About

The flu, the filters, and why we stopped trusting the box

We didn’t set out to start a company—we just wanted cleaner air at home.

So we bought air purifiers like everyone else. The boxes were confident: covers ___ sq ft, True HEPA, smart sensors, auto mode. But living with them raised a simple question that wouldn’t go away:

How would we actually know if our air was clean?

The sensors didn’t agree. “Good” didn’t come with numbers. The room-size claims felt fuzzy. The units were loud on the settings that supposedly did the work—and quiet on the settings that didn’t. Filters were expensive, and our kids would sometimes turn the purifiers off because the noise was too much.

So we stopped guessing and started measuring.

We bought PM2.5 meters and tested our rooms the way we actually live in them—different spots, different distances, real furniture, real doors. What we found was frustrating: many commercial purifiers created a “clean bubble” nearby, but the rest of the room changed more slowly. The issue wasn’t just filtration. It was airflow and mixing.

That led us to Corsi–Rosenthal boxes—filters, a box fan, duct tape. They weren’t pretty and they weren’t quiet, but they moved air like crazy and the PM2.5 numbers dropped fast across the whole room.

Then flu hit our house—and the air tested us. Normally illness spreads through everyone like dominoes. This time, one person got sick… and it stopped. That was the moment we realized clean air wasn’t just a comfort upgrade. It changed outcomes.

As we researched deeper, three disappointing truths kept showing up:

  1. CADR numbers are based on well-mixed test conditions by adding 1-2 extra fans in a small chamber, but real homes aren’t naturally well-mixed—and the box doesn’t explain that.

  2. The goal is rarely defined. What we actually needed was high ACH (Air Changes per Hour). Many modern homes are tight for energy efficiency, which can mean low natural air exchange—so families end up rebreathing shared air for hours. But no one tells you what you need for the results you're after. No one tells you 12ACH is the threshold where viruses stop spreading so easily. 

  3. HEPA isn’t required to get air extremely clean. If you move enough air and mix the room well, running air repeatedly through a high-performing filter like MERV 13 can drive particle levels down dramatically. And it can clean the air before it actually reaches you. 

So we built our own designs using MERV 13/14 filters + quiet PC fans, and tested them with real PM2.5 meters in real rooms. Our builds cleaned faster than the HEPA units we owned—again and again—not because HEPA is “bad,” but because airflow + mixing is what makes performance real at home.

DIYAirFilter.com exists for people who are tired of vague labels and want clean air they can measure, control, and trust—quietly, effectively, and sized for their space.