Does a HEPA Filter Remove PM2.5 (Fine Particles)?

Smoke Indoor Air Quality


When I began researching about how to purify the air in my home, I became very concerned about the fine particles in the air. Fine particles are pollutants that are 2.5 micrometers in size or smaller. You may have  seen them abbreviated as particulate matter 2.5 or pm2.5.

Based on my findings, the HEPA (High-Efficiency Particulate Air) filter seemed to be one of the best solutions to clean the air.

But does a HEPA filter remove fine particles? A HEPA filter is guaranteed to remove particles that are as small as 0.3 micrometers in size. Therefore, it does remove fine particles. However, there are other methods that can be used in conjunction with a HEPA filter to increase the likelihood of removing even the finer particles, such as activated carbon, UV light, and even upgrading your HVAC air filters.

A HEPA Filter Guarantees to Capture Fine Particles 0.3 Micrometers and Larger

By definition, a HEPA filter has been tested and proven to be able to filter out 99.97% of particles 0.3 micrometers in diameter and larger. Now, that doesn’t mean that a HEPA filter will absolutely be incapable of trapping those particles that are only 0.1 or 0.2 micrometers in size. It doesn’t mean that it won’t be able to remove these smaller fine particles, but rather, that its testing does not guarantee it.

A HEPA filter employs several mechanisms to trap particles, and all those methods combined are guaranteed to be effective on fine particles that are 0.3 micrometers in size and larger. In case you’re curious, a HEPA filter uses 4 methods – impaction, sieving, interception, and diffusion.

Particles smaller than 0.3 micrometers may escape until they enter the filter again in the next air change cycle. But sooner or later, the HEPA filter will be able to trap them by diffusion. Meaning that in theory many of those particles will naturally get sucked in across the membrane of the HEPA filter and eventually be filtered out by sticking in the mesh with the larger particles.

However, we have to readjust our expectations here since the HEPA filter does have its limitations. It may take several cycles for these small particles to get trapped. Some really small ones like viruses, bacteria, VOCs, smog, and other gases may just be too small even for a HEPA filter.

But before we go on about these small particles, the question is – do they really matter and what are they anyway?


Examples of Indoor Air Pollutants and Their Sizes in Micrometers

Fine particles range in size between 0.1 and 2.5 micrometers.

For reference, a single human hair is about 50 micrometers across. 50 micrometers is also roughly how large a piece of dust has to be for you to be able to see it on your furniture. It’s also roughly the size of a single grain of beach sand.

What you’ll notice is that many pollutants can vary in size, so they may be classified as either a large or fine particle. For that reason, it’s best to look at all indoor air pollutants commonly found, and discuss their general sizes:

PollutantSize (micrometers)Comments
pet dander5 to 10 micrometersThe majority are 5 micrometers and larger (some are even 100 micrometers). About 25%, however, are 2.5 micrometers and smaller, but generally more than 0.3 micrometers.
dust20 micrometersThe range tends to vary between 10 and 40 micrometers, with 20 micrometers being the average. Some dust as small as 1.0 micrometers can be found sometimes.
pollen10 to 1,000 micrometersPollen that are known to cause allergies are closer to the 10-micrometer range. It would seem that pollen is probably the easiest pollutant to filter out thanks to its large size. However, pollen grains can fragment and become fine particles.
bacteria0.2 to 10 micrometersSome can even be smaller and require an electron microscope to even be seen.
viruses0.004 to 0.1 micrometersViruses tend to be smaller than bacteria. They can be very difficult to remove from the air unless specialty, hospital-grade filtration methods are used.
mold 3 to 10 micrometersThe majority of mold spores are 3 micrometers and larger. However, there are some that are only 1.0 micrometer in size.
cigarette smoke0.1 to 1 micrometersParticle size distribution varies greatly and can even be much smaller.
volatile organic compounds (VOCs)picometersThis could technically include 200 chemicals or more, and each is different. However, VOCs are generally considered to be very small since they are individual molecules. We’re talking picometers here, meaning that these pollutants are millions of times smaller than some of the pollutants on the list. VOCs cannot be filtered by a mechanical filter.

