Filtering the Future: HEPA vs MERV 13
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WHY DOES OUR INDOOR AIR QUALITY MATTER?
People spend over 90% of their time indoors, and the effects of recent public health crises – including airborne disease outbreaks and wildfires on multiple continents – have amplified public awareness of the need for better indoor air quality standards.
However, it doesn’t require exceptional circumstances, such as a city’s air being choked by a wildfire thousands of miles away, for clean indoor air to be a necessity. There are plenty of airborne particles in the average indoor environment, including particulate matter (ie. PM 2.5), mold spores, chemical VOCs, and dust mites, that cause everyday allergies, asthma, and other health issues for millions of people. The Clean Indoor Air Revolution has made us realize something important – we don’t have to live like this!
When the air quality in your house, apartment, office, or school is bad, how do you go around removing the offending particles from the air. Many people would say to use a HEPA filter – the most accessible form of which comes in the form of a personal air purifier unit. However, is this really the best option for air purification, especially when you’re trying to clean the volume of an entire room? The work of the DIY air purification movement has shown that there is a better option out there – MERV-based air purification that hits the sweet spot of airflow and filtration efficiency!
Air Support Project is on the cusp of releasing Vello – a revolutionary flat-pack MERV-13 air purifier that can clean a roomful of air faster than HEPA, at a fraction of the cost, using furnace filters that anyone can buy at their local hardware store. But what are MERV filters, how are they different from HEPA, and what is their potential to upend the world of clean indoor air?
What do Merv ratings mean?
The Minimum Efficiency Reporting Value (MERV) rating of an air filter is a rating system derived from ANSI/ASHRAE standard 52.2 [1]. This rating describes the filter’s efficacy in capturing three particle sizes; course particles (3-10 μm) such as pollen and mold spores, fine particles (1-3 μm) such as pet dander and household dust, and ultrafine particles (0.3-1 μm), such as wildfire smoke and aerosols carrying infectious viruses [2]. As the MERV rating of a filter rises, its efficacy in catching particles increases, and it becomes capable of capturing smaller particle sizes.
MERV filters can be grouped into four main categories [3] [4]:
MERV 1-4 is the lowest tier of MERV filters, only filtering out course particles larger than 10 microns, such as debris, sand, dust bunnies, and carpet fibers.
MERV 5-8 is the standard type of MERV filter used in home and commercial HVAC systems. These filters capture primarily course particles between 3 and 10 μm, such as pollen, dust mites, mold spores, and pet dander.
MERV 9-12 are the next step up, providing efficient filtration of particles greater than 1 micron. This tier of MERV filters are effective at removing the notorious PM 2.5 particles – small, fine particles that make up a large portion of outdoor air pollution caused by automobiles and industrial activity, and have been found to exacerbate numerous air pollution-related health problems due to their ability to infiltrate the lungs [5].
MERV 13-16 provides the highest level of filtration of the MERV-rated filters. This filtration level has become the gold standard for the DIY air purifier movement that emerged to combat public health threats such as airborne disease outbreaks and wildfire smoke due to the ultrafine particle sizes (less than 1 micron) involved. The MERV-13 filter, which is used by the DIY Corsi-Rosenthal box and our flagship purifier Vello, removes 90% of course particles (3-10 μm), 80% of fine particles (1-3 μm), and 50% of ultrafine particles (0.3-1 μm) from the air passing through the filter.
HEPA (High Efficiency Particulate Air) grade filtration is a step about MERV 16 – consisting of densely packed filter media that removes 99.97-99.99% of all particulate sizes that pass through the filter. As the highest standard for air purification, HEPA filters are installed into the HVAC systems of specialized facilities that require sterile environments, such as hospitals, high-tech manufacturing, and nuclear facilities [1]. In addition to these specialized occupational uses, smaller HEPA filters are used in many commercial air purifiers, which deliver a stream of highly purified air into the surrounding area.
The Trade-Off: Filtration vs Airflow
So if HEPA provides the highest level of particle filtration, why don’t we use it to clean everyday environments? This is because filtration efficacy is not the only factor in the total amount of clean air – also known as the Clean Air Delivery Rate (CADR) – that an air filtration system can deliver! The CADR of a filtration system is determined by the filter’s efficiency times the airflow rate, and in many cases, these two factors can work against each other. That is to say, filters with higher MERV rating have more densely-packed fibers, which increases how many particles it can filter out of the air, but decreases the total volume of air flowing through the filter in the first place.
This is where HEPA runs into problems, especially when being used to clean large volumes of air (ie. cleaning a room via an HVAC system). Facilities that use HEPA filtration in their HVAC systems typically have powerful, specialized units that can handle the higher resistance imposed by HEPA filters. However, the average home HVAC system is not powerful enough to efficiently force air through a HEPA filter. Trying to do so restricts the total airflow, and requires the system’s fans to be constantly run on the highest setting – leading to increased energy consumption and utility bills [6]. In older HVAC systems, the increased resistance and decreased airflow from an HEPA filter can even damage the HVAC system and cause it to stop working [7].
