Are we all wrong about HEPA filters?

It is no secret that a DIY air cleaner is made by putting a filter and fan together. During the COVID-19 pandemic, along with other strategies, there was a need for cost-effective air purification. Even though there are many portable HEPA air purifiers on the market, Richard Corsi and Jim Rosenthal (CR) came up with the idea of a DIY air purifier unit with electrostatic filters and a fan. When they tested the efficacy of CR Box, unexpectedly they realized that particle concentration reduction was better than HEPA filter air purifiers in many cases. Even though the HEPA filter is the king of air filters, it was beaten by electrostatic MERV13 air filters. In this article, I want to get to the bottom of this by explaining how electrostatic filters outperform HEPA filters.

HEPA Filters:

HEPA is defined as “high-efficiency particulate air” [filter] that should satisfy at least 99.95% efficiency at the most penetrating particle size (typically 0.1-0.3 micron). In higher or lower airflows or for fine to coarse particles, HEPA filters are supposed to be effective at the claimed efficiency. It is well-known in the industry that HEPA filters can filter out viruses, bacteria, dirt, pollen, and dust. HEPA filters, best-in-class, are used in places where super clean air is required such as pharmaceutical and electronic manufacturing environments, clean rooms, hospitals, portable air purifiers, and cabin air filters. HEPA filters are not common in residential HVAC units due to resistance to airflow limitations while portable HEPA air purifiers are common. HEPA filtration units also have notable disadvantages, including high initial purchase costs, high costs of filter replacement, and noise levels that can exceed 50 dB.

HEPA filters are tested per ISO Standard 29463 and EN Standard 1822. Initially, HEPA filter media is tested with fine particles to determine MPPS. Correspondingly, the filter is tested at this MPSS. Then, HEPA classification is done based on the efficiencies. One important aspect is that HEPA filters are individually certified to show that they meet their target efficiencies and are also individually scanned for leakage.

HEPA filters can be made of microglass media, membrane media(ePTFE), and glass media-polymeric media combinations. Traditionally, HEPA filter media is made of fine glass fiber thus maintaining efficiency during the lifetime of the filter. Moreover, microglass media provides good dust-holding capacity because glassfibers do not collapse during particle loading.

MERV13:

MERV, minimum efficiency reporting values, is determined according to the ANSI/ASHRAE 52.2 standard. The challenge particle size ranges from 0.3 to 10 micron. The airflow rate of the test can go up to 2000 cfm, considerably higher than HEPA testing airflow. In a nutshell, MERV 13 filters should have efficiencies of at least 50% from 0.3 to 10 microns, >85% from 1 to 3 microns, and >90% from 3 to 10 microns. MERV13 filter has been used in various applications from residential to commercial filters.

MERV 13 filter media can be made of mechanical media such as nanofiber web-coated fabrics or electrostatic filter media. Electrostatic filter media is conventionally preferred due to low resistance to airflow and high dust loading capacity. MERV13 filters have become such an important product in the fight against viruses during the pandemic, that many organizations such as CDC and ASHRAE recommend upgrading filters to MERV13.

Clean Air Delivery Rates (CADR) and Air Change per Hour (ACH)

Since the comparison is conducted for air cleaners, we should introduce two important concepts here:

-?????????The extremely valuable information for predicting the effectiveness of room air cleaners is the clean air delivery rate (CADR), which is a measure of an air cleaner’s transfer of filtered clean air, articulated in cubic feet per minute (cfm). To determine CADR, three particle size ranges are normally tested in ANSI/AHAM AC-1 Test Standard, Method for Measuring the Performance of Portable Household Electric Room Air Cleaners: tobacco smoke (0.09-1 μm), dust (0.5-3 μm), and pollen (5-10 μm). Test particulates of interest are introduced into the test chamber at a known level. Throughout the test, electronic particle-counting devices monitor the concentration and size of the particles. Two tests are performed: the first test is run without the air cleaner on. This test shows the natural decay rate of the particles that is deducted from the rate established during the second test when the air cleaner is on. The difference between the two rates gives the Clean Air Delivery Rate (CADR). In some scientific studies, CADR is estimated by multiplying airflow (cfm) by particulate efficiency. The higher the CADR numbers, the better the total capability of the unit to clean your interior air, and thus the faster the air in the room will be cleaned.