Other Methods That Go Beyond a HEPA Filter

By using the list of pollutants from above, you’ll notice that a HEPA filter does has its limitations. It won’t be able to filter out VOCs, cigarette smoke, or viruses. Even other pollutants that are seemingly large in size like pet dander, pollen, and dust won’t be able to be filtered out entirely by a HEPA filter.

Don’t let that keep you away from a HEPA filter. It still is considered one of the best filters to use for an air purifier, but it shouldn’t be used alone.

Other methods should be used in conjunction with a HEPA filter. In fact, you may have noticed air purifiers that are marketed as having a HEPA filter but also having a 3-in-1, 4-in-1, or 5-in-1 filtration system.

An activated carbon filter is an absolute must together with a HEPA filter. Activated carbon is known for its high surface area. Small pollutants like VOCs, smog, and smoke, for example, are attracted and adsorbed to the pores of activated carbon. Look for heaviness in an activated carbon filter. You want actual granules, not the lightweight, spray-on kind.

Some air purifiers have ionizers to break up pollutants into negative ions, but I personally am against this since ionizers create ozone inside a home. For most homes, an air purifier with a good quality activated carbon filter and True HEPA filter will be able to clear out the majority of the fine particles in the air.

Don’t stop at the air purifier, though (or vacuum cleaner). There are other appliances in your home that you can upgrade to help you clean out fine particles from the air. Think about what hospitals are doing to prevent viruses and bacteria from spreading, then mimic that in your home.

For example, your HVAC system is a great opportunity to clean the air in your home. Keep in mind that your entire HVAC system may need to be retrofitted so that it can work with new air filters, however. Otherwise, it may be constantly on to compensate for reduced airflow. Speaking of air filters, the MERV (Minimum Efficiency Reporting Value) rating system is a good gauge to help you determine just how effective an air filter is at filtering out the air through the HVAC system.

Most average (inexpensive) fiberglass air filters are rated MERV-1 to MERV-4. They are intended to protect the HVAC from breaking down, not to purify fine particles in the room. If you want to be able to filter out fine particles, you’ll need a MERV-13 pleated air filter like this one. It’s hospital grade.

Just like activated carbon, UV light is another safe way to purify fine particles from the home. Look for air purifiers that already have a UV light function built in, along with a True HEPA filter, of course. I’m a big fan of the GermGuardian series of air purifiers. Not only do they have all the functions above (including UV light), but the products actually have a CADR rating with an AHAM Verifide label. This to me translates to an air purifier that’s been tested to work in the size of the room I need.

After all, what’s the point of trying to remove fine particles if your air purifier can’t reach across the other side of your room?


Look for a True HEPA Filter

A True HEPA filter has been rated to be able to remove 99.97% of particles that are 0.3 micrometers and larger.

Be careful with air purifiers that aren’t explicitly stated to have a True HEPA filter. You’ll often see labels like “HEPA-like,” “near-HEPA,” or “HEPA-style.” Stay away from those. It means that most of those filters can remove 99.0% of particles 2.0 micrometers and larger in size.

By looking at the table above and its list of pollutants, hopefully, I’ve convinced you enough that there are plenty of pollutants that those 99.0% filters just won’t be able to remove.


Related Questions

How do you clean a HEPA filter?

Always read the manufacturer’s instructions first to make sure your filter is washable. Otherwise, you may be ruining an expensive permanent filter. Also make sure you know what “washable” really means, as some filters are intended to be only vacuumed to be cleaned, without any water. Once you’ve made sure your filter is indeed washable, it’s best to wash the filter outside where the dust and other pollutants can air out. Then, just wash off the filter, using ample pressure from the water hose.  Let the filter dry. Never insert a filter back into an appliance while it’s still wet.


Can ionizers be harmful?

Ionizers have been marketed as an additional step in the air purifying process. The idea is that they break up the molecules of pollutants that are otherwise difficult to filter out. These molecules would be broken down into negative ions. The negative ions fall to the floor as they get attracted to dust, allowing you to vacuum them up and get rid of them.

Ionizers are becoming less frequent and less recommended since these negative ions are quite reactive and can harm the body. The ionizer also creates ozone during the process as a byproduct. If you must purchase an air purifier with an ionizer, it’s probably best to make sure that the ionizer function can be turned off.