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Coming up with standards in the air purifier industry is challenging because air, itself, is complex. Defining performance characteristics for air purifiers starts with identifying what is in the air and measuring their impact, before methods can be developed to test the possible air purification product itself.

In America, the work of establishing technical standards for air purifiers is dominated by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Association of Home Appliance Manufacturers (AHAM), which both draw on private and academic research from many sources. The standards developed by these organizations are effective for what they measure, but are mainly focused on particle capture.

Our mission at Molekule is to advance the science of indoor air, and much of our research is focused on developing standards that can more fully reflect the impact air purifiers can have on indoor air quality. The following is a compendium of this research and what form future standards can take to more fully reflect the complexity of indoor air and the technology required to clean it.

Particle capture: the only (but imperfect) standard

Currently, there are four standards used to gauge air purifier performance: ASHRAE 52.2, AHAM AC-1, AC-2, and AC-3. They measure three different sources of particles at different size ranges — pollen for large particles (3 to 10 microns), dust for mid range particles (1 to 3 microns) and smoke for the smallest particles (0.3 to 1.0 microns).

ASHRAE 52.2 measures the efficiency of particle capture by individual filter media. The results from the tests dictated by this standard could be used for both HVAC filters and portable filters, and are used to determine the MERV(Minimum Efficiency Reporting Value) rating of a filter or establish that it meets the requirements to be a true HEPA. The standard covers details like duct design, particle source, environmental conditions and other factors that need to be held steady for accurate comparison.

When measured using the methods specified in the ASHRAE 52.2 standard, technicians at a certified lab generate a report with data on a filter’s efficiency in removing smoke, dust and pollen from an airstream on a single pass to72 different data points. The way in which each filter is exposed to the pollutant is carefully specified in the standard so test results are comparable regardless of the testing facility.

However ASHRAE 52.2 has a drawback when it applies to particles passing through the filter in a specially designed duct system. When used in the real world, that filter needs to be just as well sealed to achieve the same efficiency. If the HEPA filter that tested well is poorly installed in a device or HVAC system such that air flows around the filter, the efficiency will be worse than the certified lab report.

ASHRAE occasionally changes the rules, but to be qualified as HEPA, a filter must capture at least 99.97% of particles sized 0.3 microns. This specific size was chosen because it is the most likely particle size to penetrate a HEPA filter due to the physics of particle capture. The MERV rating is primarily for furnace filters in HVAC systems, while HEPA is usually found in portable air purifiers. While they both rely on the same data, MERV is different from HEPA in that it looks at a range of particle sizes and provides a spectrum of efficiency ratings from 1 to 16. In essence, the purpose of MERV is to split furnace filters into four groups that each handle progressively smaller particle sizes in addition to the larger sizes handled by lower MERV ratings.

  • MERV 1-4. Designed to handle relatively large particles, larger than 10 microns. This includes pollen, fibers, dust mites and other almost-visible particles. Typically used in window-mounted AC units. Average single-pass capture of any particle is 65% (MERV 1) to 80% (MERV 4).
  • MERV 5-8 Designed to handle particles sized 3 to 10 microns. This would include mold spores, pet dander, dust mite detritus and other similar sized contaminants. Best for particles in residential, commercial or industrial HVAC systems. Average single-pass capture of any particle is 80% (MERV 5) to 90% (MERV 8).
  • MERV 9-12 Designed to handle particles sized 1 to 3 microns. This would remove lead dust, auto emission particulates and other tiny droplets and particles. Often found in hospital labs or for superior filtration in residential, commercial or industrial HVAC systems. Average single-pass capture of any particle is 90% (MERV 9) to 95% (MERV 12).
  • MERV 13-16 Designed to handle particles sized 0.3 to 1 micron. This would include bacteria, aerosolized cooking oil, droplets from sneezes and similar sized contaminants. Average single-pass capture of any particle is 95% (MERV 13) to greater than 98% (MERV 16). MERV 16 filters must capture 95% of particles sized 0.3 to 1 micron with each pass through the filter.

HEPA filters were once also placed on the MERV scale as 17 or higher, but currently the standard does not describe ratings above MERV 16. Aside from the discrepancies that can arise out of “real-world” installation of the filter, another major drawback of this standard lies in the fact that size is used to identify particles. In reality many different substances can share the same size but not the same impact. Viruses may be the same size as particles of soot but their risks are very different because the former could cause lasting infections with just a few hours of exposure while the latter must be inhaled for years to impact health. Similarly, allergens from pollen may be as small as 0.03 microns, which is below even the smoke particle size.