-?????????ACH measures the amount of air that is added/filtered/exchanged per hour and is also a critical parameter for indoor air quality which is calculated by dividing the airflow of the cleaning unit (cfm) by room volume. Typically, air purifiers on the market are recommended for a specific room volume. CDC, a federal agency, recommends at least 5 changes per hour in room ventilation along with min MERV13 filters because the higher the ACH the less risk of exposure to viruses. There are higher ACH recommendations for high-risk indoor environments such as hospitals by different agencies.

HEPA vs. MERV13

First, it is not right to compare filters by just looking at their supplier-provided efficiencies, there are other performance metrics as well. Second, HEPA and MERV filters are tested by different methods. Particularly, HEPA’s challenge particle size is MPPS (0.1-03) while MERV13’s challenge particle size range is 0.3-10 microns. Moreover, the test airflow rate of HEPA is typically much smaller than MERV filters. Besides, HEPA filters have higher resistance to airflow compared to MERV13 filters. That’s one of the reasons why MERV13 coupled DIY air cleaners result in much higher CADRs.

Srikrishna reported that DIY MERV13 air filters with various designs such as 3 filters with 2 fans or 4 filters with 1 fan, provide roughly double the CADR compared to HEPA filters at 0.3-micron particle size. The study shows the effectiveness of the electrostatic filters.

Cappa et al. (2022) reported the same trend in terms of CADR. For instance, the CR box with 4 MERV13 electrostatic air filters at the highest fan speed had a CADR of around 800 cfm while HEPA air purifiers had around 250 cfm CADR. The results demonstrated that the CR box with MERV13 air filters was more effective than air purifiers with HEPA filters at reducing the concentrations of particles at >0.5 micron.

In another study, Donskey et al. (2023) showed that CR Box with MERV13 air filters reduced the concentration of aerosolized bacteriophage MS2 to the same level compared to HEPA air purifiers. MERV13 CR box had similar effectiveness when the HEPA air purifier was supported with UVC photocatalytic oxidation, demonstrating the higher airflow of the CR box.

Filter effectiveness vs. Filter Efficiency

The fractional efficiency of a filter is measured in a laboratory with a controlled experiment. Everything in the test is artificial and does not represent real-world conditions. Particulate efficiency is measured with a single-pass test while airflow partially or fully recirculates in real-world conditions.

Briefly, the effectiveness of an air-cleaning unit is its ability to remove pollutants in real-world conditions. So, effectiveness is a function of airflow and efficiency. For instance, for two air cleaning units with the same MERV 13 filters, the unit with more airflow will be more effective than the other.

How does MERV 13 filter-coupled air cleaner outperform HEPA air cleaner?

To answer this question, we need to understand the relationships of single fiber filtration theory, fiber properties, and filtration mechanisms of diffusion, interception, impaction, and electrostatic attraction.

1-?????HEPA filters are mechanical filters, made of densely packed microglass fibers. Interception and diffusion are more dominant mechanisms in capturing submicron particles compared to impaction. According to the single fiber filtration theory, as fiber diameter decreases, efficiency due to interception increases while efficiency reduces due to impaction. Face velocity has no impact on interception, while impaction increases as face velocity increases. Practically, it means that airflow rate has no impact on the fractional efficiency of HEPA filters at MPPS. Regarding the air cleaners, making an air cleaner with one HEPA or 4 HEPA filters is practically no different in terms of particle capture efficiency.

