Over this last Christmas, I was at home on the couch, and my dad asked me if I’ve seen this new Molekule air purifier he saw an ad review for.
He asked not because he’s interested in purifiers, not because I’m some PhD in fluid mechanics (I’m a cultural psychologist, not a physicist), but because I’ve been building and testing DIY air purifiers ever since a Beijing airpocalypse in 2013 had me coughing for weeks.
Here’s one of the more “beautiful” machines I built and tested:
Why would a psychologist spend four years building and testing purifiers? Because the air outside my Beijing apartment was “beyond index”…
…and the view in my online shopping cart looked like this.
I was scared, and these companies were saying that clean air costs $500 or $1,000. Soon I found I could make my own purifier for about $30 (see the full data on how it effectively reduced even tiny PM2.5 particles in my room!)
The lesson I’ve been learning throughout my testing is that big purifier companies actually love smoke. That smoke comes in the form of claims that we need filters imported from Switzerland (learn more about that ‘fancy’ Swiss company I’m talking about here), not because they’re high profit margin, but because the technology is better.
That’s the backdrop that was in my mind as my dad handed me his iPad, and I clicked on the Molekule air purifier ad. So let me review the claims:
Molekule Air Purifier Claim #1
Immediately, I saw this claim.
These guys are so technologically advanced they haven’t read the Wikipedia page about HEPA filters. If they had, they would’ve learned that HEPA filters are actually fantastic at capturing particles under 0.3 microns.
See, the problem is our intuition is to think of HEPA filters like a net. If a particle is smaller than the holes in the net, it gets through. So the smaller the particle, the harder it is to capture. Makes sense.
That logic works for objects as big as marbles. But when we get to really small particles—like particles under 0.3 microns—things start getting weird. Particles that small have so little mass that they actually get bounced around like a pinball when they hit gas molecules (known as Brownian Motion). So they move in random zigzag patterns.
Why that matters is that tiny particles are small enough to fit through HEPA filters—if they flew straight. But because they fly in zigzag patterns, they end up hitting the fibers and getting stuck.
OK, that’s a lot of theoretical explanation. Is there any test evidence for this? There’s lots! One test I like is this thesis on duct filters (never thought I’d say that about a thesis on duct filters).
That’s the percentage of particles the filters captured at different sizes. See how the effectiveness goes down, but then starts to go up again? That’s below 0.3 microns—the particles Molekule claims that regular purifiers can’t get.
By the way, this study was on furnace filters, which are lower grade than HEPA filters. But the principle works for any fiber filter. It works for the HEPA filters that are in purifiers (see this guy’s tests down to .01 microns) and even masks (I describe mask tests down to .007 microns here). That means you don’t even need a fancy HEPA filter for Brownian motion to work its magic.
So why is Molekule claiming HEPA filters can’t capture any particles below 0.3 microns?
Either they don’t know this basic fact about purifiers (in which case, I’m highly skeptical of their claims of being experts), or they’re straight up lying about purifiers to sell you a more expensive one (in which case, I don’t trust them).
Bottom line: Their claim to superiority about particle sizes is false.
Molekule Air Purifier Claim #2
Their second big claim is about bacteria. They claim that bacteria will grow on regular HEPA filters and then get released back into the air, so you need their Molekule technology to kill that bacteria.
That includes lots of people in hot and humid Shanghai and Shenzhen, not to mention basically all of India (which qualifies as hot and mostly humid). I’ve never once seen or heard of mold or bacteria growing on these HEPA filters. That’s despite the fact that I had a mold problem in my Beijing apartment before I started using purifiers.
Bottom line: Is it theoretically possible for bacteria to grow on a HEPA? I suppose so. But even in these extreme environments, I’ve seen absolutely zero evidence that it actually happens. If it were a problem, I don’t see why airplanes and hospitals use HEPA filters to trap bacteria.
Molekule Air Purifier Claim #3
Molekule says HEPA filters can’t capture viruses because they’re too small. But the Molekule air purifier can!
This gets back to point #1. HEPA filters (and really most filters) are great at capturing viruses.
Viruses are solidly in the range of very, very high efficiency for HEPA filters and even masks.
Molekule Air Purifier Claim #4
About allergens, Molekule says HEPA filters “only traps larger allergens” and then releases them back into the air anyway.
We’ve seen the claim about small particles before, so we know that’s wrong. But what about releasing them back into the air?
I’ve seen people worry about this, but I’ve never seen any evidence for it. And in the tests I’ve done, all I find is evidence of the opposite. Let’s take this 200-day test longevity test. See, I wanted to know how long a HEPA filter would last on my DIY in real Beijing air. So my Smart Air co-founder Gus tested it everyday in his Beijing bedroom for 200 days.
Here’s what a regular test day looked like. There’s particulate in Gus’s bedroom (red line) and outside (blue line).
Here are 10-day averages all the way out to 200 days (all original data available here).
Even out to day 200, the HEPA filter was managing 50% reductions in room particulate. At no point was the HEPA filter making room air worse.
That’s just not how filters work. They capture particles. They don’t release them. Unless you’re shaking them or cutting them open, they’re not going to release those particles back into the air.
Molekule Air Purifier Claim #5
There is one claim that strikes me as “could be true.” It’s their claim that the machine “destroys” chemical gases known as volatile organic compounds (VOCs). That includes stuff like formaldehyde and toluene, which come out of products that we buy, like carpeting.
They’re right that HEPA filters don’t capture gas pollutants. HEPAs are for particles, not gases. They’re weirdly claiming that carbon is only effective for 80 minutes. That must be an incredibly tiny carbon filter. It doesn’t match my tests with the carbon filters on regular purifiers.
But does their machine actually “destroy” VOCs? Maybe it does! If so, that would be the only actual selling point of this machine. Unfortunately, they don’t explain what the filter is (beyond using fancy-sound terms like “nano”), and they don’t provide any data with enough detail that I could assess it.
Bottom line: The Molekule might be better at capturing VOCs, but they do not provide convincing evidence for it. The other information on their site is so bad that it makes me skeptical of this claim.
So what’s an air breather to do?
I’ve been nerding out on purifiers ever since that 2013 Beijing airpocalypse. If I can sum up all of my testing into one message, it’d be this:
Air purifiers are shockingly simple. They’re a fan and a filter. Cheaper purifiers perform just as well—and often outperform—the ridiculously expensive machines.
So brush aside marketing claims, beautiful Nordic families living in all-white homes, and keep your money.
Instead, check out any one of several sources of independent purifier tests and choose the best combination of effectiveness and price. I outline several sources of tests here:
P.S. I’ve ranted for too long about Molekule’s claims, but please indulge me the claim at the top of their homepage.
Finally a purifier “that actually works.” Click on the link above to see dozens of purifiers that cost far less and “actually work.”
View the original article on Quora here.
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Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.