Air Purifier Ratings, Industry Standards, and What You Should Know

Today’s industry standards on air filter effectiveness have largely remained the same for the past 40 years, despite the fact they only reflect how well particles are trapped and not necessarily how clean the air is. Molekule’s PECO technology goes beyond simply capturing particles on filters, to actually destroy pollutants in the air. Here’s our take on what today’s air purifier rating systems can and can’t tell you, and a broader vision of what clean air truly looks like.

What current standards ignore

Depending on what’s in the device, air purifiers can filter particles of matter or gases, but the standards you see on the market today do not reflect gas filtration at all. Common technologies like HEPA and ionizers will only deal with particles, and if they claim to address gases as well, will usually have an added technology like a carbon filter to aid in this. Theoretically, the industry could create a standard for gases like it has for particles (see CADR below), though that would require sophisticated laboratory equipment and methodology.

For now, if you want to compare potential effectiveness on gases, you should consider:

  • Some technologies that claim to address gases, like PCO, ozone generators and plasma, can produce ozone (see safety standards section below).
  • Does the air purifier contain a substantial weight of carbon, measured in pounds? A thin layer will saturate quickly.
  • Also, carbon filters do not permanently remove gases. Changing conditions in the room, such as elevated moisture or temperature, can cause gaseous pollutants to “unstick” from the filter surface and redisperse into the air.

A third-party laboratory confirmed that Molekule’s PECO technology destroys VOCs, permanently removing these harmful chemicals from the air.

The CADR standard

At its core, an air purifier has two basic components: some form of filtration technology, and a fan that blows air through the unit. What makes a traditional filter “efficient” at capturing particles depends on various factors, like the size of the particles it captures[1] and how much electricity it consumes. However, having a more “efficient” filter does not necessarily translate to cleaner air.

Since the early 1980s, the industry has tried to bridge the gap between filter efficiency and actual impact on healthy air. In an attempt to shed light for the consumer, a standard was developed that ties filter efficiency with the air flow of the unit. The Clean Air Delivery Rate (CADR) measures how quickly a unit can trap certain particles, expressed as the volume of “clean air”  delivered to a room. The standard focuses on three particle sizes[2]: smoke (small), dust (medium), and pollen (large). Developed by a private trade association, the standard is voluntary in the industry so you may see an inconsistency in its usage on air purifiers being sold.

Facts you should know about CADR:

  • The test conditions are controlled in a laboratory, meant to provide the best estimate but cannot fully represent real-world situations.
  • Filter “efficiency” is simply the percentage of particles collected after air flows once through the media. This is what determines “clean air” even though the definition of clean air in the real world is much more complex.
  • The reported CADR is usually the highest it can achieve at the highest airflow setting, which is also the noisiest and uses the most power.
  • CADR does not account for any new particles being introduced into the space from outside sources, which can vary significantly from room to room.
  • Particles of smoke, dust and pollen are not the only pollutants found in an average home. There are many other substances that pollute the air, such as carbon monoxide, volatile organic compounds (VOCs), ozone, sulfur oxides, nitrogen oxides and others that are not counted as particles and will not be accounted for in a CADR reading.

Though CADR cannot reflect the full picture, it can be used to compare one unit to another on removal of certain sized particles. For example, an air purifier without a fan will have a low air flow rate, and may not be effective for an average-sized room.

CADR and its ambiguity on microbes and allergens

Mold, viruses and pet dander have this in common: they all count as particles. When you inhale even very small amounts of microbes or allergens, these particles may react with your body and impact allergies or other sickness. Just a few allergens may cause an asthma attack, and just one virus particle can make you sick. If grouped into one category, there is no way to tell if an air purifier is removing the bulk of inert particles but leaving behind enough biologically active particles to cause harm, except through expensive laboratory testing. It would be impractical to create a standard for each individual allergen or microbe.

Though CADR reports a filter’s efficiency according to particle size, it cannot reflect the true nature of most particles or their health implications:

  • Though a unit may have a high CADR for a large-sized pollen grain, tiny allergy-producing particles can break off in real-world situations and can pass through the filter, thus, re-entering the air you breathe.
  • Mold particles can potentially multiply on the filter surface and be released back into the air (see more details in “HEPA” section).

Molekule’s PECO technology can destroy biological contaminants like allergens, viruses and bacteria so that they are completely removed from the air, leaving no possibility for microbes or mold to multiply on a filter surface and be released back into the air.

