Have you heard of ultra fine particles? Ultra fine particles are smaller than 0.1 microns, or 0.0001 millimetre. That is smaller than most bacteria and dust mites! But because they are so small, these particles can penetrate deep into your lungs and enter your bloodstream.

But the actual science tells a different story.

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

Let’s get into it.

Expert Recap: What is the best air filter for CIRS?

Marketing for HEPA filters is so prevalent that they are the automatic go to when searching for the best air filter for CIRS. However, studies show the low cost MERV 13 furnace filter is just as efficient. When used in a Corsi-Rosenthal DIY air purifier, the MERV 13 air filter surpasses the HEPA filter for air filtration.

The most penetrating particle size phenomenon

There’s a concept in filtration called the most penetrating particle size, or MPPS. MPPS is the particle size that has the most success penetrating a medium. It’s hardest particle for filters to capture, and sits around 0.3 microns. Particles larger than MPPS either get stuck in an air filter, or are intercepted. Particles smaller than MPPS get caught by diffusion. But particles right around 0.3 microns are small enough to not get caught, yet large enough to avoid diffusion – making them the trickiest to filter.

If you’ve started doing research on the best air filter for CIRS, you may have read that HEPA filters are better at capturing particles both larger and smaller than 0.3 microns. This includes excellent ultrafine particle capture.

But you may not know that MERV furnace filters are just as efficient. A study looking at the movement of outdoor air particles and how they are captured showed MERV-rated furnace filters could efficiently remove particles ranging from 0.0001 microns to 10 microns.

Just like a HEPA filter, MERV 16, MERV 14, and even MERV 12 filters are almost 100% efficient for capturing ultrafine particles.

Another study looked at the efficiency of MERV furnace filters in an indoor environment. This study showed a MERV 13 filter had similar particle size efficiency.

Do MERV-13 filters work in a DIY air purifier?

Those previous studies measured the efficiency of one MERV filter used in a furnace under lab conditions. How do multiple MERV filters perform when used as an air purifier?

As I mentioned in my previous blog, The Best Air Purifier for CIRS, I abandoned expensive HEPA purifiers in favour of a DIY model called a Corsi-Rosenthal (CR) box. A CR box is a low cost air purifier made of four or five MERV-13 furnace filters. The filters are duct taped in a cube with a box fan to draw air through the filters. The cost is much lower than a HEPA purifier, and multiple studies show the CR box is vastly more efficient at cleaning the air.

A study out of Rutgers University measured the Clean Air Delivery Rate (CADR) of CR boxes across particle sizes from 0.01 microns to 1.0 microns. When it comes to air purification, CADR is the only metric that matters. CADR measures how quickly clean air is supplied to a space – essentially, how fast pollutants are removed from your room. To significantly lower particle concentration, the removal rate must be high enough to counteract the rate at which particles are generated – whether that is from cooking, outdoor air, mold, skin cells, or movement.

The results showed that the CADR of a Corsi-Rosenthal box was lowest with particle sizes of 0.3 microns, and increased as particle size decreased to 0.01 microns. Just like HEPA, CR boxes are actually better at capturing ultra fine 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 just as 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 (Best Air Purifier for CIRS)? 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 ultra fine particles. In other words, 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 ultra fine particles.

MERV-13 filters and airborne viruses 

A presentation by Dr. Katherine Ratliff from the American Environmental Protection Agency (EPA) took a look at how a MERV-13 CR box performed when faced with airborne non-pathogenic viruses in simulated saliva. The CR box removed 97% of the 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, a low cost, DIY MERV-13 air purifier can protect you from airborne viruses.

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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: do-it-yourself MERV-13 purifiers beat commercial HEPAs in Clean Air Delivery Rate (CADR), cost per CADR, and CADR per watt. The DIY purifiers 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, you have some options for air filters in your home:

1. Buy from companies like Clean Air Kits that use MERV-13 filters with PC fans for high CADR, low noise, and excellent energy efficiency. Use the code ALEXKESSLER for 5% off.
2. Check out 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).
3. Build your own CR box using four MERV-13 filters and a box fan or higher-quality fan. Check out my guide to build one on 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 the Association of Home Appliance Manufacturer’s (AHAM) two-thirds rule: CADR should equal at least two-thirds of your room’s square footage (if there is wildfire smoke, CADR should equal the full square footage).

Next week, 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

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Book a consult with our team today.

Alex Kessler, of healthyhomeguide.com, is an indoor/outdoor environmental consultant and translational scientist. After surviving CIRS across multiple toxic buildings, he pivoted ten years of precision medicine training into a public mission: offer guidance on mold and indoor air quality grounded in real-world practicality, not marketing. His YouTube channel @HealthyHomeGuide (2M+ views) and 1:1 virtual consultations serve renters, homeowners, and the chronically ill (CIRS, etc).

Find him on Instagram @healthy.home.guide, Tik Tok @healthyhomeguide and YouTube @HealthyHomeGuide.