How Do I Know If an Air Filter Really Works?
- Thomas Dearden
- Aug 6, 2025
- 3 min read
Updated: Sep 9, 2025
Can I test it myself?
Response by Dan Dearden, CAFS – Certified Air Filtration Specialist, Essential Air Products Founder, NovusAer Inventor
Great question—and one that more people should be asking.
One of my biggest frustrations in the indoor air quality industry is that some companies exaggerate or flat-out mislead people about what their products actually do. Take, for example, a wildly popular “air purifier” on the market today. Let’s call the manufacturer Company R.
The Problem with Lab Results
Company R claims their purifier “inactivates” 99.9% of the MS2 virus within two hours in a sealed laboratory test chamber. Sounds impressive, right?
But here’s what they don’t tell you: They never ran a control test—that is, they didn’t measure what would happen to the virus without the purifier running. In science, this control group is essential. Without it, the test results don’t mean much.
See the Company R graph below:
Let me show you why that matters.
Real Examples: Same Test, Different Stories
Company T ran a similar test. They reported a 98% reduction of the MS2 virus after one hour. Sounds great—until you look at their control test. Without the purifier running, 70% of the virus was already inactivated by natural decay. So in reality, their product only improved things by 28%.
See Company T Results below:
Company P went a step further. They claimed a 99.99% reduction of SARS-CoV-2 in one hour. But their control test showed that 81% of the virus decayed naturally without any purifier. Their real improvement? Just 19%.
See Company P Results below:
As for Company R? They didn’t publish a control result at all. But based on the other two, we can reasonably assume that 70% to 80% of the virus would have decayed on its own in the same amount of time.
Lab Chambers Are Not Real Homes
These tests were run in sealed 10’ x 10’ x 10’ chambers. Your home isn’t sealed. Air is constantly moving in and out—through windows, doors, and ducts. No test results exist showing these purifiers actually work in real-world home environments with ducted HVAC systems and air infiltration.
Why Filters Are Different
So why can’t manufacturers be held more accountable?
Because most homeowners and even contractors can’t test these high-tech “air purifiers” on their own. But when it comes to air filters, that’s a different story. You can test them yourself—with a simple tool called a laser particle counter.
Yes, You Can Test Your Air Filter at Home
Twenty years ago, the first laser particle counter I bought cost $800. My second one—scientific grade—was almost $4,000.
But today, you can buy:
A contractor-grade handheld particle counter for about $69, or
A tabletop model for around $38.
They’re easy to use, and here’s how:
Turn your HVAC system fan on.
Measure the return air. Place the particle counter near one or more return air grilles and average the PM2.5 readings.
Measure the supply air. Remove a supply register and place the counter inside the duct. Let it read for a minute.
Do the math. Example:
Return air: 12.3 µg/m³
Supply air: 1.2 µg/m³
1.2 ÷ 12.3 = 0.098 → Move the decimal two places → 9.8%
That means 90.2% of the harmful PM2.5 particles were removed by your filter.
Here's a helpful link to Consumer Reports' reviews of Indoor Air Quality Monitors. You'll need a subscription to see all the ratings, but you can see the list of monitors without a subscription.
Bottom Line
You don’t have to rely on marketing claims. You can see the results for yourself—in your own home, with your own system. And yes, you can even test our filters this way.
So if you're serious about clean indoor air, skip the hype and test what actually works.
Disclaimer: I don’t earn any money from the products listed above. They are simply examples of tools you can use to measure indoor air quality. Some I’ve tried personally, others I haven’t. Listing them here should not be considered an endorsement. Please make purchase and usage decisions at your own discretion and risk.
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