Yet another other major drawback is the lack of consideration for gaseous pollutants. VOCs such as formaldehyde can penetrate filters far more effectively than particles of smoke, which are the most challenging pollutants to capture in the ASHRAE 52.2 standard. Formaldehyde and ozone removal would add a powerful dimension to understanding filter efficiency.

The AHAM AC series eliminates one drawback of the ASHRAE standard by specifically testing air purifiers, not just their filters. These methodologies can generate Clean Air Delivery Rates (CADR) and measure noise produced by the fan in a unit. There is a similar level of detail on sources and equipment as in ASHRAE 52.2, and also particle capture efficiency as the only criteria to measure impact. However, in this standard the units are placed in chambers for extended periods of time. The theory is that the performance from inside the chamber can be generalized to the real world.

Like ASHRAE 52.2, this series has the same drawback of not considering gaseous pollutants and using size to determine particle composition. As a result, CADR values currently cannot give a fully accurate measurement of efficacy. Moreover, the standard calls for a sealed chamber and a single introduction of pollutants, whereas in the real world many sources of pollutants are constant.

These methods are helpful to understand smoke and dust removal, but do not give any information on protection from infections, gases or allergens.

Looking beyond borders for better standards

The International Organization for Standardization (ISO) comprises many committees from different countries that meet to establish a variety of standards of protecting and informing the public about manufactured products. One series of standards the organization has set is the ISO 16000 series, with its reproducible methods pertaining to how chambers should be designed and equipped to test air purifier efficiency on removing gaseous toxins like formaldehyde or airborne bacteria. These ISO standards could be used in the future to provide CADR values for pollutants other than particles, which is not currently done.

China has taken a lead on air quality research and developed standards to consider more than smoke, dust and pollen efficiency. The Guobiao standards (GB or GB/T) have been developed by the Chinese government to draw on established research and the country’s participation in ISO. The GB/T 18801 standard introduces a new metric, Cumulate Clean Mass (CCM), which measures the amount of pollutants a filter can remove over its entire lifetime instead of over a single pass in a duct or single introduction in a chamber.

Using standards like GB/T 18801 could enable a better range of CADR values instead of just the three associated with the different sized particles. For instance, prior to the Covid-19 global pandemic, it would have been useful to know the CADR values of air purifiers for virus infectivity. At Molekule we recently partnered with the University of Minnesota on a peer-reviewed paper based on ASHRAE 52.2 that was designed to determine animal coronavirus and influenza capture and deactivation*. The research team was able to differentiate between the viruses captured by our purifier and those few that penetrated, but were deactivated by, the PECO catalyst.

The University of Minnesota researchers used metrics more familiar to the microbiological world — i.e. log reduction, which shows the fraction of detectable microbes in the air. Since exposure to just a few viruses can cause a full-blown infection, it is important to define how quickly or easily they are removed from the air to a very fine degree — say, on the order of .001%, which was the precision of the research at University of Minnesota.

During the Covid-19 pandemic, the FDA recommended air purifiers that could reduce viruses in an appropriately sized room by log 4, which is 99.99%. This type of indication would be effective in a national standard because it would allow consumers to select purifiers that meet the criteria organizations like the FDA recommend for protection. PECO air purifiers meet this requirement with data showing not just capture but also destruction of viruses.

Molekule is also working with AHAM to develop better methods for measuring microbial performance in air purifiers by serving on the committees that research new standards. The international community is a rich resource for this type of research and the studies that went into the GB/T, and ISO standards are influencing the recommendations of the committee in which we participate.

We are calling on organizations that review electronic products like air purifiers to consider pollutants beyond particles. Smoke and dust can be problems, but consumers are increasingly concerned about chemicals in the indoor environment, viruses, allergies and asthma triggers. Incorporating additional methods that cover how well an air purifier can deal with these threats can not only help consumers make more economical choices, but also select products that can have the most positive impact on them.


*In 2020, the University of Minnesota College of Science and Engineering and College of Veterinary Medicine tested Molekule’s PECO technology using an Air Mini device and its operating flow rate in single-pass efficiency testing with viable viruses in aerosol transmission. Methods developed for the research allowed measurement and characterization of airborne virus detection and inactivation by air purification technologies. For the PECO technology, testing showed near 99.9% removal and total removal rates (virus titer based) approaching 99.99% for influenza A and exceeding 99.9% for bovine coronavirus. Dr. Chris Hogan and Dr. Montserrat Torremorell led this research. While air purifiers can provide an additional layer of protection to help reduce potential viral exposure, Molekule encourages use of medical countermeasures suggested by government authorities.

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