What does “PM” mean?
PM stands for particulate matter–particles in the air. These particles can be solids (like dust) or liquids (like drops of water). Some common sources of PM are car exhaust, smoke from coal-fired power plants, pollen spores, and dust from construction sites.
Sometimes you’ll see “PM” followed by a number, like PM2.5. That refers to particles of diameter 2.5 microns, or micrometers, or less. Microns are small – 1 millionth of a meter. Pollen particles are often 10 microns or bigger. Bacteria are often about 1 micron.
While PM of any size can cause adverse effects to our health, particles below 2.5 microns in size are especially dangerous. These are small enough to be absorbed directly to our bloodstream and enter our lungs, heart, and brains.
Why is there so much fuss about 0.3 microns?
If you start Googling air filters, you’ll find a lot of fuss about 0.3 microns. For example, big purifier companies will tell you how their special HEPA filters can capture particles down to 0.3 microns.
Other purifier companies like Molekule claim that only their proprietary technology can filter particles under 0.3 microns.
The air purifier company AirDog also claims to have special technology that captures particles beyond what HEPAs can capture.
However, the reality is that you don’t need any fancy technology or special HEPA because regular HEPA filters are fantastic at capturing particles under 0.3 microns.
In fact, smaller particles, like 0.01-micron particles are even easier to capture than those 0.3-micron particles!
Why Are 0.01-Micron Particles Actually Easier to Capture?
It seems counterintuitive that 0.3 microns would be harder to capture than 0.01 microns—that’s 30 times smaller. But the root of the problem is our intuition 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 big objects like marbles. And it’s basically how HEPA filters work for particles bigger than 0.3 microns. These particles either can’t fit through or their inertia causes them to hit the filter fibers (processes called impaction and interception).
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.
These 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.
Here’s why that 0.3 micron number comes up all the time. The weirdness of Brownian motion works its magic under 0.3 microns. The more easily understandable filtering works its magic above 0.3 microns. But where those two processes overlap is the weak spot. Particles at 0.3 microns lie in between the two, and that makes them the hardest particle size to capture. Researchers call this the most penetrating particle size (MPPS).
Is there any actual evidence that HEPAs capture tiny particles?
Scientists from NASA have tested the particle capturing efficiency of filters and found that 0.3 microns is the lowest point. Another piece of evidence comes from hospitals and airplanes, many of which use HEPAs to capture viruses as the “removal efficiency is generally greater for particles both larger and smaller” than 0.3 microns.
The theory of 0.3 microns being hardest to capture works for fiber filters of all types, including masks and lower-grade MERV furnace filter.
Can HEPA filters capture 0.3 micron particles?
But “weak spot” is overstating things. Even though 0.3 microns is the hardest size to capture, HEPA and masks filters are still great at capturing these particles.
How I protect myself
Smart Air is a certified B Corp committed to combating the myths big companies use to artificially inflate the price of clean air.
Smart Air provides empirically backed, no-nonsense purifiers and masks, that use the same H12 and H13 HEPA filters (that filter 99.5-99.95% of particles of size 0.3 microns) for a fraction of the cost of big companies.
Kang Wei is a chemical engineer from the National University of Singapore. He’s currently working on R&D and engineering at Smart Air.