technology. Some flaws were beginning to surface in these early testing procedures.

First, Burroughs explained that the dust spot method uses atmospheric air to test the filter efficiency. He said this creates a problem because outdoor air is different everywhere. Performing the test in different areas of the country skews the test results.

Next, Burroughs said that determining filter efficiency through discoloration does not take into account particle size, especially tiny particles.

“As particles get smaller and smaller, they are less colored,” he said. “But these are the important particles from a comfort, health, and safety standpoint.”

Burroughs pointed out that the dust spot test averages results over the loading life cycle of the filter. According to Burroughs, this does not help in determining the minimum efficiency of the filter, only the maximum efficiency at peak conditions.


When the ASHRAE standard committee finally tackled the issue in 1992, it was decided to keep Standard 52 in place while creating a separate standard for the improved methodology. The old testing methods are referred to in Standard 52.1, while the new guidelines for filter testing are found in Standard 52.2.

Burroughs said that Standard 52.1 could not be completely replaced because the arrestance testing procedure is still relevant, especially for older filters that are still out in the field. But for the majority of filters on the market today, the testing procedures fall into Standard 52.2, which was published in 1999.

Standard 52.2 corrected testing procedures and created a new way of summarizing the filter efficiency.

First, it was decided that in order to improve control of laboratory testing, instead of using atmospheric air, the test would use a laboratory-generated aerosol of potassium chloride to help prevent the inconsistencies that might result from using regular outdoor air. The test duct was also improved, and the new test method employs optical particle counters for upstream and downstream measurements of the filter efficiency.

Burroughs said that 52.2 also created a new way of documenting the efficiency of filters and established the Minimum Efficiency Reporting Value (MERV).

To find this value, the test looks at 12 different particle size bands from 0.3 microns to 10 microns. The method of testing documents the minimum efficiency curves of filters by using six different loading stages. The standard uses a method for loading the air cleaner with synthetic dust to simulate field conditions.

By comparing these 12 different particle sizes to the six different loading stages, testers can determine a minimum performance curve. This Composite Minimum Efficiency Curve is the primary data product of the procedure. The curve also determines a MERV level that is derived by averaging the efficiencies within three size bands. This helps to communicate to contractors and consumers in a simple manner how the air filter will perform under minimum requirements against specific sized particles. Burroughs refers to this MERV number as “a worst-case scenario” — it tells the very least amount of efficiency one can expect from the filter over time.


Even with all of these improvements, it is still necessary to keep Standard 52.1 alive, at least for the time being.

Burroughs said that most of the leading manufacturers are now testing their filters using the latest standard, but there are many filters still in the field being used that can only be tested under the old gravimetric technique. He also said that the old standard describes several methods that are pertinent to new and improved testing procedures, such as loading of the air filter, pressure drop determination, equipment guidance, and so forth.

Burroughs expects that in the near future, Standard 52.2 will eventually adopt the parts of 52.1 that are still valuable and pertinent. Once that is done, Standard 52.1 and its atmospheric dust spot test will fade away.


Burroughs believes that since Standard 52.2 is still quite new, it has taken time for the information to be disseminated.

“It takes a while for a new standard to come around,” he said. “This standard was published only two years ago, and it takes time for the information to trickle down.”

Burroughs said that just about every manufacturer is using and promoting the testing procedures of Standard 52.2. Now it is time for contractors to educate themselves on the new ratings and communicate to customers why improved air filtration is important.

“Contractors need to be aware of how to educate customers intelligently,” said Burroughs.

He explained that this is important because it behooves contractors to be more proactive and aggressive when it comes to IAQ concerns.

“If you upgrade filtration in a space, you upgrade the IAQ. If you upgrade IAQ in a space, you have reduced your risk of contaminants damaging the system and exposing the occupants,” said Burroughs.

He also suggested that contractors should attempt to reduce the risk of liability from problems that could occur in a building space.

“Because they are on the scene with the consumer and are the leading edge of HVAC information and service, adverse IAQ can turn around and bite a contractor,” Burroughs warned, “and that bite can have a terrible sting.”

Publication date: 12/23/2002