For this reason, HEPA filters are not recommended for the average home HVAC system. Instead, the larger, more porous MERV filters are recommended, in order to maintain sufficient airflow within the home’s ventilation system and not over-strain the HVAC system’s fan and motor. Expanding the differences between these two filter types to air purifier units, personal HEPA filters provide a high level of filtration for a small volume of air. This is great for providing a pocket of purified air to a small space, such as that surrounding a person, but is less ideal for removing pollutants from a large volume of air – such as that of an entire room. However, this is where a MERV-13 based air purifier, such as a Corsi-Rosenthal Box, excels – in situations where a high level of airflow is needed to quickly and efficiently clean the air of an entire room.
The Corsi-Rosenthal Box emerged as a simple, yet groundbreaking concept during the early 2020s – a DIY box made of readily available materials for under $100 that outperformed the leading HEPA filters on the market in terms of its raw ability to clean the air. Research by Dal Porto et al in 2022 found that a standard Corsi-Rosenthal box, made of four MERV-13 filters and a box fan, delivered three times as much clean air as comparable HEPA units [8].
The Corsi-Rosenthal Box emerged as a simple, yet groundbreaking concept during the early 2020s – a DIY box made of readily available materials for under $100 that outperformed the leading HEPA filters on the market in terms of its raw ability to clean the air. Research by Dal Porto et al in 2022 found that a standard Corsi-Rosenthal box, made of four MERV-13 filters and a box fan, delivered three times as much clean air as comparable HEPA units [8].
A comparison of the CADR between MERV-13 based Corsi-Rosenthal boxes versus personal HEPA air cleaners found that the CR Box exhibited a Clean Air Delivery Rate (CADR) of 600 on a low setting, and over 800 on a high setting. This was nearly 3 times the efficacy of the comparable HEPA units. What this means is that in a real world setting, a MERV-13 air purifier can clean the air in a room three times faster than a HEPA unit.
But you may be wondering – if a MERV-13 filter only removes 90% of the particulates that you want to remove from the air, doesn’t that leave 10% of those particulates to continue circulating around the room, free to infiltrate your lungs? Not necessarily! The higher airflow of a MERV-13 based air purifier enables multi-pass filtration – essentially, all of the air in a room with one of these purifiers is being put through the filter multiple times. And if you’ll think back to your elementary school math class, those percentages add up quickly!
It is this perfect balance between airflow and filter efficiency that we are leveraging in our upcoming MERV-13 based air purifier, the Vello, to deliver a clean air solution for everyday environments – at an everyday price!
Hello Vello
Soon on Kickstarter: Vello, our flagship flat-pack MERV-13 purifier, inspired by the proven Corsi-Rosenthal Box and engineered for maximum clean-air impact per dollar. Vello uses off-the-shelf, non-proprietary MERV-13 filters to filter particles and contaminants from the air three times faster than a personal HEPA unit, and at a fraction of the cost of specialized HEPA filtration systems.
Sign up for our mailing list to be the first to receive updates about the Vello Kickstarter – including behind-the-scenes insights and exclusive early-bird pricing for the first backers!
The greatest victory is the one which requires no battle.
[1] National Air Filter Association (2014). Understanding MERV: NAFA User’s Guide for ANSI/ASHRAE Standard 52.2-2012. https://amfco.com/wp-content/uploads/2020/10/MERV-RATING-CHART-52.2.pdf
[2] IQAir (2021). Particle size matters. https://www.iqair.com/us/newsroom/particle-size-matters
[3] FilterBuy (2024). All About MERV Ratings & MERV Rating Charts. https://filterbuy.com/resources/air-filter-basics/all-about-merv-ratings/
[4] Environmental Protection Agency (2025). What is a MERV rating? https://www.epa.gov/indoor-air-quality-iaq/what-merv-rating
[5] Environmental Protection Agency (2025). Particulate Matter (PM) Basics. https://www.epa.gov/pm-pollution/particulate-matter-pm-basics
[6] Uchital, A. (2021). Can I Install a HEPA Filter in My HVAC System? Around the Clock Heating and Air Conditioning. https://aroundclock.com/blog/can-i-install-a-hepa-filter-in-my-hvac-system/
[7] Purl’s Sheet Metal (2018). The Wrong High-Efficiency Air Filter Might Spell Disaster for Your HVAC System! https://www.purlsheetmetal.com/blog/wrong-efficiency-air-filter-might-disaster-hvac-system/
[8] Dal Porto, R., Kunz, M. N., Pistochini, T., Corsi, R. L., & Cappa, C. D. (2022). Characterizing the performance of a do-it-yourself (DIY) box fan air filter. Aerosol Science and Technology, 56(6), 564–572. https://doi.org/10.1080/02786826.2022.2054674
Melissa Smallwood
Melissa Smallwood is Science and Technology Policy researcher with a background in neuroscience, psychology, and social science, and the author of The Future of Long COVID: A Threatcasting Approach. They are interested in how disability and healthcare justice intersect with emerging technologies, and are an advocate for COVID mitigation strategies to help end the pandemic.