The particle size myth HEPA companies sell you

Can MERV-13 filters capture ultrafine particles smaller than 0.1 microns?

According to HEPA marketing and many indoor air quality professionals, the answer is no. They claim you absolutely need HEPA filtration for ultrafine particles - those tiny particles that can penetrate deep into your lungs and enter your bloodstream. But here's what the actual science shows: MERV-13 filters exhibit the exact same particle capture phenomenon as HEPA filters, including excellent performance on ultrafine particles.

Today, I'm going to dismantle the biggest myth in the air purification industry and show you why filter efficiency curves tell a story HEPA companies don't want you to know.

Let's get into it.

The most penetrating particle size phenomenon applies to all filters.

There's a concept in filtration called the most penetrating particle size, or MPPS, which sits around 0.3 microns. This is the particle size that's hardest for filters to capture. Particles larger than MPPS get caught by impaction and interception. Particles smaller than MPPS get caught by diffusion. But particles right around 0.3 microns are small enough to avoid impaction yet large enough to avoid diffusion - making them the trickiest to filter.

Here's what HEPA advocates always mention: HEPA filters are better at capturing particles both larger and smaller than 0.3 microns, including excellent ultrafine particle capture. But here's what they conveniently leave out: MERV filters exhibit the same exact phenomenon. A study by Parham Azimi et al. shows size-resolved removal efficiencies for MERV-rated filters from 0.001 microns (deep into ultrafine range) to 10 microns. MERV 16, MERV 14, and even MERV 12 all approach near 100% efficiency for ultrafine particles, just like HEPA [1].

Another study by B. Stephens demonstrates similar particle size efficiency curves for MERV-13 filters specifically [2].

But do MERV-13 filters work in real DIY air purifiers?

Those previous studies measured single-pass efficiency for standalone filters in lab conditions. What about in the context of an actual DIY air purifier with four MERV-13 filters?

A study out of Rutgers University measured the CADR of Corsi-Rosenthal boxes across particle sizes from 0.01 microns to 1.0 microns. The results showed that CADR was lowest at the MPPS of around 0.3 microns and increased as particles got more ultrafine down to 0.01 microns. Just like HEPA, CR boxes are actually better at capturing ultrafine particles than they are at capturing particles around 0.3 microns [3].

Translation: HEPA is not needed for ultrafine particle capture. MERV-13 filters in CR box configurations capture them effectively.

Lower air velocity makes MERV-13 even more effective.

Remember how I mentioned that HEPA units often run at their highest speed to achieve their marketed CADR numbers? There's a problem with that. A study from Aerosol and Air Quality Research demonstrates that filter penetration increases as air velocity increases, especially for ultrafine particles. The faster air flows through a filter, the more particles can slip through without getting captured [4].

This is actually good news for MERV-13 CR box designs. Because CR boxes have much larger total filter surface areas than compact HEPA units, air velocity through each filter remains much lower. Lower velocity means better particle capture, particularly for those tricky ultrafine particles. The study even used 3M Filtrete filters, which multiple testers have identified as the best-performing HVAC filters currently available.

MERV-13 filters can even protect you from viruses.

A presentation by Dr. Katherine Ratliff from the EPA tested a MERV-13 CR box against aerosolized non-pathogenic viruses in simulated saliva in a large chamber. The CR box removed 97% of bio-aerosols in 30 minutes and 99.4% in 60 minutes, with a CADR of 234 CFM - far better than the control condition with the CR box turned off.

So yes, DIY MERV-13 air purifiers can protect you from airborne viruses.

The comprehensive data is overwhelming.

A major review compiled data from numerous studies on DIY air purifiers versus commercial HEPAs. The findings were consistent across every metric: DIY MERV-13 purifiers beat commercial HEPAs in CADR, cost per CADR, and CADR per watt. They clean air faster, cost less upfront, and use less electricity - even though MERV-13 filters have lower single-pass efficiency than HEPA.

This isn't cherry-picked data. This is the scientific consensus based on multiple independent research teams testing real-world performance.

Why were you told HEPA is necessary?

I believe most people recommending HEPA filters genuinely think they're giving good advice. They've been taught that higher efficiency equals better performance. It's intuitive. It makes sense on paper. But it's wrong in practice for in-room air purification.

The HEPA industry has spent decades conditioning consumers and professionals to focus on filtration efficiency while ignoring airflow. They've created a market where people pay premium prices for machines that move minimal air volumes through ultra-efficient filters - resulting in slow, ineffective air cleaning despite impressive-sounding specifications.

The three practical solutions for actually cleaning your air.

Now that you understand the science, here are your options: First, you could buy from companies like Clean Air Kits (and get 5% off with my code ALEXKESSLER) that use MERV-13 filters with PC fans for high CADR, low noise, and excellent energy efficiency. Second, you could check cleanairstars.com, which compiles performance data for hundreds of air purifiers and has a selection tool (there are some decent HEPA units, like SmartAir Blasts, but they're more expensive). Third, you could build your own CR box using four MERV-13 filters and a box fan or higher-quality fan. -> Here’s my guide on how to build your own: How to Make a Better DIY Air Purifier

Whichever option you choose, make sure the unit's CADR meets the space requirements using AHAM's two-thirds rule: CADR should equal at least two-thirds of your room's square footage (for wildfire smoke, CADR should equal the full square footage).

Click the link to part 3 below, where I'll share the Clean Air Kits story, discuss real-world implementation strategies, and explain why getting this right is literally a matter of life and death.

Keep questioning everything,

Alex Kessler

P.S. The scientific evidence is crystal clear: indoor air quality is now among the leading causes of death worldwide, right alongside high blood pressure, poor diet, smoking, and alcohol use. This isn't about preference or opinion - it's about health outcomes.

[1] Parham Azimi, Dan Zhao, Brent Stephens, Estimates of HVAC filtration efficiency for fine and ultrafine particles of outdoor origin, Atmospheric Environment, Volume 98, 2014, Pages 337-346, ISSN 1352-2310, https://doi.org/10.1016/j.atmosenv.2014.09.007.

[2] Stephens, B., & Siegel, J. A. (2013). Ultrafine particle removal by residential heating, ventilating, and air-conditioning filters. Indoor air, 23(6), 488–497. https://doi.org/10.1111/ina.12045

[3] Myers, N. T., Dillon, K. P., Han, T. T., & Mainelis, G. (2023). Performance evaluation of different low-cost DIY air cleaner configurations. Aerosol Science and Technology, 57(11), 1128–1141. https://doi.org/10.1080/02786826.2023.2249963

[4] Chang, D.Q., Chen, S.C. and Pui, D.Y.H. (2016). Capture of Sub-500 nm Particles Using Residential Electret HVAC Filter Media-Experiments and Modeling. Aerosol Air Qual. Res. 16: 3349-3357. https://doi.org/10.4209/aaqr.2016.10.0437