What is PM0.3 and Why Is It Important?

What Does “PM” Mean?

To understand what PM0.3 is and why it is important, we should start with understanding what PM is. “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.


PM2.5 PM10 Hair sand


Sometimes you’ll see “PM” followed by a number, like PM2.5. That refers to particles of diameter 2.5 microns, (AKA “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.


size of particles microns


While PM of any size can cause adverse effects to our health, particles below 2.5 microns 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. So why this particular size so important? For example, big purifier companies will tell you how their special HEPA filters can capture particles down to 0.3 microns.


HEPA Filter Post Filter


Other purifier companies like Molekule claim that only their proprietary technology can filter particles under 0.3 microns, other filters can’t.


HEPA Filters cannot trap 0.3 microns Molekule


The air purifier company AirDog also claims to have special technology that captures particles beyond what HEPAs can capture.


AirDog HEPA Air Purifier


Since this 0.3-micron cutoff is so important in these air purifier advertisements, it must be quite hard to capture particles smaller than that, right? It turns out, reality is far weirder. The data shows that you don’t need any fancy technology or special HEPA because regular HEPA filters are fantastic at capturing particles under 0.3 microns.


HEPA wikipedia definition


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!


net capture particles


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).


interception and impact


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.


brownian motion


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. In another study, researchers tested what percentage of particles slipped through HVAC filters (actually weaker than HEPA filters). The smaller the particle, the fewer slipped through–even down to 0.025 microns.


HVAC Furnace filters capture nanoparticles 0.3 microns


This is good news for us when we’re in hospitals and airplanes. That’s because airplanes and many hospitals use HEPAs to clean the air. That means they clear our air of viruses, where the “removal efficiency is generally greater for particles both larger and smaller” than 0.3 microns.


hepa used airplane pm0.3


The theory of 0.3 microns being hardest to capture works for fiber filters of all types, including masks and lower-grade MERV furnace filters.


filtration efficiency


Can HEPA Filters Capture 0.3 Micron Particles?

But “weak spot” is overstating things. Even though PM0.3 is important and is the hardest to capture, HEPA and masks filters are still great at capturing these particles.


penetrating size 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 backedno-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.


air filters and efficiency


CADR air purifier

Bottom Line: What is PM0.3 and Why Is It Important

PM0.3 is particulate matters of diameter 0.3 microns.

Brownian motion works its magic under 0.3 microns. The filtering works its magic above 0.3 microns. Particles at 0.3 microns lie in between the two, and that makes them the hardest particle size to capture.

HEPA and masks filters are still great at capturing these particles.

Breathe safe!

Covid19 coronavirus best masks materials fundraiser

Get the latest clean air tips!

Get updates on masks, air purifiers and air quality delivered straight to your inbox.

Leave a Reply

Notify of
Muthiah P. Muthiah

Fantastic explanation! You are a great teacher!!
Bottom line: particle size, static force attraction, Brownian movement all contribute to filtration.
Some experts are stuck on particle size alone, downplaying the importance of masks and filtration devices.
Using the concept of Minimal Infective Dose, REDUCING the viral particles inhaled is pivotal in reducing the chance of an infection, even if we are exposed.


In current filters, electrostatic forces matter, not just Brownian diffusion, and the actual MMPS is lower, around 30-100nm. Nice review here: doi:10.1007/s11051-009-9649-3.

anil magdum

Me too thought same. The N95 and other masks provide protection against COVID due to these electrostatic forces also. What’s your opinion about this.

Roberto Blanco

Hello, I’ve been a nurse for 25 years, have lived thru one pandemic, and am mortified by the fact that medical workers don’t have the proper PM masks to keep them safe.. I have a degree in design as well and would like to create, for my self, and my coworkers the proper PPE’S. Do you sell this fabric and if not could you refer me to some one who does. Any information you provide me would be greatly appreciated.

Liz - Smart Air

Hi Roberto, Liz here! Unfortunately we do not sell the materials tested. We do have the HEPA filter but not the HEPA sheet. If you plan to get HEPA sheet as DIY mask material, the best thing is to contact with a HEPA manufacturer. Make sure that the HEPA is made of PP/PET instead of glass fiber! Stay safe!

anil magdum

Roberto please guide us if you find something to protect yourself. We too want to distribute something which will protect us and fellow medics, paramedics and police people against this.


Hello Roberto: I am immunocompromised and have many underlying conditions. I am trying to find this filter as well as my son will be returning to work as well as 520 employees without room for social distancing. if you will kindly email me with the mask and filter solution you may have found I would be forever grateful. Thank you.


Hi, two points from doi:10.1007/s11051-009-9649-3 :
– “Most respirators made in the last decade containing N95 filter media:use electret filter media” … “imparting a static electric charge on non-woven polymer fibers (e.g., polypropylene”. I would guess this is more important than Brownian diffusion.
– Their experimental MMPS is in the range 30–100 nm

Carla Hart

Hi. Myself and many of my friends want to help our local healthcare providers during the viral pandemic. They have run out of face masks in many areas but it will only get worse. I’m now working from home as are many of my friends. We also have many friends who do not work but are home in isolation. What I would like to know is if it would be possible to get the materials that you use for air filters BEFORE they are pleated so that we could use them to make face masks to donate to our healthcare… Read more »