2-?????MERV13 filters were reported as electrostatic in some studies. In this case, everything changes. Using 4x MERV13 filters with a fan increases the total filtration area thus reducing face velocity (= airflow/total media area). As face velocity reduces, the residence time of particles in the filtration medium increases and thus capturing efficiency increases. Particularly, not only submicron particle capture efficiency increases but also resistance to airflow decreases by reducing face velocity. That’s why an electrostatic MERV13 filter becomes more efficient, in these cases, as efficient as HEPA filters. Inherently, an electrostatic MERV13 filter has a significantly lower resistance to airflow compared to HEPA filters. Moreover, reducing the face velocity of DIY MERV13 air cleaner reduces resistance to airflow, resulting in even more CADR (=efficiency x cfm). The more CADR, the more ACH, thus lower the particle concentration. Therefore, the combined effects of increased efficiency and higher CADR make MERV13 filter coupled air cleaner more effective. Hypothetically, mechanical MERV13 filters coupled air cleaners would not be as effective as HEPA air cleaners for the reasons explained above. So, the success of DIY CR air cleaners is tied to electrostatic filter media.

Takeaways

Electrostatic filtration media is such an underrated product yet with the right design, such products outperform HEPA and do effective air cleaning. One strategy to reach the full potential of electrostatic media technology is to reduce the face velocity. Additionally, due to many technological advancements, producing electrostatic media is more convenient compared to HEPA filter media technologies.

Regarding the question in the title, no, we are not wrong about HEPA filters. From an efficiency standpoint, they are performing as claimed. However electrostatic filters are so underappreciated that many organizations started recommending mechanical filter products. With the right design, electrostatic MERV13 installed air cleaner could be more effective than HEPA air cleaner. Filtration efficiency and airflow should be considered together for the air cleaner's effectiveness.

DIY air purifiers could be optimized based on the design (- Filter x Fan- 3x2 or 4x1), filter thickness (1”, 2” or 4”), fan airflow, etc. Sealing or leak situations should also be considered seriously. For an effective DIY air cleaner, there should be no leak. Keep in mind that each DIY air purifier will have its own effectiveness.

HEPA filter media technology has not gone through many breakthrough advancements in the last 50 years. There will be a moment in the future when synthetic fibers will start replacing glass fibers in HEPA filter media. After that, we will see less and less fiberglass HEPA filter media on the market. On the other hand, electrostatic filter media technologies advanced significantly: it has more charge density than before along with a stable charge retention which results in a satisfactory shelf life. Isn’t it the right time to reconsider the throne of HEPA in the filtration industry?

Indoor air quality is real. Single pass efficiency is not super critical for indoor air cleaners where air recirculates. If you want to protect your loved ones from the next wildfire smoke or next pandemic, filtered air can help.?

References:

https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html

Srikrishna, Devabhaktuni. "Can 10× cheaper, lower-efficiency particulate air filters and box fans complement High-Efficiency Particulate Air (HEPA) purifiers to help control the COVID-19 pandemic?."?Science of the Total Environment?838 (2022): 155884.

Cadnum, Jennifer L., et al. "Effectiveness of commercial portable air cleaners and a do-it-yourself minimum efficiency reporting value (MERV)-13 filter box fan air cleaner in reducing aerosolized bacteriophage MS2."?Infection Control & Hospital Epidemiology?44.4 (2023): 663-665.

Derk, Raymond C., et al. "Efficacy of Do-It-Yourself air filtration units in reducing exposure to simulated respiratory aerosols."?Building and environment?229 (2023): 109920.

https://ahamverifide.org/wp-content/uploads/2020/06/Air-Cleaner-Performance-FAQs.pdf

https://www.webmd.com/allergies/corsi-rosenthal-box

Dal Porto, Rachael, et al. "Characterizing the performance of a do-it-yourself (DIY) box fan air filter."?Aerosol Science and Technology?56.6 (2022): 564-572.

Yasar Kiyak, Ph.D. is the R&D Manager at Gessner Filtration, a MATIV Brand. He is a USPTO registered patent agent. He received his bachelor's and master’s degrees in Textile Engineering from Istanbul Technical University and a Ph.D. degree in Fiber and Polymer Science from NC State University. During his graduate studies, Yasar specialized in nonwovens for filtration applications. He is an innovator with a strong background in the fiber and filtration industries. Yasar can be reached at [email protected].