The MERV standard

MERV, which stands for Minimum Efficiency Reporting Value, is the most widely used filtration standard for particle removal by an HVAC system[3]. MERV ratings run from 1 to 16 and the higher numbered ratings are more efficient. It is very important to have the right MERV rating for the right HVAC system, so if you are responsible for replacing a furnace filter, be sure to know what MERV is right for you. This is because although higher-rated filters have narrower spaces between fibers to trap smaller particles, they allow less air to flow. If the filter rating is too high for the blower in the system, then it will not be able to move enough air to maintain air quality and temperature. Conversely, if the MERV rating is too low, the blower may push contaminants all the way through the looser fibers.

So when considering a MERV rating, keep the following in mind:

Different MERV ratings are most appropriate for certain rates of air flow, so be sure you or your HVAC professional choose the right filter for the system.

  • As a filter gets full of particles, its flow rate will drop over time and there is no industry standard on filter lifetime, so be sure to change them on time.

From our research in the industry, we know it’s been assumed less than 20% of air filters are changed on time, which means that many people are breathing air that is dirtier than it needs to be and overtaxing their HVAC systems. To ensure your system runs at its best, we recommend that you change your filters on time, which is the same recommendation we make to our customers about Molekule filter replacement.

dirty-and-clean-filter-closeup

What HEPA might miss

HEPA filters are not rated by MERV, but rather assure a 99.97% capture rate of particles sized 0.3 micrometers (µm) which the EPA notes roughly corresponds to MERV 16. HEPA filters used in industrial and healthcare settings have more stringent requirements, but there is no firm standard for consumer HEPA filter performance. Most manufacturers assure the 99.97% capture rate by measuring the filter’s ability to capture a “test dust” composed of a known range of particle sizes including the target 0.3 µm.

There are several caveats to consider when choosing a HEPA filter:

  • HEPA and high MERV rated filters (over 11) are best used in portable air filters; their effect on air flow is too great for most HVAC systems.
  • Some manufacturers sell filters labeled as “HEPA-type” or “HEPA-like.” Unless specifically stated, these filters do not remove 99.97% of particles sized 0.3 µm. There is no standard for these types of filters, so the only information available is what is provided by the manufacturer on their packaging or website.
  • While HEPA filters can capture 99.97% of 0.3 µm particles from test dust, in real-world conditions the efficiency of capturing biological and other particles will be different.
  • All gaseous pollutants, such as VOCs, carbon monoxide, ozone, and many others, are too small to be stopped by HEPA filters.
  • The tightly spaced fibers of HEPA filters will collect mold and bacteria by design, and these organisms thrive on moisture and dust. It is possible for them to grow in the filter and redisperse themselves in the air. So all fibrous filters should be monitored, particularly those in humid climates.

CARB and other regulatory standards for ozone/byproducts

Controversial and misleading claims over the years have brought air purifier safety into question, especially because some units generate harmful byproducts.

Here is what you should know when it comes to safety and air purifiers:

  • Ionizers, ozone generators and UV-C light technologies can generate ozone, a harmful air pollutant, either intentionally or as a byproduct.
  • There is currently no federal regulation on air purifiers that produce ozone.
  • If an air purifier is sold in California, it must be certified by the California Air Resources Board (CARB) for electrical safety, and must not generate more than 50 parts per billion (ppb) of ozone concentration. California is the only state with this requirement, and lists which air purifiers are CARB certified.
  • Harmful byproducts other than ozone, such as formaldehyde and ultrafine particles, can be emitted by air purifier technologies like plasma, PCO and ozone generators.

When choosing an air purifier, watch out for air purifier technologies that produce ozone intentionally (ozone generators), or those where ozone can be a byproduct.

What the true standard should look like

Today’s air purifier ratings judge how well particles are trapped onto a filter, but they fail to provide the full picture of whether an air purifier can deliver truly clean indoor air to your home. Simply capturing particles on filters does not ensure they are permanently removed from the air. To solve this problem, Molekule PECO technology is the air purifier reinvented, going beyond simple particle capture to pollutant destruction. We believe that clean indoor air should be free from pollutants that harm us the most, which often pass through traditional filters. The definition of clean air as it currently stands is just not enough. The future of air purifier rating systems will look a lot different and incorporate not just particle removal, but pollutant destruction, and Molekule is committed to being at the forefront of this evolution.


  1. EPA, Residential Air Cleaners – A Technical Summary 3rd Edition (July 2018)
  2. AHAM, Method of Measuring Performance of Portable Household Electric Room Air Cleaners (January 2013)
  3. ASHRAE, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size (January 2017)
Written by

Dr. D. Yogi Goswami is a Distinguished Professor and the Director of the Clean Energy Research Center at the University of South Florida. He has over 40 years of experience in education, research, entrepreneurship and policy in the areas of clean and renewable energy, as well as air quality.